Martensitic Transformation And Mechanical Behavior Of Ti-V-Al Based Lightweight Shape Memory Alloys

Ti-V-Al alloy, as a good potential candidate for lightweight shape memory alloy, has a low denstity and great workability. But the martensitic transformation is unstable during thermal cycling and its shape memory effect needs to be improved. The effects of content and treatment on structure, transformation and properties of Ti-V-Al alloy are still unclear. In this study, proper content is found and thermomechanical treatment is adopted to improve transformation cycle stability and shape memory effect. The effects of Al, Fe adoping and thermomechanical treatment on the microstructure, martensitic transformation, mechanical behavior and shape memory effect of Ti-V-Al alloy and their mechanisms were investigated systematically by XRD, TEM, DSC and tensile test.It is shown that Ti-13 V alloy consists of bcc β phase and hcp athermal ω phase which is uniformly distributed in it. Al doping suppresses the formation of athermal ω phase. When Al content is over 3at.%, the alloy consists of α " martensite only. After annealing, cold-rolled Ti-13V-3Al alloy consists of α" martensite and a little hcp α phase. With the increase of annealing temperature, the amount of α phase decreases and its small, uniformly distributed morphology changes to coar se, concentrated distributed morphology.Al content and thermomechanical treatment have a great influence on martensitic transformation, mechanical behavior and shape memory effect of Ti-V-Al alloy. Reverse martensitic tranformation temperature decreases with increase of Al content. Reverse martensitic tranformation temperature of cold-rolled Ti-13V-3Al alloy first decreases then increases as annealing temperature rises. Increase of Al and thermomechanical treatement increase martensitic transfomation stability during thermal cycling. With the increase of Al content, stress plateau of solution-treated Ti-V-Al alloys decreases while tensile strength, elongation and recoverable strain first increases then dicreases, and they all get maximum when Al content is 3at.%. Stress plateau of cold-rolled Ti-13V-3Al alloy decreases while elongation increases as annealing temperature increases. Obvious stress plateau corresponding to martensitic reorientation can be seen when annealing temperature is 700°C while elongation is 17%. Thermomechanical treatment improves shape memory effect. Cold-rolled Ti-13V-3Al alloy annealed at 700°C has a fully recoverable strain of 7.5%.A little amount of Fe reduces the martensitic transformation temperature of Ti-V-Al-Fe alloy rapidly, increases elongation and improves shape memory effect. 1at.% Fe addition leads to a drop of 250°C in reverse transformation temperature. Ti-13V-3Al-0.5Fe alloy consists of α " martensite and a little β phase. The amount of α" martensite decreases and β phase increases as Fe content increases. Ti-13V-3Al-1.5Fe alloy consists of β phase only. Plateau stress and elongation increases with the increase of Fe content while recoverable strain first increases then decreases. Thermomechanical treatment improves shape memory effect. After annealing at 750°C, cold-rolled Ti-13V-3Al-1Fe alloy shows a recoverable strain of 7.3% with a pre-strain of 8% and it has a large enlongation of 35%.This paper reveals mechanisms of improvement of shape memory effect and martensitic transformation cycle stability caused by Al, Fe addition and thermomechanical treatment. Fine grains, proper amount of dislocations, small unifom α phase gained by thermomechanical treatment and solution strenghening gained by Al and Fe increases the strength of parent phase, suppressing plastic deformation during martensite reorientation. Meanwhile, small martensite variants divided by α phase have a better interface mobility. These factors contribute to the enhancement of shape memory effect. Increase of Al and Fe as well as thermomechanical treatment suppresses the formation of ω phase during thermal cycling, improving martensitic tranformation cyle stability.