What the Beta Transus Means
The beta transus of a titanium alloy is the temperature at which the last alpha phase transforms to beta on heating, and it is the boundary between the alpha-beta field and the single-phase beta field. Above this temperature the alloy is fully beta, and the structure formed on cooling depends on the cooling rate and the degree of deformation.
For TC4, Ti-6Al-4V, the beta transus is the most important process reference because nearly every forging and heat treatment decision is defined relative to it.
Typical Values for TC4
For standard TC4 with a normal oxygen content, the beta transus typically lies between 980 and 1000 degrees Celsius, with about 995 degrees Celsius being a representative value. The exact value for a specific heat is determined by its actual chemistry rather than assumed from the nominal composition.
Because the transus varies from heat to heat, production practice usually measures the value for each melt, and the measured value is recorded on the material certificate.
Factors That Shift the Transus
Aluminium, which is an alpha stabiliser, raises the beta transus, while vanadium, a beta stabiliser, lowers it. The interstitial elements have a strong effect per unit weight: oxygen raises the transus significantly, and iron and nitrogen also influence the value.
Small variations in these elements between heats produce transus differences of 10 to 20 degrees Celsius, which is enough to change the optimum forging temperature and the resulting microstructure.
Determination Methods
The metallographic method is the most common technique. Small samples are heated to a series of temperatures near the expected transus, held for a fixed time, quenched, and examined under a microscope to find the highest temperature at which primary alpha remains.
Continuous methods such as differential thermal analysis and dilatometry detect the transformation by thermal or dimensional effects, and they give faster results. Whichever method is used, the sample must be representative of the heat and the thermal history must be controlled.
Practical Use in Forging and Heat Treatment
In forging, working below the transus in the alpha-beta field produces a fine equiaxed alpha structure with a good balance of strength and ductility, while working above the transus produces a lamellar structure with better creep and fracture toughness but lower ductility.
In heat treatment, solution treatment temperatures are defined relative to the transus, and quenching from above or below it produces very different microstructures. Knowing the exact transus of the heat is therefore essential for reproducible properties.
Frequently Asked Questions
Q: What is the beta transus of TC4 titanium alloy?
A: The beta transus of standard TC4 typically lies between 980 and 1000 degrees Celsius, with about 995 degrees Celsius as a representative value.
Q: Why is the beta transus important?
A: It defines the boundary between the alpha-beta and beta fields and is the reference for all forging and heat treatment decisions.
Q: What raises the beta transus of TC4?
A: Alpha stabilisers such as aluminium and the interstitial element oxygen raise the beta transus.
Q: How is the beta transus measured?
A: It is measured by the metallographic method, differential thermal analysis or dilatometry on representative samples.
Q: What happens if TC4 is forged above the beta transus?
A: Forging above the transus produces a lamellar structure with better creep and fracture toughness but lower ductility.
