Inconel 718

By Nick Buck, Final Year MEng student, Department of Mechanical Engineering Sciences, University of Surrey.

Introduction

Inconel 718 is a material of choice for various components of aeroengines, where critically, the material has excellent high temperature properties that enable the increase of engine efficiency. In some engines the alloy makes up almost 50% of the alloys used! Aside from the ability to operate at over 0.7 times its melting temperature, other useful properties include oxidation and corrosion resistance. This makes the alloy important in areas such as turbomachinery components in steam turbines and parts for nuclear power stations.

Microstructure

Inconel 718 is termed a nickel-based superalloy. This means that the main alloy constituent is nickel as shown in the composition in Table 1. The nickel content forms a matrix with an FCC crystal structure in which other alloying additions can dissolve and is denoted γ. The aluminium, titanium, tantalum, and niobium all help create the primary strengthening precipitates. The first three combine with the nickel to form Ni3(Al,Ti,Ta) which is known as γ’. These precipitates have an FCC structure and provide precipitation hardening to the alloy. Niobium and nickel form a second precipitate called γ’’ that also provides precipitation strengthening, this has a BCT crystal structure.

Table 1 Composition ranges of IN718 for aerospace use (Data from Deng, 2018).

A further important phase in the microstructure, are the MC carbides. In Inconel 718, these carbides take the form of (Nb,Ti)C and they are a key strengthener that improves the creep strength of the alloy Substitutional solid solution strengthening is important in addition to the strengthening precipitates. Table 2 summarises all the elements and the roles they play in strengthening the alloy.

Table 2 The role of alloying elements in Inconel 718. Closely based on Deng, 2018 .

ElementEffect
CrSolid-solution strengthener, M7C3 and M23C6 carbides former, improves oxidation and hot corrosion resistance.
CoSolid-solution strengthener, raises solvus temperature of  γ′ Ni3 (Al,Ti).
AlStrengthening phase γ′, Ni3 (Al,Ti) former, improves oxidation and hot corrosion resistance.
TiTi Strengthening phase γ′ Ni3 (Al,Ti) former.
NbStrengthening phase γ′′, Ni3Nb former, MC and M6C carbides former.
FeSolid-solution strengthener.
MoSolid-solution strengthener, MC, M23C6 and M6C carbides former
WSolid-solution strengthener, MC, M23C6 and M6C carbides former.
TaSolid-solution strengthener, MC carbide former, improves creep properties.
CM(C,N) carbonitrides former, grain-boundary strengthener.
BGrain-boundary strengthener, improve creep properties and rupture strength

Processing

Traditionally IN718 is processed using both casting and wrought techniques. Casting is mainly used to produce turbine blades and allows the simultaneous creation of complex shapes and microstructural control. Advanced methods such as directional solidification have been developed by the aerospace industry to further improve the properties of these parts. In contrast, wrought processing; or forging as it is also known, uses heat and deformation to produce the desired shape. This produces parts that have very good mechanical properties, but it is tricky to produce complex geometries. Powder metallurgy is a third method of manufacturing Inconel 718. This uses the powdered alloy and uses methods such as hot isostatic pressing to produce the part. These parts can be cheaper to make than casting and forging while allowing good microstructural control.

The alloy is hard to machine and causes rapid tool wear. In recent years there has also been development into 3D printing Inconel 718 which addresses this problem. The ability of additive manufacturing to produce a geometrically accurate parts that needs little processing is very desirable.

Summary

Although a brief look at one of the most common superalloys used today, this post highlights the unique properties of Inconel 718 and the elements that make up its complex microstructure. With the advances in manufacturing such as with 3d printing, perhaps we will soon see this alloy finding even more widespread use and allow engineers to produce increasingly efficient turbomachinery.

Reference

Deng, D. (2018), ‘Additively Manufactured Inconel 718: Microstructures and Mechanical Properties’, PhD thesis, Linköping University.

