Effect of Hf on the microstructure and martensitic transformation behavior in Ti–Pd–Hf alloy

We investigated the microstructure and the crystallography of martensite variants in Ti–Pd–Hf alloy using electron microscopy. The martensitic transformation temperature decreased with increasing amount of Hf substituted into the Ti_50−xPd₅₀Hf_x alloy. No indication of a martensitic transformation was observed in the as-quenched Ti₂₅Pd₅₀Hf₂₅ and Ti₁₀Pd₅₀Hf₄₀ alloys, even when the samples were cooled to 150 K. The Ti₄₀Pd₅₀Hf₁₀, Ti₃₅Pd₅₀Hf₁₅, and Ti₃₀Pd₅₀Hf₂₀ alloys were composed of a B19 orthorhombic phase having plate-like habit-plane variants consisting of both major and minor correspondence variants several hundred nanometers in width and several tens of micrometers in length at room temperature. Lattice-invariant shear of these alloys was a {111}_B19 Type I twin. A solution-treated sample of the Ti₂₅Pd₅₀Hf₂₅ alloy consisted of a B2 cubic matrix with local atomic displacement and lattice distortions, and rounded H-phase particles several tens of nanometers in diameter were precipitated inside the B2 matrix upon aging of the specimen at 823 K for 10.8 ks. The as-quenched Ti₁₀Pd₅₀Hf₄₀ alloy contained rounded H-phase particles several tens of nanometers in diameter embedded in a B2 matrix at room temperature. The martensitic transformation temperature in the Ti_50−xPd₅₀Hf_x alloy was drastically decreased by both the fully coherent nanosized H-phase particles and the short-range order originating from clustering of solute atoms.