#phaseposts

The Iron-Nickel-Chromium Ternary System

Despite the labeling of the phase diagram from Phase Poll #8, the Fe-Ni-Cr diagram isn’t strictly a diagram of stainless steel compositions. True, stainless steels are iron based alloys containing both nickel and chromium in varying amounts, but if we move to the nickel side of the phase diagram, for example, we start moving toward nickel-based superalloys, not steels, which by definition are predominantly iron. Which isn’t even taking into account the numerous other elements present in steels and the other alloys touched upon by this system, including manganese, molybdenum, titanium, aluminum, etc.

Alloys within this space include the following (all compositions are approximate averages of the many alloys within each category):

Austenitic stainless steels usually have around 18% Cr and 10% Ni, sometimes with some Mo or Ti. Ferritic stainless steels and martensitic stainless steels usually have around 16% Cr, but they don’t always even contain nickel. Duplex stainless steels have around 25% Cr and lower Ni at around 5%, in addition to some Mo. Precipitation hardening steels again have around 16% Cr, but can vary widely in their Ni content, from 5-25%. Moving away from steels, Ni-based superalloys such as Inconels can have 15-30% Cr and 5-10% Fe, with the balance as nickel. High chromium alloys, however, aren’t common. While chromium is added for it’s corrosion resistance, Cr is has a BCC crystal structure, which, while strong, isn’t as ductile as the FCC structure favored by Ni*.

That means the composition chosen by the poll, 42% Cr, 36% Fe, and 22% Ni, isn’t really going to be found in any commercial alloys. However, we can discuss the microstructure at that composition, which just barely stays in the Cr + γFeNi region of the phase diagram at that temperature. γFeNi, or the gamma phase, is a disordered solid solution FCC structure that forms the matrix of most Fe-Ni superalloys, though you typically need slightly more Ni than we have to stabilize the phase. The Cr phase here is also sometimes labeled as α’, and it is a Cr-rich BCC phase (where α would be the Fe-rich BCC phase).

Sources/Further reading: ( 1 ) ( 2 - image 2 ) ( 3 ) ( 4 )

Image sources: ( image 3 ) ( image 4 )

More phase diagrams: ( image 1 ) ( Cr-Fe-Ni, 500K ) ( Cr-Fe-Ni, 1000K ) ( Cr-Fe-Ni, 1073K ) ( Cr-Fe-Ni, 1473K ) ( Cr-Fe-Ni, 1500K ) ( Cr-Fe-Ni, 2000K ) ( Cr-Fe-Ni, liquidus projection ) ( Cr-Fe ) ( Cr-Ni ) ( Fe-Ni )

The Magnesium-Zinc Binary System

Phase Poll #5 has concluded - and nobody pointed out the glaring error I made! The phase diagram is for the magnesium (Mg)-zinc system, even though I typed out manganese (Mn) for some reason. So, I’m just going to pretend that you all would have voted as much for magnesium as you did for manganese! The final result was the composition above (41 mol% Zinc; 59 mol% Mg), with no specific votes for temperature.

The numerous intermetallics formed in this binary system are not well studied, but we’ll discuss what is known! Along that compositional line, we encounter the following phases as we decrease in temperature: liquid, C14 Laves (MgZn2), Mg2Zn3, Mg12Zn13, Mg51Zn20 and an HCP solid solution of the two elements. Of the above listed intermetallics, the crystal structures of MgZn2 and Mg51Zn20 are known (space groups numbers 164 and 71, respectively). Data about other phases, such as entropy and enthalpy of formation has been calculated, however I couldn’t find any structural information.

If we approach the alloy space from the perspective of magnesium with zinc added, magnesium is a common material used for its light weight and its biodegradability in the form of metallic implants. (Magnesium can be dissolved after implantation, eliminating a need for a second surgery to remove an implant.) As zinc is also biocompatible, the combination of these two elements in medical technology is an area of interest. The addition of zinc to magnesium, generally speaking, increases strength but induces brittleness. Alloys near the middle of the phase diagram also form significant intermetallics, adding to this effect.

Sources/Further Reading: ( 1 ) ( 2 - image 2 ) ( 3 ) ( 4 )

Intermediate Compounds and the Nickel-Uranium Binary System

Phase Poll #11 concluded yesterday with a fairly even split between nickel (53%) and uranium (47%). We also got one vote for a temperature of 1000K, thanks to @grimmtaupe. The Ni-U diagram is really only considered in the space of nuclear alloys, and isn’t very popular even then, so we’re going to spend some time on this post talking about intermediate compounds.

I’ve already briefly discussed congruently melting compounds vs. incongruently melting compounds, after the conclusion of phase poll #7. These definitions focus on phase transitions, while defining intermediate compounds starts to delve into the realm of solid solubility. Uranium and nickel have very low solid solubility below ~600°C (or ~873K). This can be seen from the numerous vertical lines on the phase diagram, each of which represents a specific intermediate compound, defined as simply phases with compositions that fall between two neighboring phases. Intermediate compounds almost always have a different crystal structure than the neighboring phases as well.

