TC4 Alloy Basics
TC4 is the Chinese designation for the Ti-6Al-4V alloy, the most widely used alpha plus beta titanium material in the world. It contains about 6% aluminium as the alpha phase stabiliser and about 4% vanadium as the beta phase stabiliser, and it combines a density of roughly 4.43 g/cm³ with a tensile strength comparable to many alloy steels. The balance between the two phases is not fixed: heat treatment and deformation change the proportion, size and morphology of alpha and beta, and those changes set the mechanical properties of the finished plate.
| Nominal composition | Al 5.5–6.75%, V 3.5–4.5%, Ti balance |
| Alloy type | Alpha plus beta titanium alloy |
| Beta transus temperature | About 995 °C for standard chemistry |
| Melting range | About 1604–1660 °C |
| Density | 4.43 g/cm³ |
| Typical annealed plate properties | UTS 896 MPa minimum, yield 828 MPa minimum, elongation 10% minimum |
The beta transus is worth a comment, because handbooks quote values from about 940 °C to about 1000 °C. The spread is a chemistry effect: extra oxygen and aluminium push the transus up, while beta stabilisers push it down, so two plates of nominally identical grade can differ by tens of degrees. Every heat treatment schedule should therefore be set against the transus measured for the actual heat rather than a generic figure.
What the Heating Rate Controls
During heating, alpha transforms to beta, and the rate at which the plate crosses the two phase field decides how much of each phase is present when the temperature is reached. Fast resistance heating, where current output and gas flow are adjusted to drive the plate temperature, keeps the high temperature beta phase from being converted back into coarse alpha on the way up. Slow heating gives the transformation time to complete and produces a more equilibrated structure. Heating rate and hold temperature therefore work together, and comparing test results is only valid when both are recorded.
What the Cooling Rate Controls
Cooling is the stronger lever. When beta phase is cooled, the transformation product depends on how far the atoms can diffuse before the temperature falls out of the transformation range.
| Cooling route | Approximate cooling rate | Transformation product | Effect on the plate |
|---|---|---|---|
| Furnace cooling | Below 0.1 °C/s | Diffusion controlled lamellar alpha plus beta | Lower strength, highest ductility and toughness |
| Air cooling | Roughly 1–10 °C/s | Equiaxed to lamellar alpha plus beta | Balanced strength and ductility |
| Intermediate fast cooling | Roughly 20–400 °C/s | Mixed product containing massive alpha | Strength rises, ductility falls |
| Very fast cooling, water quench | Above about 400 °C/s | Hexagonal alpha prime martensite | Highest strength, reduced ductility, distortion risk |
Widely cited cooling rate studies on Ti-6Al-4V report these three regimes: above roughly 400 °C/s the beta phase transforms almost entirely to alpha prime martensite, between about 20 and 400 °C/s the product is a mixture that includes massive alpha, and below about 20 °C/s the transformation is diffusion controlled and gives lamellar alpha plus beta. Thin plate reaches high cooling rates far more easily than thick plate, so plate thickness must be part of the treatment record; a section that quenches at 200 °C/s in one thickness may only manage air cooling rates in another.
From Microstructure to Mechanical Properties
Martensite in titanium is not the same as martensite in steel. Alpha prime is a diffusionless hexagonal transformation product that is strong but relatively brittle, and it is normally only an intermediate step, because the plate is aged afterwards to let dispersed alpha precipitate from the retained beta and restore a useful combination of strength and ductility. This is the basis of the classic route for TC4: solution treatment in the two phase field, quenching, then ageing at a lower temperature to raise strength while keeping elongation acceptable. Where maximum toughness is needed instead, an annealed alpha plus beta structure produced by slow cooling is preferred.
Higher solution temperature and faster quench: more metastable beta retained, more martensite on cooling, higher strength after ageing.
Slower heating to the same temperature: more complete transformation before the quench, less retained beta and lower strength after ageing.
Longer or hotter ageing: more alpha precipitated, higher strength, lower ductility, and a risk of overageing if pushed too far.
Thicker plate: slower cooling through the section, weaker response to the quench, so properties vary between surface and centre.
Practical Heat Treatment Guidance for Plate
Stress relief is normally performed in the range of 600 to 650 °C, annealing of the alpha plus beta structure around 700 to 800 °C, and solution treatment in the two phase field around 940 to 970 °C followed by quenching and ageing at approximately 480 to 600 °C. Above about 600 °C in air the plate forms a hard oxygen enriched surface layer, so furnace atmosphere, gas flow and time at temperature have to be controlled, and the layer is usually removed chemically or mechanically before service. Temperature uniformity across the plate is as important as the nominal set point, because a gradient produces a structural gradient and, with it, distortion during the quench and residual stresses in the finished part.
Frequently Asked Questions
Q: What does the beta transus of TC4 actually mean?
It is the temperature above which the alloy is fully beta phase. For standard Ti-6Al-4V it is about 995 °C, but oxygen and aluminium raise it and beta stabilisers lower it, so heat treatment should be based on the measured value for the heat.
Q: Why does faster cooling make TC4 plate stronger?
Fast cooling suppresses diffusion, so beta either transforms to hard alpha prime martensite or is retained as metastable beta that later precipitates fine alpha during ageing, both of which raise strength.
Q: Does fast cooling ever reduce performance?
Yes. Too fast a quench lowers ductility and fracture toughness, introduces distortion and residual stress, and can produce a structure that is difficult to machine, so cooling rate must be matched to the part and the specification.
Q: Is plate thickness important for the results?
It is decisive, because thickness sets the actual cooling rate achieved in the section. Two plates given the same furnace schedule can end up with quite different structures if their thicknesses differ.
Q: What happens if TC4 is heated above the beta transus?
The structure becomes fully beta and, on cooling, coarse lamellar or martensitic structures form, which normally reduces ductility and fatigue performance, so beta processing is reserved for specific purposes.
Q: How is the oxygen enriched surface layer dealt with?
It is controlled by limiting time at high temperature in air and by using a protective atmosphere, then removed by pickling or machining before the plate goes into service.
