Precipitation of Heusler phase (Ni2TiAl) from B2-TiNi in Ni-Ti-Al and Ni-Ti-Al-X (X = Hf, Zr) Alloys

The precipitation of Heusler phase (L2(1): Ni2TiAl) from a supersaturated B2 (TiNi-based) matrix at 600degreesC and 800degreesC is studied using transmission electron microscopy (TEM), analytical electron microscopy (AEM), and three-dimensional atom-probe (3DAP) microscopy in Ni-Ti-Al and Ni-Ti-Al-X (X = Hf and Zr) alloys. The B2/L2(1) two-phase system, with ordered structures based on the bee lattice, is chosen for its microstructural analogy to the classical gamma/gamma' system with an fee lattice. Knowledge of the temperature-dependent partitioning of alloying elements and their atomic volumes in the B2-TiNi and L2(1) phases is desired to support design of high-performance shape-memory alloys (SMAs) with controlled misfit strain and transformation temperatures. After aging at 600degreesC for up to 2000 hours, the L2(1) precipitates remain fully coherent at a particle diameter of similar to20 nm. The observed effects of a misfit strain of -1.9 pct on the microstructure of the B2/L21 system are similar to those theoretically predicted and experimentally observed for the gamma/gamma' system. The similarities are demonstrated in terms of the precipitate shape, spatial distribution, and minimum distance of separation between L2(1) precipitates. However, all these effects disappear after aging the alloys at 800degreesC for 1000 hours, when the L2(1) precipitates become semicoherent at particle diameters above similar to400 nm. A simple analysis of the size evolution of L21 precipitates after an isochronal aging (1000 hours) experiment suggests that they follow coarsening kinetics at 600 C and growth kinetics at 800 C, consistent with the Langer-Schwartz theory of precipitation kinetics, which predicts that a high supersaturation suppresses the growth regime. Microanalysis using AEM and 3DAP microscopy define the TiNi-Ni2TiAl phase boundaries at 800degreesC and 600degreesC. At 800degreesC, Hf and Zr partition to the B2-TiNi, while at 600degreesC, they partition slightly to the L2(1) phase, reducing the lattice misfit to -1.7 and -0.011 pct, respectively, and partition strongly to the metastable phase Ti2Ni3. To describe the composition dependence of the lattice parameter of multicomponent B2 and L2(1) phases, the atomic volumes of Al, Hf, Ni, Ti, and Zr in the B2-TiNi and L2(1) phases are determined. A simple model is proposed to predict the lattice parameters of these phases in multicomponent systems.