Solid solutions, on the other hand, can be continuous (like in the Ni-Cu phase diagram), primary/terminal (the fcc Ni phase and the bcc U phase at either side of the Ni-U phase diagram), or intermediate (like the hcp phase in the Ag-Al phase diagram). The primarily vertical lines on a phase diagram can thus give a visual indication of the solid-solubility of two elements across their compositional space.

Back to the chosen composition, our alloy would be primarily Ni9U7. Not much is known about this compound, but the thing about phase diagrams is that they’re difficult to create, even if computational tools have accelerated the process in modern times. Some research (ref. 3 below) into nickel-uranium alloys suggests that the phase is actually U10Ni13, and crystalizes in a monoclinic crystal structure. If we consider the temperature as well, we can see from our previous definitions that Ni9U7 is an incongruently melting compound. For the nickel uranium system in general, small additions of nickel have been shown to promote sintering of uranium carbides.

Sources/Further reading: ( Ni-U: 1/image 2 ) ( Ni-U: 2 ) ( Ni-U: 3 ) ( Phase diagrams: 1 ) ( Phase diagrams: 2 ) ( Phase diagrams: 3 )

The Nickel-Zinc Binary System

Phase Poll #13 concluded yesterday with a pretty even split between the two elements. Nickel got slightly more votes with 51.5%, and zinc slightly less with 48.5%. Before we move into the properties and applications of nickel-zinc alloys, let’s talk a little about what we’ve already learned about phase diagrams.

From the left and right sides, we can see that zinc is fairly soluble in nickel, but that nickel is not soluble in zinc. We can also tell, without knowing anything about the periodic table, that zinc and nickel are close to each other, given that the atomic and weight percents are fairly similar. Despite their proximity on the periodic table though, we can see from the phase diagram they have vastly different melting temperatures, with zinc melting at over 1000°C less than nickel.

The primary application of this binary system is in corrosion-resistant coatings. While pure zinc coatings were once more common, small amounts of nickel help to increase the corrosion resistance. These coatings usually contain around 85% zinc or 15% nickel, resulting in the γ phase shown in the above diagram. Not only does this phase provide better corrosion resistance, but it is more ductile as well, allowing for easier coating. However, nickel remains much more expensive than zinc, limiting applications of such coatings.

The middle of the phase diagram is less well studied, and alloys with equal parts nickel and zinc are uncommon. It is known that the β1 phase crystalizes with a CuAu type structure.

Sources/Further reading: ( 1 - image 1 ) ( 2 - image 2 ) ( 3 - images 3 and 4 ) ( 4 ) ( 5 )

The Titanium-Chromium Binary System

Phase Poll #9 concluded yesterday with a fairly even split between titanium (55%) and chromium (45%) and a single vote for temperature at 1400°C (thanks @bow-ties-and-daydreams​!). Practical applications of this binary system mainly occur on the left side of our phase diagram, as the properties of Ti are typically considered improved with small additions of Cr (increased strength and hardness, as well as corrosion resistance). Both elements have a bcc structure at higher temperatures, and although Ti has a different low temperature phase, in the form of hcp, Cr retains its bcc structure (stronger, but more brittle than fcc) and thus is rarely used as the main component of structural alloys.

Directly below our chosen temperature of 1400°C, the Ti-Cr system forms a continuous solid solution, though this is only maintained between 1350°C-1400°C for values of ~50-70wt.% Cr. At our composition, this solid solution would be a pretty even mixture of both elements.

At slightly lower temperatures, solid-solution decomposition leads to precipitation of the TiCr2 intermetallic phase that can be maintained down to room temperature (not shown in the above diagram), forming a two-phase material with beta titanium as the second phase. The structure of this intermetallic is a C15 Laves phase, with Ti as the minority component, as is typically for alloys of Ti with other transition metals. The microstructures resulting on cooling often contain twinning, notably within the Laves phase.

Titanium has long since been considered a prime material choice, but the aluminum and vanadium alloys used in the past have come under scrutiny more recently, with concerns of long-term health effects from the alloying elements. As such, Ti-Cr alloys are under consideration for biomedical applications, including in the dentistry field, as the search for the best biocompatible alloys continues.

Sources/Further Reading: ( 1 - image 1 ) ( 2 - image 2 ) ( 3 - image 3 ) ( 4 - image 4 )

The Aluminum-Magnesium-Silicon Ternary System

The winner of Phase Poll #4 was magnesium by a clear margin (41%), while the other two elements were decently split (25% Al, 33% Si). As a result, that puts us squarely in the Mg2Si phase on the phase diagram above. Before we spend time discussing this phase diagram, though, I want to talk about ternary phase diagrams in general, and how binary phase diagrams can be seen in liquidus projections.