Engineering Materials: Trumps

An Introductory Exercise for ENG3164

Diamond is, in many way, a model material. Its strong stiff covalent bonds make its physical properties spectacular. It has the highest hardness and stiffness of any engineering material. It has a very high theoretical strength and is chemically inert. Its thermal conductivity is incredibly high. Despite these spectacular properties the carbon atoms are packed inefficiently—assuming they are hard spheres they fill 34% of a unit cell. This inefficient packing arises due to the nature of the carbon to carbon bond, and carbon’s desire to bond to four other carbon atoms. Because the bonds are mutually repelling (they are identical because of a hybridisation process) the atoms cannot pack efficiently. In a game of trumps diamond would be a real winner.

Despite diamond’s amazing properties it has some issues as an engineering material. These include how much it costs and the difficulty of making it at length-scales greater than a millimetre, or two. High density polyethylene, or HDPE, is at the other end of both the rarity and cost scales. In many ways it is a model polymer. Its high density is, of course, very much a relative term. It is high density with respect to low density polyethylene. HDPE has a higher density than its low density cousin simply by virtue of the geometry of the polymer chains. In HDPE the polymer chains are largely linear, having very few side branches. An even higher density results when extreme care is taken in polymer synthesis and the polymer chains are especially long. A material with this structure is known as ultra-high molecular weight polyethylene or UHMWPE. This more costly variant is used in biomedical applications and in other contexts where resistance to moisture diffusing into the material is important.

The strength of commercial purity aluminium, see below, is a helpful reminder of how pure metallic elements have limited strength (and hardness). Like all common engineering metals, aluminium is used in its commercially pure form when formability and low cost are more important than strength. Though more expensive than the cheapest steels, commercial purity aluminium does not require any surface treatment or protective coatings to survive many years in mundane applications such as the frame of a classroom whiteboard. This material is commonly fabricated by extrusion of the metal at 400-500°C. Extrusion allows for the formation of intricate and thin cross sections.

Compared to aluminium, alumina poses a very different property balance. This property balance, like all property balances poses both opportunities and limitations. The very strong mixed (between ionic and covalent) bonds in alumina make it hard, stiff and very wear resistant. These are excellent attributes for wear resistance in applications such as the ball of a total hip replacement. Shaping and machining alumina to very precise tolerance and appropriate surface finish is, however, a real challenge. These shaping and finishing processes are made even more challenging given the embrittlement created by any porosity in the material. The sintering of a high melting point material so that it has no porosity requires high temperature and the application of pressure. This essential requirement is costly. In other applications such as tool tips, porosity must also be eliminated if the alumina is to exhibit appropriate robustness. In some other applications, such as the insulating exterior of a spark plug, the demands are only a little less stringent.

The four materials presented in the article embody some key opportunities and challenges for the materials engineer and they each illustrate key features of the materials class they in part represent.

Revision

  1. How representative of engineering materials do you think the four materials chosen above are? Would you have chosen differently?
  2. Sketch their relative performance in a tensile test. To do this the data above is helpful but you have to use some knowledge of their likely plasticity too.
  3. Can you explain the relative properties of the four materials above?

Some Questions

  • Would actual trump cards like those introduced above be a helpful learning resource?
  • What materials should be included?
  • Is the design chosen helpful?
  • Can you suggest something better?

Please add some comments below.

AA7075

Wrought Alloys

Aluminium alloys are traditionally grouped into (i) cast alloys, and (ii) wrought alloys. This post concerns one of the strongest wrought alloys. Before explaining why this alloy has such a high strength the term ‘wrought’ needs to be explained.

The term wrought, as an adjective applied to metals, dates back to the industrial revolution. At this time wrought iron—iron that was mechanically shaped at high temperature—was found to have superior ductility and toughness to cast iron. Wrought, in general, means ‘shaped’ or ‘made’. In materials engineering it refers to the plastic shaping of metals at elevated temperature. More specifically when using the term to refer to aluminium alloys it implies a more complex story. Prior to being wrought, aluminium alloys of this type will be cast as large ingots (or produced by a continuous casting process). These large slabs are then further processed by hot working. Hot working is most commonly hot forging (hitting with a large hammer) or hot rolling. The term ‘hot’ also has a very specific meaning. It refers to the material being above the recrystallisation temperature. This will be the subject of a new post in the next week, or so. This hot rolling is the step that makes this a wrought alloy. After hot rolling, the material is generally finished (made into its final shape) by cold rolling. The term ‘cold’ meaning the material is below its recrystallisation temperature.

