Influence of H-phase precipitation on the microstructure and functional and mechanical properties in a Ni-rich NiTiZr shape memory alloy

Abstract

The relationship between precipitate evolution, martensitic transformation temperatures, hardness, and functional load-bias behavior has been analyzed for a Ni50.8Ti34.2Zr15 (at.%) alloy. In the solutionized condition, the alloy was fully austenitic and no transformation (at least to −90 °C) was observed. Upon aging at 550 °C, the onset of a martensitic transformation (Ms), as determined by differential scanning calorimetry, was observed at approximately −10 °C and 75 °C after 24 and 300 h, respectively. Electron diffraction identified the precipitation of the orthorhombic H-phase within the B2 matrix. When the inter-precipitate spacing was ~12 nm, a greater undercooling was necessary to initiate the martensitic transformation due to overlapping strain fields of the precipitates. As the precipitates coarsened with aging time, a corresponding increase in the inter-precipitate spacing occurred and the chemical partitioning effects between the matrix and precipitate, as determined by atom probe tomography, began to dominate the transformation behavior resulting in an increase in transformation temperatures. For selected aging conditions, the load-biased shape memory behavior was determined under compressive and tensile loading using uniaxial constant-force thermal cycling experiments. A tension-compression asymmetry was noted with larger transformation strains in tension than compression at constant stresses up to 400 MPa. A recoverable transformation strain of 3% was observed in the sample aged for 4 h at 550 °C under a tensile stress of 400 MPa, which is the largest recoverable strain currently reported for a precipitation-strengthened NiTiZr alloy.