Microstructure and interface structure of Ti₃₆Ni₄₉Hf₁₅ high temperature shape memory alloy

The microstructure and the interface structure of martensite variants in the Ti₃₆Ni₄₉Hf₁₅ alloy have been investigated by TEM, XRD, EPMA, DSC and HREM techniques. The experimental results showed that the constitutional phases of the Ti₃₆Ni₄₉Hf₁₅ alloy at room temperature after solid solution treatment were martensite matrix and the (Ticursive Greek chiHf_{1-cursive Greek chi})₂Ni particles exhibiting spherical or rectangular shape. The structure of the martensite belong to monoclinic B19′ type, with the lattice parameters a=0.2454 nm, b=0.4087 nm, c=0.4791 nm, β=99.32°. The substructure of martensite variant was confirmed to be (001) compound twin. The martensite variants exhibit self-accommodated morphology and have spear-like, mosaic-like and wedge-like characteristics. Spear-like and mosaic-like martensite variants were found to be (011) type I or 〈011〉 type II twin related, and there are small wedge-like (111) type I twinning martensite variants within them. The (011) type I twinning boundary is straight and well coherent, and 〈011〉 type II twinning boundary gradually curved, whereas the (111) type I twinning interface is wavy.