7075 and its Friends

Wrought alloys fall into a number of series. One of these the Durals are denoted as the 2,000 series, thanks to an internationally agreed numbering scheme for wrought aluminium. You can find our more about this series in ‘D is for Duralumin’. Rather obviously, 7075 belongs to the 7,000 series. This series of alloys, like the 2,000 series, are age hardenable as we’ll explain below. The compositions of 7075 and 7475 (a similar alloy) are shown in Table 1.

Table 1 The typical compositions of AA7075 and AA7475 in wt%.

Table composition 7075

Table 1 indicates that these two alloys are very similar in composition. Indeed they are so similar that they might unhelpfully be thought to be the same. They are not. They have significantly different values of plane strain fracture toughness. Before we explain this, age hardening behaviour needs to be explained.

Age Hardening and 7075

The three elements Cu, Mg and Zn are the three elements responsible for the age hardening response of 7075 and 7475. Because both alloys have nominally very similar amounts of these elements, denoted in green in Table 1, they have very similar aging responses. These three elements have two key features with regard to their solubility in aluminium:

  1. At high temperature (c. 480°C) they have high solubility in aluminium—at the amounts shown in Table 1 they will be fully in solution.
  2. At typical ambient temperature (–40 to +40°C) they have very limited solubility in aluminium. A significant wt% of them will tend to form intermetallic precipitates at lower temperatures.

The upshot of these two facts is the possibility of applying a three-stage heat treatment to the material, see Figure 1.

Blog T6 Nov 2018

Figure 1 The three stages of the T6 heat treatment of 7075.

Similar heat treatments have been described elsewhere on these pages (see for example ‘D is for Duralumin’ and ‘X is for X-750’). The goal of the first step in Figure 1 is simply to ensure that the Cu, Mg and Zn are in solution. Enough time is required to ensure that any pre-existing precipitates containing these here elements have dissolved. This simple microstructure is sown in Figure 2(a). The second step is a fast cool, known as a quench. The purpose of this is to hold, or ‘freeze’, the solute (i.e. the Cu, Mg and Zn) in solution. The microstructure would still look like that in Figure 2(a) after a successful quench, because there is insufficient time for precipitation to take place. Rapid cooling is necessary as the three elements want to form precipitates as the material cools. Sometimes it is difficult to get the heat out of a material quickly enough to hold all the solute in solution. On other occasions the rapid cooling can cause other issues, such a distortion and/or stress corrosion cracking. Both of these issues arise from the ‘frozen-in’ stresses that arise from differential cooling. If a quench is unsuccessful then uncontrolled precipitation can occur. In the extreme, slow cooling produces large but ineffectual precipitates, see Figure 2(b).

The third step is the aging itself. Aging is so-called because it tends to be a lengthy treatment compared to other heat treatments. 24 hours, for example, is quite typical for 7075. The aging is the step that gives the hardness and strength increase, but it only works because of the two prior steps. The key issue about the aging is that the time and temperature have been carefully selected to give control of the precipitation and so a desirable property balance. Figure 2(c) shows the fine precipitates that give the desired high strength for which 7075 was designed.

Blog micros 7075 Nov 2018 Full Figure

Figure 2 Some schematic microstructures of 7075. (a) The high temperature microstructure which arises during solution treatment once any prior precipitates are dissolved, (b) the undesirable large precipitates that arise during slow cooling, (c) the desirable fine precipitates that form during aging, (d) a more realistic microstructure which includes the inclusions which cannot be altered by heat treatment.

The concept of property balance is very important as sometimes strength is required at the expense of other properties, but on other occasions those other properties have to be accounted for. One of the most challenging aspects of the treatment in Figure 1, known as a T6 heat treatment, is that the peak in strength coincides with very poor corrosion resistance. The same precipitate distribution that gives high strength gives poor corrosion performance. In practice a large number of variations on the T6 are possible to ensure that properties such as tensile proof stress, ultimate tensile strength, fatigue strength, ductility, fracture toughness and corrosion resistance can all be balanced for different applications.

