Phase Constitution and Heat Treatment Behavior of Titanium-Manganese Alloys

摘要

Although titanium is considered to be a ubiquitous element since it has the tenth highest Clarke number of all elements, it is classified as a rare metal because the current refinement process is more environmentally damaging than the processes used to refine iron and aluminum. Furthermore, the beta stabilizing elements of titanium alloys (e.g., V, Mo, Nb, and Ta) are very expensive due to their low crustal abundances. Manganese is also considered to be a ubiquitous element, since it has the 12th highest Clarke number of all elements. Therefore, manganese is a promising alloying element for titanium, especially as a beta-stabilizer. In order to develop beta titanium alloys as ubiquitous metallic materials, it is very important to investigate the properties of Ti-Mn alloys. In this study, the phase constitution of and the effect of heat treatment on Ti-3.3 to 8.7 mass% Mn alloys were investigated by electrical resistivity and Vickers hardness (HV) measurements and by X-ray diffraction (XRD) analysis and optical microscopy. In 3.3, 5.1, and 6.0 mass% Mn alloys quenched from 1173 K, α' martensite and β phase were identified by XRD, whereas in the 8.7 mass% alloy, only the (3 phase was detected. The resistivities at both temperatures increased with increasing Mn content up to 6.0 mass% Mn and the positive temperature dependence of resistivity became negative at 6.0 mass% Mn. ρ_(LN) increased gradually with increasing Mn content up to 8.7 mass% Mn, whereas ρ_(RT) decreased considerably at a Mn content of 8.7 mass% Mn. HV increased with increasing Mn content up to 5.1 mass%, after which it began to decrease. In Ti-3.3 mass%Mn and 5.1 mass%Mn alloys, the resistivity and the resistivity ratio decreased with increasing temperature of isochronal heat treatment because of decomposition of α' martensite. In 6.0Mn and 8.7Mn alloys, the resistivity and the resistivity ratio decreased, while Vickers hardness increased with increasing temperature of isochronal heat treatment because of isothermal ω precipitation. Furthermore, the temperature for the onset of precipitation increased with higher Mn content.