Microstructural Evolution and Transformation Behavior Of An Aged Ti-Ni-Au Shape Memory Alloy

Aging of a solution annealed, Ni-rich, Ti₄₉Ni₂₆Au₂₅ (at.%) shape memory alloy at 400°C and 550°C results in high hardness near 700 VHN and above, indicating exceptional precipitate hardenability.  This is due to the formation of L1₂ and Ti-rich C11_b precipitates in the high temperature cubic B2 phase.  This microstructural evolution with thermal treatment was characterized via scanning electron microscopy, transmission electron microscopy, x-ray diffraction, and differential scanning calorimetry (DSC).  Based on TEM-EDS, the average chemistries of the L1₂ and C11_b phases are (41.20 ± 0.42) Ti – (25.47 ± 1.49) Ni – (33.33 ± 1.55) and (65.09 ± 0.96) Ti – (0.95 ± 0.34) Ni – (33.98 ± 0.84) Au (at. %), respectively.  The L1₂ phase (Pm3m) exhibits the lattice parameter a = 6.8 Å.  The C11_b phase (I4/mmm) exhibits the lattice parameters a = b = 3.1 Å and c = 11.4 Å.  These two proposed crystal structures will be derived from electron diffraction tilting experiments.  DSC results show that the as-cast alloy exhibits reversible martensitic transformation with M_s ~ 155°C and A_f ~ 188°C.  Interestingly, no transformations were observed in the solution annealed and aged specimens, suggesting that the newly identified precipitates act to impede martensitic transformation.