Impurities and Plane Strain Fracture Toughness

Cu, Mg and Zn are the elements chosen for strengthening 7075 and many other high performance 7,000 series alloys. These elements are shown green in Table 1 as they are good news in an alloy designed for strength. Three elements are shown in red. The red signals some bad news. The elements Fe, Si and Mn are impurities in aluminium alloys that are a result of primary aluminium production. They are detrimental to ductility and especially fracture toughness. The reason why they lower the fracture toughness is their tendency to form large and brittle particles, see Figure 2(d). There are three separate pieces of bad news here:

  1. Such brittle particles are stress raisers and often have cracks in them arising from the previous mechanical working steps. Both of these factors make problematic crack initiation and propagation more likely.
  2. They form during the casting of the alloy and cannot be altered by heat treatment.
  3. Reducing the levels of Fe, Si and Mn (refining) is expensive.

Although removing these elements is expensive, it is sometimes worth doing. The alloys 7475 was created as an improvement on 7075. The design philosophy was to lower the volume fraction and size of the inclusions. They was achieved by limiting the Fe, Si and Mn in this alloy much more strictly than in 7075, see Table 1 above.

Other Alloying Additions

The enthusiastic reader will notice that the roles of Cr, Ti and Zr have not been addressed. The full story will have to await another post. For now we can note that they help limit the grain size of the alloy.

Closing Comments

The basic story unfolded here provides a starting point for understanding all age hardenable aluminium alloys. Further afield the stages of solution treatment, rapid cooling and aging are central to some steels, some titanium alloys and creep resistant nickel alloys.

 

 

 

Zamak

This post is the first guest post on this site. Many thanks to Alex Nelson, Undergraduate in Mechanical Engineering Sciences, University of Surrey.

Are you a zinc alloy, ‘cause you’re Zamak-ing me crazy.

This is the first post on this site focusing on a zinc alloy, although zinc has been introduced as a coating on steel. The world of engineering alloys is dominated by ferrous, aluminium, titanium and copper alloys. Zinc alloys, including Zamak, are niche materials.

Zamak, also known as Zamac, is named in recognition of its constituent elements: zinc, aluminium, magnesium, and copper. The ‘k’ originates because copper is kupfer in German. Strictly speaking Zamak is a family of alloys rather than a single alloy. The most common Zamak alloy is Zamak 3, with Zamak 2, 5 and 7 also still used commercially [1]. In terms of basic metallurgy Zamak alloys are based on Zn-4wt%Al alloys—the other elements are present in very small quantities. Zamak 3, for example, contains approximately 0.2 wt% copper and 0.02–0.05 wt% magnesium. Like many other commercial alloys there are trace amounts of impurities, and in the case of Zamak these are typically iron and lead.

Zamak was developed in 1929 by the New Jersey Zinc Company [2] and then, as now, its primary application was/is die casting. The majority of readers of this blog are very likely to have encountered Zamak as the case of a hand stapler and/or as die cast toys, most commonly cars.

When considering zinc alloys for die casting, Zamak 3 is typically the first choice due to its excellent cast-ability—an elusive property which combines flowability which in turn depends on melt viscosity and freezing range. It has also been found to have good finishing characteristics for both plating and painting/lacquering. This latter point is especially important in the toy sector where decorative presentation has to provide not only good aesthetics but needs to be robust too.

Figure 1 shows the attractive microstructure typical of Zamak, in this case from an ingot of Zamak 3. Elsewhere in the same ongot porosity is present, see Figure 2. Such porosity can be probelematic for the integtrity and surface finish which is why die casting is the main processing route for Zamak. Die casting utlises high pressures to limit shrinkage porosity.

Figure 1 Zamak

Figure 1 A backscattered electron micrograph of the microstructure of a Zamak 3 ingot. The light primary phase is clearly seen in a matrix of eutectic microconstituent.

Figure 2 Zamak

Figure 2 A secondary electron micrograph of a pore in Zamak 3.

