Experimental investigation and thermodynamic calculation of Ti-Co-Hf system

Phase relationship of Ti-Co-Hf ternary system, which may contribute to the development of both new Ti-based biomaterials and Co11Hf2 based rare-earth-free magnetic materials, have been studied by using Ti-Co-Hf diffusion triple and selected equilibrated alloys. Isothermal sections of the Ti-Co-Hf ternary system at both 1173 and 1073 K were experimentally determined by means of electron probe microanalysis (EPMA), scanning electron microscope (SEM) and X-ray diffraction (XRD). Meanwhile, structure refinements of three continuous solid solutions, i.e. Co(Ti, Hf)2, Co2(Ti, Hf) and BCC_B2 which were formed between CoTi2 and CoHf2, Co2Ti(c) and Co2Hf as well as CoTi and CoHf, respectively, were performed by Rietveld method. The stable temperature range of Co11Hf2 was revised to be from 1310 ± 12.5-1516 K. And Ti (about 3.7 at%) partly dissolves in Co23Hf6 at 1173 K while the solubility of Hf in Co3Ti at both 1173 and 1073 K is relatively limited. Based on the available data of binary system, thermodynamic modeling of Ti-Co-Hf ternary system was carried out, with Co(Ti, Hf)2, Co2(Ti, Hf) and BCC_B2 being respectively described with models, (Co, Hf)1(Co, Ti, Hf)2, (Co, Ti, Hf)2(Co, Ti, Hf)1 and (Co, Ti, Hf, VA)0.5(Co, Ti, Hf, VA)0.5. A set of self-consistent thermodynamic parameters, which can reasonably reproduce the measured results, has been obtained.