Structure and properties of Ti-Ni-Au shape memory alloys

Abstract

Ternary Ti-Ni-X based alloys, where X = Pt, Pd, Hf, Au or Zr, show promise as high temperature shape memory alloys (HTSMAs). In comparison to binary Ni-Ti alloys, some hypo-stoichiometric versions of these ternary compositions exhibit higher transformation temperatures and better mechanical stability due to the formation of nano-scale precipitates. In this study, a Ti49Ni26Au25 (at.%) alloy was solution annealed at 1050°C for 3 hours and isothermally aged at 400°C and 550°C. A specimen was also annealed at 1050°C for 3 hours and furnace cooled. Ageing resulted in a very high peak micro-hardness for both temperatures. The structures and chemistries of the phases formed during ageing were characterized by wavelength dispersive x-ray spectroscopy (WDS), scanning electron microscopy (SEM), energy dispersive x-ray spectroscopy (EDS), transmission electron microscopy (TEM), three-dimensional atom probe tomography (3DAP), x-ray diffraction (XRD), and differential scanning calorimetry (DSC). It was found that ageing at both 400°C and 550°C resulted in the formation of two different precipitates. First, two variants of a (Au,Ni)4Ti3 type phase form with SADPs similar to tetragonal D1a. The proposed orientation relationships with the matrix are the following: [001]D1a // [100]B2 with (011)B2 // (310)D1a and [00-1]D1a // [100]B2 with {011}B2 // (310)D1a . It is then postulated that the (Au,Ni)-rich phase creates local Ti-rich regions that promote the precipitation of two Ti2(Ni,Au) variants with tetragonal (I4/mmm) type symmetry. Their proposed orientation relationships with the matrix are the following: [100]Ti2(Ni,Au) // [100]B2 with (001)B2 // (001)Ti2(Ni.Au) and (001)B2 // (100)Ti2(Ni,Au). The combination of both phases appears to inhibit martensitic transformation by stabilizing the high temperature austenite phase, as evident by no transformation peaks in the aged specimens via DSC. However, it is interesting to note that the as-cast and 1050°C furnace cooled specimens did exhibit martensitic transformation with Ms values of approximately 135° and 160°C, respectively. Such behavior is thought to be attributed to their microstructure, which both appeared to consist mainly of (Au,Ni)4Ti3 platelets, as evident via SEM. This ultimately increases the Ti concentration in the matrix and raises the transformation temperatures. Two precipitate buttons with target compositions (at %) of 66.00 Ti - 3.50 Ni - 30.50 Au (Ti2(Ni,Au)) and 41.70 Ti - 27.40 Ni - 30.90 Au ((Au,Ni)4Ti3) were also heat treated at 550°C for 96 hours. The structures and chemistries of the phases formed during ageing were characterized by scanning electron microscopy (SEM), energy dispersive x-ray spectroscopy (EDS), and x-ray diffraction (XRD. It was found that the as-cast Ti-rich button consisted of a Ti3Au phase and a Ti-Ni-Au B2 phase with small Ti-rich platelets, as evident via SEM. Ageing at 550°C for 96 hours promoted precipitate growth in the B2 regions. The crystallography of the additional Ti-rich platelets was found to fit well with I4/mmm symmetry, as evident via XRD. The as-cast (Au,Ni)-rich button consisted of a Au2Ti phase and a Ti-Ni-Au B2 phase with additional (Au,Ni)-rich platelets, as evident via SEM. Ageing at 550°C for 96 hours promoted the growth of the (Au,Ni)-rich platelets. The crystallography of the (Au,Ni)-rich platelets appeared to fit well with the proposed peak locations for (Au,Ni)4Ti3, as evident via XRD.