 

References

  1. Eazall.com. (2018). Eastern Alloys – Zinc Die casting Alloy. [online] Available at: https://www.eazall.com/zinc-die-casting-alloys [Accessed 20 Jun. 2018].
  2. Viasetti, G. (2018). Advantages of the Zamak die-casting | Zinc alloy die casting. [online] Pressfinmetal.it. Available at: http://www.pressfinmetal.it/eng/information-about-zamak.html [Accessed 20 Jun. 2018].

 

 

 

 

 

Additive Manufacture of Engineering Alloys

3D Printing, or Additive Manufacture, is a transformative technology. This means that it is revolutionary in its implications for delivering local and made-to-measure products. More than that, it promises to challenge the way things have been made since the consolidation of the Industrial Revolution in the mass production of steel and other engineering alloys. At the present time the ‘revolution’ has been in niche areas. For example, modern hearings aids are now mostly made by 3D Printing. It is the 3D printing of polymeric materials that has thus far had the biggest impact for the public. Not only are some products now made this way, but having a 3D printer at home has become a possibility. At the moment such printers are purchased by enthusiastic hobbyists of one type or another. It is not much a stretch, however, to imagine the majority of homes having one in the UK within a decade or so,

The term 3D Printing is not without its problems but it is probably here to stay as it captures the experience that many people have with this technology. I prefer the term Additive Manufacture as this term helpfully reminds us that much traditional manufacture is subtractive. Some of the most extreme examples of Subtractive Manufacture are found in the aerospace sector. The structural integrity of many civil and military aircraft depend on very large components made from aluminium alloys. Given the importance of minimising the weight of an aircraft these structural components are shaped so as to deliver the necessary strength and stiffness without any excess material. In engineering, the shapes required to achieve this goal tend to be very complex (a big story for a later post!). Because of this necessity to save weight it has become normal to produce a very large forging only to then machine it away bit-by-bit so that sometimes only 5–10% of the material remains in the finished component. So prevalent is this process of Subtractive Manufacture that a technical term exists which describes this problem. This aerospace terminology is the buy-to-fly ratio. Our 5–10% material remaining after subtraction equates to buy-to-fly ratios in the range of 10:1 to 20:1. It is self-evident that there is a lot of expensive machining required as well as a large amount of material wasted. Even when we factor in recycling, the recycled material cannot always be used to make such high value-added components next time round.

For applications with such troublesome buy-to-fly ratios it is easy to see the appeal of additive manufacture as it offers the hope of near-net shape components that require very little machining and therefore material loss and recycling. If that was the whole story, Additive Manufacture would of course been adopted wholesale and the revolution mentioned above would be over. There are however a collection of other challenges in moving from subtractive traditional processes of manufacture to 3D Printing. The challenge is also different for the various materials classes. For polymeric materials there are many applications that do not require high strength, such as the hearing aids already mentioned. For this reason additive manufacture of polymeric material is relatively mature approach. For metals there is a bigger hurdle. We use metals, or more correctly engineering alloys, when we require strength and toughness. Making engineering alloys by additive processes to have reproducibly high strength and toughness to match traditionally made materials is challenging.

Over the next few posts we will explore these challenges and point to various ways to address them. The challenges include porosity, microstructural issues, reproducibility and environmental impact. Before we explore these various challenges the second post will introduce the range of Additive Manufacturing Process that are being used to make engineering alloys, and a third post will outline a major advantage of additive manufacture: topological optimisation.

 

Nickel-Chromium 625

Introduction

This alloy is often known as Inconel® 625 or just 625. It is a widely used nickel-chromium alloy in applications were outstanding corrosion resistance is critically important. In addition to its excellent corrosion resistance it also exhibits high strength—even in the annealed state it has a 0.2% proof stress of around 400 MPa and an ultimate tensile strength (UTS) of 800 MPa, or so. It can be cold worked to achieve a UTS of 1,100 MPa. It is also relatively easy to fabricate and can be joined readily by a variety of welding processes. Unlike many high strength alloys its microstructure does not tend to be susceptible to changes that lead to a significant loss in either corrosion resistance or strength.

