Abstract:The precipitation behavior of α phase from β phase and the variation of mechanical properties during aging in the severely cold rolled alloy were studied. The results show that nanostructure is formed in some regions of the cold deformed alloy. α phases precipitating from β nanostructure region are thin-needle like during aging from 450 oC to 650 oC and those from β non-nanostructure region grow from the needle to the lath-shaped and finally become lenticular plate. After cold rolling and heat treatment at 450 oC for 4 h, the hardness of the alloy reaches the peak value (5328 MPa). The yield strength and ultimate strength of the alloy aged at 450 oC for 4 h are 1483 and 1562 MPa, respectively. The aging time for peak value is shortened and the hardness decreases sharply with the increase of aging temperature. The strength and hardness of the alloy after aging at 650 oC is lower than that before aging. This is attributed to the coarse thin-plate precipitates, the growth of nanostructure crystals and the decrease of dislocation density in the alloy during aging at 650 oC. The change tendency of the strength and the hardness of the alloy treated by different aging procedures are similar and they are related to the state of the precipitates formed during aging.

Abstract:For the cold pre-deformed Ti-10Mo-8V-1Fe-3.5Al (TB3) alloy, the high density of dislocation could still be observed after traditional aging treatment, which could lead to detrimental influences on mechanical properties. To resolve this problem, a second aging treatment process was proposed. Transmitting electron microscopy (TEM) investigations show that the dislocation density may be dramatically decreased by the secondary aging treatment in predeformed TB3 samples. Variant selection effect was observed in predeformed and aged TB3 samples. Low cycle fatigue (LCF) tests were carried out to the conventional peak aged TB3 samples (FA) and the predeformed and secondary aging treated TB3 samples(SA). The LCF results show that fatigue lifetime is slightly higher for SA samples when the total strain amplitude is lower than 1%, but it has an opposite result when the total strain amplitude is higher than 1%. TEM investigations on the TB3 samples after fatigue tests show that dislocation slip is the predominant deformation mode. The dislocation bands resulting form cyclic deformation are wide and discontinuous in FA sample, however they are fine and homogeneous in SA samples.

Abstract:The influence of prior cold deformation after solution treatment on the α phase precipitation behavior during the subsequent aging treatment and final mechanical properties of TB3(Ti-10Mo-8V-1Fe-3.5Al) alloys was investigated. In the predeformed samples, network patterns were observed after aging treatment by metallographic microscope. Transmission electron microscopic (TEM) observation shows that in the predeformed-aged samples, the plate-shaped α precipitates prefer to nucleate and grow in high density dislocation sites, such as grain boundaries and slip bands. In addition, variant selection effects were observed in these samples. Results of X-ray diffraction and metallographic microscope indicate that α phase precipitation process could be accelerated in predeformed samples. Tensile test results reveal that by cold deformation the aging time could be shortened and the tensile strength could be enhanced, while the ductility could be reduced, in comparison with the freely aged sample.

Abstract:After solution treatment, Ti-10Mo-8V-1Fe-3.5Al (TB3) alloy was aged at 500 oC under a compressive stress. TEM results show that several α variants are observed in conventionally aged samples. However, in the stress-aged TB3 samples, most α plates are parallel to each other in one β grain, which indicates a variant selection effect. Two kinds of TB3 samples were selected for low cycle fatigue tests, which had been heat treated to the same strength level. The one was conventionally aged at 500 oC for 9 h, and the other was aged at 500 oC for 6 h with about 200 MPa compressive stress. Low cycle fatigue (LCF) test results show that the stress-aged TB3 samples exhibit a longer LCF lifetime than the conventionally aged ones

Abstract:Titanium and its alloys have been widely used in biomedical field due to their excellent biocompatibility, low elastic modulus, good comprehensive mechanical properties and corrosion resistance. It was one of the most promising medical metal materials than stainless steel and cobalt chromium alloys. From the perspective of biomechanical properties, biological corrosion resistance, biocompatibility and antibacterial properties, the development status of metastable β-titanium alloys and its research process are summarized. Especially, the research status of improving the mechanical properties of β-titanium alloys by altering the alloy elements, changing heat treatment process and alloy forming methods are well focused on.