Effect of cold rolling on the aging phenomena in beta titanium alloy prepared by vacuum arc re-melting process

摘要

Beta titanium alloys have desirable mechanical properties, such as tensile properties, ductility, hardenability, and biocompatibility. The combination of these superior properties make beta titanium alloys an outstanding candidate for the use in biomedical implant applications. Beta titanium alloys without aluminum were considered in many recent studies due to its nurological problem. The new metastable beta alloy without Al, Ti-6Mo-6V-5Cr-3Sn-2.5Zr(wt.%) alloy, was fabricated by vacuum arc re-melting (VAR) process and homogenized at a temperature of 1123 K for 4 hours in argon atmosphere. The specimens were cold rolled with reduction of 10%, 30%, 50% and 70% ,intermediate recovery annealing, subsequent aging treated at 573 K, 623 K, 673 K, 723 K and 773 K. The microstructure of alloys during solution treatment and cold rolling were observed in this study, since mechanical properties are strongly influenced by the structure and distribution of the phases. Texture evolution and orientation relationship between the phases were studied by X-ray diffraction, electron backscatter diffraction (EBSD) and transmission electron microscopy (TEM). The relationship between microstructure and mechanical properties was investigated through Vickers hardness. The increased amount of cold deformation led to an increase in the hardness due to elongation and severely deformed beta phase, which was confirmed by optical images. The solution treatment for 3 minutes at 1053 K above beta transus temperature after cold rolling produced in fine recrystallized beta phase according to EBSD results. Moreover, the average grain size of the specimen decreased as the amount of cold deformation increased. The grain size of specimen cold rolled after 70 % reduction followed by annealing for 3 minutes at a temperature of 1053 K resulted in size of approximately 20 μm. A fine acicular alpha phase occurred at beta grain boundaries after aging treatment at temperatures higher than 673K, which was then preceded by intragranular alpha phase. The size of secondary alpha phase was strongly dependent on the aging temperature and time. In conclusion, increasing rolling reduction by cold rolling leads to a fine beta grain size after annealing, which can result in good ductility. Fine acicular alpha phase after aging treatment was well distributed, which can be expected to result in good creep resistance and fracture toughness.