A consideration of its composition, shown in Table 8, helps explain its corrosion resistance, fabricability and strength.

Table 8 The nominal composition in wt% of Inconcel® 625.

625 Composition

1Ta is also present but not distinguished from Nb.

Corrosion Resistance

As with all high corrosion resistance nickel alloys, the addition of Cr is key. 22 wt% Cr represents about the highest addition of Cr possible without any significant risk of formation of the embrittling intermetallic, Ni2Cr. Such a high level of Cr confers excellent general corrosion resistance. The high Mo content is key to limiting localised corrosion such as pitting and crevice corrosion—although the mechanisms for this role of Mo are still the subject of controversy.

Fabricability

Nickel-chromium alloys can be usefully grouped into two categories, those that exhibit some precipitation strengthening and those that do not—which category they fall into is evident from their composition. Because gamma prime is the intermetallic phase Ni3(Al,Ti) then the total amount of the elements Al and Ti reveals whether there is the possibility of gamma prime precipitation. This can be seen schematically in the graph in Figure 16, where it is evident that gamma prime formation is only possible for additions of around 2.5 at% Al+Ti or more.

Gamma Prime Content in Ni AlloysFigure 16 A schematic graph showing the volume fraction of gamma prime resulting from additions of (i) aluminium and (ii) aluminium + titanium.

In Alloy 625 the maximum combined total of aluminium and titanium is 0.8 wt%, c. 1.3 at%. This has important implications for the processing and fabricability of Alloy 625, including:

  1. The lack of the possibility of precipitation hardening, by gamma prime formation, means that it is only heat treated in a single stage designed to induce recrystallization.
  2. The lack of reliance on precipitation hardening makes further mechanical processing easier as there are no precipitates present to harden the material and lower the ductility.
  3. The lack of precipitates also makes the microstructure tolerant of temperature excursions such as those created by welding, thereby making the alloy weldable.

Strength and Strengthening Mechanisms

Having established that Alloy 625 is a formable alloy, lacking any gamma prime precipitates, we can note that it primarily gains its strength from solid solution strengthening (or matrix strengthening). This is a significant effect because of the large amount of additions, more than 40 wt%, and the range of sizes of the atoms in solution. The lattice structure has a high density of elastic strain fields around the atoms which are smaller and larger than the average. These strain fields impede dislocation glide and thereby cause strengthening and hardening.

The only other ways to strengthen Alloy 625 are by work hardening and ensuring that it does not experience grain coarsening during any annealing/recrystallization heat treatment.

 

Vanadium

Background

When this blog originated in April 2016 there were 26 posts in the first month. These provided an A to Z of metals and alloys that have ‘changed the world’. It was a literal A to Z in that it started with ‘A for Aluminium’ and proceeded through the alphabet to ‘Z for Zircaloy’. These first posts also attempted to provide a coherent ‘story’ by introducing and developing some key concepts. Anyone new to this blog can still read these original posts in A to Z order by starting with the Aluminium post and clicking successive ‘next page’ buttons at the bottom of each page. Increasingly I find that most readers make use of the Index to find a post or posts of interest—this index is ideal for the casual visitor or for those with a very specific interest.

When the original posts appeared some readers expected that when the letter V was reached the metal of choice would be vanadium. It wasn’t. Instead I wrote a rather light-hearted post about the fictional alloy vibranium. This joke took an unexpected turn and eventually became a chapter in a book. Now finally I have written the eagerly anticipated ‘V is for Vanadium’.

Unlike many metallic elements, such as gold and uranium, vanadium has not made a great impact on the world in its own right. Neither has it given rise to a significant class of alloys in which it is the majority element. Yet, despite this, vanadium is critical to the success of some important alloys. This is the case in titanium alloys, where it is used in all three major groups: alpha, alpha/beta (duplex) and beta alloys. The most widely exploited titanium alloy, Ti-6Al-4V, contains 4wt% vanadium and accounts for well over half of all commercial titanium sales. There are also steels where small to medium additions of vanadium enable a property balance that is difficult to match with other alloys. Before this post considers the importance of vanadium in these titanium and ferrous alloys there a few things that need to said about the element vanadium.

