Papers by Keyword: Ni-Rich NiTi

Abstract: A predictive microscale-informed model that takes into account the precipitate–shape memory performance relations and allows for the evaluation of the effective thermomechanical response of precipitated Ni-rich NiTi shape memory alloys on the basis of composition and heat treatment is presented. The model considers the structural effect of the precipitates (coherency stresses due to the lattice mismatch between the precipitates and the matrix material and precipitate volume fraction), as well as the effect of the Ni-concentration gradient in the matrix, resulting from Ni-depletion during precipitate growth. The predictive capability of the model is tested against experimental data obtained fromNi₅₀.₇Ti (at. %) that has been heat treated under different conditions and good agreement is shown.

Abstract: Thermomechanical properties of precipitated NiTi shape memory alloys are investigated using the finite element method. The precipitated material microstructure is explored using a representative volume element with embedded Ni4Ti3 precipitates. Features such as precipitate coherency and distribution of Ni within the matrix due to the precipitation process are individually explored and characterized. Changes in the material’s macroscopic thermomechanical response due to this precipitation are determined.

Abstract: NiTi alloys containing more than 55%wt nickel undergo precipitation of Ni4Ti3, Ni3Ti2, and Ni3Ti phases during various heat treatments which could have a great effect on the chemical composition of the matrix and behavior of alloy. In this investigation, a NiTi alloy with Ti-57.5%wt nickel content, produced by vacuum induction melting in a graphite crucible, were subjected to the homogenization heat treatments in 1100oC and for various time periods (0.5, 1, 2, and 4 hours). The subsequent cooling was conducted in different cooling media (furnace and air) in order to examine the effect of cooling rate. Microstructural investigations show Ni4Ti3 particles with bimodal size distribution in furnace cooling. Differential scanning calorimetry demonstrates the correlation between homogenization time and transformation temperatures of the alloy.