Reverse Transformation Behavior of Ti-6Al-4V After Deformation in the Two-Phase Region
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namic transformation was first reported to take place in steel by Yada and co-workers.[1] This phenomenon (i.e., the conversion of alpha into beta) was later observed in titanium alloys.[2–7] For example, an increase in volume fraction of the beta phase during the deformation of Ti-5.5Al-1.5Fe in the two-phase region was described by Koike et al. in 2000[2] and similar observations of phase transformations have also been published by Yang et al.,[3] Zhang et al.,[4] Matsumoto et al.,[5] Prada et al.,[6] and Jonas et al.[7] The reverse transformation (i.e., beta to alpha) during isothermal holding after hot compression was also described by Koike.[2] This phenomenon has been shown to take place in steel as well and to be diffusion controlled.[8,9] However, only limited data regarding this phenomenon are available on titanium alloys. The present tests were conducted to fill this gap. In this
BAOQI GUO, CLODUALDO ARANAS Jr., BINHAN SUN, and JOHN J. JONAS are with the Materials Engineering, McGill University, 3610 University St, Montreal, H3A 0C5, Canada. Contact e-mail: [email protected] XIANKUN JI is with the College of Materials Science and Engineering, Hunan University, Changsha, 410082, China. Manuscript submitted September 11 2017.
METALLURGICAL AND MATERIALS TRANSACTIONS A
work, the dependence of phase fraction on holding time after deformation was determined, which enabled the construction of a time–temperature–reverse transformation (TTRT) diagram. The letter describes the microstructural evolution after deformation in Ti-6Al-4V. The material for the experiments was Ti-6Al-4V with a composition (in weight percent) of 6.53 aluminum, 4.10 vanadium, 0.17 iron, 0.17 oxygen, 0.03 carbon, 0.03 nitrogen, the balance being titanium. Its (alpha + beta) to beta transus temperature is 1015 °C according to measurements by differential thermal analysis (DTA). The as-received material was characterized by an equiaxed microstructure and was machined into compression cylinders with heights of 9.6 mm and diameters of 6.4 mm. Hot compression tests were performed on a 100 kN MTS servohydraulic compression machine equipped with a radiation furnace. As shown in Figure 1, the samples were heated at 2 °C/s to the compression temperature and then held for 900 seconds prior to deformation. A suspension of boron nitride in ethanol was applied to the top and bottom surfaces of the samples for lubrication purposes. An argon protective atmosphere was used to minimize oxidation during compression. Strains of 0.9 were applied at a strain rate of 0.01 s1. After deformation, the samples were held at temperature to permit reverse transformation of the metastable beta into the thermodynamically stable alpha. The deformation temperatures were 940 °C, 970 °C, and 1000 °C and the holding times were 0, 18, 180, and 1800 seconds, respectively. They were water quenched after holding. Samples were cut along the longitudinal direction and prepared for examination using standard metallographic techniques. The optical microstructures were taken usin
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