The Effect of Hafnium Content on the Transformation Temperatures of Ni₄₉Ti_51-xHf_x. Shape Memory Alloys

Johnson Controls, Inc., Central Research Laboratory, 1701 W. Civic Dr. MS A-37, Milwaukee, WI 53209, U.S.A.

Abstract

Ternary alloys of NiTiHf, having higher transformation temperatures than binary NiTi shape memory alloys, have been produced and analyzed. Beginning with a base composition of Ni₄₉Ti_51’, Hf was substituted for Ti up to 30 atomic percent. Differential scanning calorimetry was used to determine the transformation temperatures of the as-cast alloys. The peak martensite temperature of the Ni₄₉Ti₅₁alloy was 69°C and increased to 525°C for the Ni₄₉Ti₂₁Hf₃₀ alloy. The peak austenite temperature of the Ni₄₉Ti₅₁alloy was 114°C and increased to 622°C for the Ni₄₉Ti₂₁Hf₃₀ alloy. An apparent minimum in the peak transformation temperatures occurred between 0 and 3 atomic percent Hf. Preliminary experiments were also conducted to determine the effect of thermomechanical processing on the shape memory properties of the Ni₄₉Ti₄₁Hf₁₀. Data are presented on the effect of cold work and heat treatment on the transformation temperatures of this alloy.