Vanadium the Metal

Vanadium is a transition metal and is sandwiched between titanium (atomic number 22) and chromium (atomic number 24) on the periodic table. Like chromium it has a simple body-centered cubic (bcc) crystal structure. This makes vanadium a reasonably ductile metal because of the high symmetry of this crystal structure. Its density of 6.0 g cm-3 means that it is denser than titanium but less dense than the Cr, Mn, Fe, Co, Ni, Cu and Zn. The name vanadium arises not from its properties as a metal but rather from the characteristics of its compounds and salts. Various attempts were made to name vanadium but uncertainties about whether it was a distinct element to chromium meant a variety of proposals were made. It was the Swedish chemist Nils Gabriel who firmly established that vanadium was a new elements and he proposed the now accepted name. He chose vanadium because he had worked on creating various chlorides of vanadium and he wanted to acknowledge the remarkable range of bright colours adopted by these and other vanadium salts and compounds. The immense variety of colours that Sefström observed are due to the ability of vanadium to form compounds in which it can have one of four oxidation states: +2, +3, +4 and +5.  In ancient Norse mythology Vanadis is the goddess of fertility and beauty (among other things)—vanadium’s compounds are at once both beautiful in colour and abundant in their variety of hues. One especially important vanadium compound which students of A Level Chemistry should remember is vanadium pentoxide, V2O5, which catalyses the oxidation of sulphur dioxide to sulphur trioxide in the contact process:

2.SO2 + O2 ⇔ 2.SO3

Vanadium also has some interesting biological functions in some marine life forms, such as algae. This has the consequence that some fossil fuels contain significant amount of vanadium impurities. But our concern is with some important engineering alloys.

Vanadium in Titanium Alloys

As we saw in ‘B is for Beta Titanium Alloys’ vanadium is a beta stabiliser, i.e. it encourages titanium to be bcc rather than hexagonally close-packed (hcp). Vanadium is used in some alpha alloys although only ever at levels of around 2.5wt% or less due to its beta stabilising character. This the case with the very popular near alpha alloy Ti-3Al-2.5V which is sometimes known as Grade 9 titanium. This alloy has an excellent balance between strength and formability. It is widely exploited for this reason in tubular form. Commercial purity titanium, or Grade 2, is more workable but not as strong as Grade 9. The infamous alloy Ti-6Al-4V, also known as Grade 5, is higher in strength than Grade 9 but is both more expensive and more difficult to shape.

In Grade 9 vanadium hardens and strengthens the alpha phase solid solution by substituting in the hcp lattice. In Grade 5 having promoted the formation of the formation of some beta phase it favours dissolving in this phase. It, however, solid solution hardens both metallic phases in this alpha/beta alloy. In some beta alloys vanadium is joined by other elements that also substitutional solid solution the alloys. It is the combination of these different elements and the large percentage of them which enables beta titanium alloys to exhibit very high strength. It should be noted however that such alloys have a higher denser than commercial purity titanium. Table 7 shows the composition of the beta alloy Grade 19 titanium which contains 8wt% vanadium.

 

Table 7 The composition of Grade 19 titanium in wt%.

Ti

Al

V Cr Mo

Zr

balance

3%

8% 6% 4%

4%

alpha stabiliser beta stabilisers neutral addition

 

The Earliest Vanadium Steels

It was early in the twentieth century that the discovery was made that small additions of vanadium could harden and strengthen steel. One of the very earliest applications of vanadium containing steel was in the chassis of the first car manufactured on an assembly line, the famous Model T Ford (1909–1927). The mechanism of the strengthening is not the same as that of vanadium in titanium alloys. Rather than staying in solution, vanadium in ferrous alloys is a strong carbide former. Vanadium carbide can precipitate, depending on the steel, in just about every other constituent, for example, ferrite, pearlite and martensite. In all cases the often submicron sized carbides impart a precipitation strengthening effect of several tens of MPa or more. In more recent alloys the understanding of carbide formation has led to the use of additions of nitrogen so as to create vanadium nitrides or vanadium cabonitrides too.

