Ti 6al 4v Phase Diagram

Getting the Phase Diagram Right for Ti-6Al-4V

The Ti-6Al-4V phase diagram is one of those things everyone uses and almost no one actually reads. You grab it from Callister or ASM Handbook, plug it into your heat treatment calculations, and call it a day. It works most of the time. Then it doesn't, and you spend three days wondering why your alpha + beta microstructure looks nothing like the diagram predicted. Here is how to actually use it without losing your mind. The standard Ti-6Al-4V phase diagram you find in textbooks is a binary approximation. Real Ti-6Al-4V has about 6% aluminum, 4% vanadium, and the rest titanium with trace interstitials. The diagram you need is the metastable beta elimination diagram combined with the equilibrium alpha-beta boundary. If you are only looking at one or the other, you are working blind.

What the Ti 6al 4v Phase Diagram Actually Shows

The alpha phase field dominates below roughly 995°C, which is your beta transus temperature for this alloy. Above that, you are in the single-phase beta region. The aluminum raises the beta transus. The vanadium lowers it. They roughly cancel each other out in this particular composition, which is why the transus sits where it does. Add more aluminum, the transus climbs. Add more vanadium, it drops. This matters when you are processing different heats of material and the supplier sends you a cert with slightly off chemistry. Below the beta transus, the alpha and beta phases coexist. The fraction of each phase depends on where you sit in the two-phase field. At room temperature, you get a lot of alpha and a smaller fraction of beta that is enriched in vanadium. The beta phase is what makes this alloy respond to heat treatment. Without it, you would just have weldable titanium and nothing else. When you cool from the beta field, the transformation path matters more than the diagram itself. Equilibrium cooling gives you Widmanstatten alpha plus transformed beta. But you are never doing equilibrium cooling. The cooling rate shifts everything.

I ran into this problem last year on a batch of forged Ti-6Al-4V bars. The spec called for a fine alpha plus beta microstructure with good fatigue properties. I heat treated at 950°C, held for two hours, and air cooled. Came back and the microstructure was coarse basketweave. The phase diagram said I should be in the alpha + beta field at that temperature. It didn't mention that the prior beta grain size at 950°C had grown to about 80 microns because I hadn't controlled the furnace atmosphere properly and the surface alpha case was thicker than expected. The diagram doesn't account for your poor furnace maintenance. It can't. I fixed it by going to a solution treat at 920°C instead, which kept the prior beta grains under 30 microns, followed by a slower furnace cool. The phase diagram still applied. I just picked a different point on it. One thing people consistently miss about the Ti-6Al-4V Phase Diagram is that the alpha solvus and the beta transus are not the same thing. The beta transus is where all alpha dissolves on heating. The alpha solvus is the boundary below which alpha starts precipitating from beta on cooling. Between those two lines, you are in the two-phase field, and the width of that field determines how much microstructural control you have. For Ti-6Al-4V, that gap is roughly 20 to 30°C depending on cooling rate and exact composition. That is a narrow window if you are trying to do something precise like stress relief without changing the strength. Another counter-intuitive point: the phase diagram is barely useful for predicting hardness in this alloy. Hardness in Ti-6Al-4V is driven more by the amount of interstitial oxygen and the morphology of the alpha plates than by the simple phase fractions the diagram gives you. Two samples with identical phase fractions from the diagram can have vastly different hardness if one cooled faster through the beta transus and the other cooled slowly. The phase diagram tells you what phases exist. It does not tell you how they are arranged.

If you want the actual diagram for reference, the most reliable source is the ASM Handbook Volume 3 or the NASA Titanium Design Manual. Neither is free. The online versions you find on materials sites are usually degraded scans with wrong axis labels. I have seen at least three versions of the Ti-6Al-4V diagram floating around where the beta transus is marked at 955°C, then 1005°C, then 920°C on different sites. Check the composition range the diagram was calculated for. If it does not specify, assume it is wrong for your material. The main limitation of relying on the Ti 6al 4v Phase Diagram is that it assumes equilibrium. Your process is not equilibrium. Any heat treatment you run, especially aging or fast cooling, pushes you off the diagram. The diagram is a map, not a GPS. It tells you the terrain. It does not tell you how fast you are moving or whether you are driving off a cliff. Use it to identify the phase fields and estimate phase fractions. Use Scheil-type simulations or computational thermodynamics tools like Thermo-Calc with the TITAL database if you need to track non-equilibrium transformations. The phase diagram alone will get you 60% of the way there. The other 40% comes from experience with the actual material and process you are using.