Effects of impurities content (oxygen, carbon, nitrogen) on microstructure and phase transformation temperatures of near equiatomic TiNi shape memory alloys

Abstract

Investigation of precipitation is performed on several near equiatomic TiNi alloys elaborated by arc melting or by powder metallurgy (combustion synthesis mode). These alloys contain various amount of impurities (oxygen, carbon, nitrogen). We show that such impurities result in both oxides (Ti4Ni2O3, with x ≤ 1) and carbonitrides (TiC3N1-x, with x ≤ 1) precipitation. X-ray diffraction measurements are performed on residues of electrochemical extraction to determine the structure and the lattice parameters of these precipitates. The precipitated fractions are quantified for various amount of impurities by image analysis. A very good correlation is obtained between the nominal content of impurities determined by chemical analysis and the precipitated fraction measured by image analysis and X-ray measurements. Our results indicate that almost all the nominal oxygen has precipited to form Ti4Ni2O type oxides. As a consequence, an increase of the impurities content in near equiatomic TiNi alloys implies a lowering of the phase transformation temperatures. This phenomenon is attributed to the precipitates stoichiometry which induces a deficit of titanium content in solid solution.. On the other hand, we find that fine precipitation improve the grain size reduction during hot working. Possible explanation is that the precipitates act as grain refiners through a "pinning" or a "grinding" process.