Ti-Cr系合金の相構成と等温時効挙動

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By virtue of their high strength and excellent cold workability, β titanium alloys have been used for various applications, e.g. parts of airplanes and eyeglass frames. Generally, β titanium alloys contain vanadium and molybdenum as alloying elements. However, the official quotations of vanadium and molybdenum are unstable. Therefore, use of those alloying elements is limited. We adopted chromium and iron, whose official quotations are more stable than those of vanadium and molybdenum, and reported some parts of the research of Ti-13Cr-1Fe-3Al alloy. Cr is one of the attractive elements to develop the cost affordable titanium alloys. Thus, it is very important to investigate the Ti-Cr binary alloys as basic study. It is also important to study the heat treatment behavior of the Ti-Cr alloys. In this study, phase constitution and isothermal aging behavior of Ti-Cr alloys was investigated by measurements of electrical resistivity and Vickers hardness, X-ray diffraction, optical and scanning electron microscope observations and tensile tests. The obtained results are as follows. In solution treated and quenched stated (STQed) state, hcp martensite, α’ was only identified at 3Cr alloy. β phase and α’ were identified at 5Cr alloys. Above 5Cr, β phase was identified. In 7Cr and 10Cr, athermal ω was also identified. Maximum of HV appeared at 7Cr and then HV decreased with increase of Cr content up to 15Cr. Above 15Cr, HV slightly increased due to solution hardening by Cr addition. On isothermal aging, precipitation of the α phase was delayed by Cr addition. In STQed state, tensile strength is 961 MPa in 10Cr, 988 MPa in 13Cr and 967 MPa in 15Cr, respectively. Elongation is 27.1 % in 10Cr, 26.8 % in 13Cr and 23.9 % in 15Cr, respectively. In 15Cr alloy, nominal stress-nominal strain curve showed work-softening phenomenon after yield stress, whereas S-S curve of 10Cr alloy did not show that phenomenon. In 773 K-12 ks aged state, tensile test specimens of 10Cr and 13Cr were broken with no elongation, while the specimen of 15Cr alloy was broken after about 5 % in elongation. It is considered that difference of tensile properties between the 15Cr and others is due to differences of shape and volume fraction of precipitated α.
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Ti-Cr系合金の相構成と等温時効挙動

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