Amorphization of Titanium Nickelide by Means of Shear under Pressure and Crystallization at the Subsequent Heating

摘要

Methods of transmission electron microscopy and X-ray diffraction analysis were used to study structural changes in the alloy Ti-50.5at.%Ni upon severe plastic deformation by high pressure torsion and subsequent heating. It has been established that an increase in the strain can lead to a martensite transformation, twinning of martensite crystals, formation of reorientation bands, development of rotational modes of deformation, formation of a nanocrystalline structure, as well as to the amorphization of the alloy. A scheme of the formation of the amorphous structure of the alloy during deformation is suggested as based on the observed structural changes. It has been found that the reverse martensitic transformation can be one of the mechanisms of plastic deformation of the alloy. Therefore, as the strain increases, firstly the forward and subsequently the reverse martensitic transformation can occur. The formation of an amorphous structure starts after attainment of the strain 4.2 (one revolution of Bridgman anvils); at a strain of 6.8 (5 revolutions of the anvil), the process is virtually completed. The crystallization of the amorphous alloy upon heating starts even at 200°C. However, upon heating up to 300°C (for 0.5-h holdings), the kinetics of crystallization is sluggish. After annealing at 350°C, the complete crystallization with the formation of a nanocrystalline structure with a grain size of 20-70 nm takes place.