Although vanadium is produced as pure metal for use in the manufacture of alloys, because the majority of vanadium is used in steels a feedstock known as Ferrovandium is often made. This alloy precursor has varying amounts of vanadium from as low as 35wt% to as high as 80wt%.

Vanadium in High Speed Steels

High Speed Steels (HSS) gained their name as they were developed for tools and tool components and in the most demanding of these applications they material they cut or drill moves rapidly relative the tool. The first half of the twentieth century saw a rapid proliferation of HSS. Although vanadium is generally a minor addition to these alloys it still plays a very important role in though the creation of vanadium carbide (VC). This can be seen in Table 8 where the compositions of three popular HSS are shown.

Table 8 The composition in wt% of three popular MSS.

 

Fe C V Cr Mo W Mn Si

T1

balance 0.65% 1% 4% 18.0% 0.2 0.3

M1

balance

0.80%

1%

4%

8%

1.5%

M2 balance 0.96% 2% 4% 5% 6.0%

 

Vanadium in High Strength Low Alloys Steels

If the first half of the twentieth century was characterised by the addition of many things to steel to create a vast array of alloys, the second half often focused on removing things from steel and/or adding the bare minimum. We have already seen this with maraging steels where C, Mn, Si, P and S are all minimised to improve plane-strain fracture toughness.

The combination of removing impurities and getting the best from small additions is the basis for the High Strength Low Alloy (HSLA) steels. These steels have been critically important for decades in automotive applications as well as being used in large structure, such as bridges and rollercoasters. Their importance in the automotive sector is seen in the most widely adopted naming system where HSLA steels are given SAE designations, where this denotes The Society of Automotive Engineers.

In this way these HSLA steels evolved from vanadium high-carbon steel alloys contain 0.15% to 0.25% vanadium. Rather than give some example we can note that a typical HSLA steel will have around 0.2wt% C and 1–2wt%Mn. Three other elements are normally minimised such that there is less than 0.04wt%P, <0.05wt%S and <0.9wt%Si. So where is the vanadium in these steels? Well not all of them have vanadium additions but many do. Once again when added it is a carbide former. The highest strength HSLA steels tend to have an addition of vanadium or niobium. Niobium is a story for another day.

 

Is it a Ceramic? Is it Graphene? No it’s Vibranium!

This last week saw the official launch of the book, ‘The Secret Science of Superheroes‘, edited by Mark Lorch and Andy Miah and published by the Royal Society of Chemistry. I was fortunate enough to be asked to contribute a chapter to this book.

The book covers a diverse range of scientific disciplines, including Chemistry, Physics, Materials Science, Computer Science and Biology. It uses Western cultures preocuppation with superheroes as a point of departure for showcasing serious science at an accessible level. The basic premise of the book is: how far can science go in explaining the remarkable powers and feats of superheroes?

In my chapter I chose to deal with a material rather than a superhero per se. That material, as the above title suggests, was vibranium. The figure below shows something of the journey of the book chapter. Reading clockwise the figure outlines the methodology I adopted:

  1. The Marvel Cinematic Universe (MCU) was chosen as the source of information rather than the comics.
  2. The MCU’s most useful films for exploring vibranium are those in which Captain America uses his vibranium shield as a defensive and offensive tool.
  3. When these five films (see figure below) are studied some key facts emerge about vibranium. We can add high fracture toughness and high hardness to the properties in the figure.
  4. The book makes a case for what type of materials vibranium could be, see figure. This is complex material currently beyond conventional materials science and engineering.
  5. Further data will soon be available to test the hypothsis as the MCU’s focus turns to Wakanda, the ‘home’ of vibranium.

Vibranium M J Whiting Saturday 7th Oct 2017

New Posts Coming Soon

It is the time of year when this blog wakes up. It will be providing some background material for a University of Surrey module (Engineering Materials, ENG3164) which I teach. In addition it will be informative to anyone interested in metallic materials. The posts are mostly factual but occasionally fictitious metals are featured too.

Future posts will include:

  • The Ni-Ti shape memory alloy nitinol.
  • The long-promised post on vanadium.
  • An update on vibranium.

Requests also considered!