To start with, the Al-Si binary system is a eutectic system, with the eutectic point at a little over 10% (mol%, here) Si. This can be seen pretty clearly in ternary phase diagrams by looking at the Al-Si side of the plot. Above, we can see the line that branches from just over 10% Si - and we can see it again if we look at other ternary systems, like the Al-Cu-Si system and the Al-Si-Zn system. An example projection of a binary system with two eutectics, projected onto a ternary system, is shown below:

In this way, we can still gain some understanding about what a binary system would look like, even if we only have the liquidus projection of a ternary system containing that binary.

But, onto Al-Mg-Si in particular. The Al-Si alloy classification is a broad one, leading to alloys with low density, high corrosion resistance, and good castability, but these alloys are not easily heat-treatable, which is where the Mg comes in. With the addition of Mg, alloys in this ternary system are thus well used, offering strength in addition to being light weight. They are often used in the automotive and aerospace industries, with the addition of other elements (Fe, Cu, etc.) as desired, and are typically produced through casting. One common alloy used for additive manufacturing is known as Al-Si10-Mg.

The voted for phase, Mg2Si (ignoring the Al content), is an inorganic compound that functions as a narrow bandgap semiconductor and has applications in thermoelectrics. In Al-Mg-Si alloys, the phase acts as a precipitation hardening mechanism, brittle but strong. In addition, it is sometimes used as feedstock material for the creation of aluminum alloys.

Sources/Further Reading: ( 1 - image 1 ) ( 2 - image 2 ) ( 3 - image 3 ) ( 4 - image 4 ) ( ternary phase diagrams ) ( Mg2Si )

The Silver-Calcium Binary System

Phase Poll #7 concluded last week - apologies for being a little late with this post! You guys were almost equally split this time between silver (at 51% of the votes) and calcium (at 49%), which puts us squarely in the middle of the phase diagram, pretty near to the equiatomic intermetallic AgCa.

Unfortunately, when it comes to the Ag-Ca system, there’s not much to say. Not to worry though, I wanted to use this opportunity to talk a little more about the components of phase diagrams! Invariant points have already been covered by this blog, but I’m going to touch on them again to give the definition as points on phase diagrams with no degrees of freedom - they only occur at specific compositions, temperatures, and pressures, etc. In the above diagram, there are several eutectic points, and a couple peritectic points.

Congruent melting, meanwhile, is a phase transition wherein the composition of the liquid is the same as the composition of the solid. In incongruent melting, contrarily, a phase melts to a composition of liquid different than that of the solid. The former case is common in binary systems, with intermediary phases and two-phase regions wherein solids precipitate from the liquid before full melting occurs.

Returning to the silver-calcium system specifically, there aren’t many applications that make use of binary mixtures of the two elements. Silver-calcium batteries, despite the name, are still primarily lead-acid batteries with small amounts of calcium and silver added (as in, even less than 1%). Historically, silver-calcium alloys have been prepared as an intermediary step in the creation of silver catalysts. Another interesting feature of note in this phase diagram is the high temperature calcium phase. While calcium is fcc at room temperature, it has a phase transition above 450°C.

Sources/Further reading: ( 1 ) ( 2 ) ( 3 ) ( 4 ) ( congruent melting ) ( understanding phase diagrams ) ( phase diagram definitions )

The Bismuth-Indium-Tin Ternary System

Phase poll #2 concluded yesterday, with bismuth being the fan favorite! The final composition of the poll was ~46% Bi, 30% In, and 24% Sn, which will be discussed at the end of the post. Our phase diagram this time was a ternary phase diagram, with the composition in weight percent of each element. A few more examples of the ternary system diagram can be found in the sources and the more phase diagram links below.

As this is the first ternary phase diagram I’ve posted, first, a little background information. As each of the three axes are composition only, ternary phase diagrams have two main modes of being displayed: isothermal plots, where the temperature (and pressure) are constant, or liquidus surface projections, which often, but not always, include isothermal lines. The heaviest element is often, though not always, placed at the bottom left, with the lightest element placed at the top peak.

The primary applications of this alloy system take advantage of the low melting points achievable. There are two eutectic points - compositions in which the melting temperature is lower then the melting temperature of the individual components - on the phase diagram, one with a melting temperature of ~59°C and the other with a melting temperature of ~76°C. Field’s metal, also known as Field’s alloy, has a composition close to the lower-melting point eutectic. It, and similar alloys, have been investigated as lead-free solders, as well as applications including phase change materials, rapid prototyping, and uses as a liquid metal.

The (Bi), (In), and (Sn) phases above have the crystal structures of their primary components: rhombedral for bismuth, and body-centered tetragonal for indium and β-tin. The phases on the left edge of the phase diagram are primarily bismuth-indium intermetallics. The two named phases are β and γ. The poll-selected γ phase has the chemical formula of In1-xSnx, and can be a component of the lower melting point eutectic.

Sources/Further Reading: ( 1 - images 1 and 4 ) ( 2 ) ( 3 ) ( 4 ) ( ternary systems )

Image sources: ( image 2 ) ( image 3 )

More phase diagrams: ( isothermal diagram at 500K/1bar ) ( isothermal diagram at 300K/1bar ) ( liquidus projection in mole fraction )