The effects of microstructures on notch fatigue propertes of a Ti-29Nb-13Ta-4.6Zr alloy (TNTZ) subjected to various thermo-mechanical treatments were investigated in this study. The notch fatigue strengths of TNTZ subjected to aging at 673 K and 723 K after solution treatment or cold rolling at stress concentration factors of 1 (without the notch), 2 and 6 in both low and high cycle fatigue life regions are greater than those of as-cold rolled and as-solutionized TNTZ, and TNTZ subjected to aging at 598 K after solution treatment or cold rolling. A single fatigue crack initiates on a specimen surface and at the tip of the notch at stress concentration factors of 1 and 2. While, a few cracks initiate the tip of the notch having a stress concentration factor of 6. Notch sensitivity factor of TNTZ is almost equal to or smaller than those of other materials such as Ti-6Al-4V, CP-Ti, S35C, S45C and SUS304. It is considered that the notch sensitivity factor decreases with decreasing the width of lath like α phase and its volume fraction.

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As basic study to develop low cost βTi alloys, changes in phase constitution, electrical resistivity and Vickers hardness with increase of Cr content were investigated in Ti-Cr binary alloys quenched from 1173 K. In solution treated and quenched state, hexagonal martensite, α' was identified at 3Cr and 5Cr alloys, whereas β phase was identified from 5Cr to 20Cr. In 7Cr and 10Cr alloys, athermal ω was also identified. Resistivity increased with increase of Cr content up to 10 mass% Cr and then slightly decreased. Above 10 mass% Cr, resistivity at liquid nitrogen temperature was larger than that at room temperature. Maximum of HV appeared at 7 mass% Cr and then HV decreased with increase of Cr content up to 13 mass% Cr. Above 13 mass% Cr, HV slightly increased due to solution hardening by Cr addition. Temperature-resistivity curves of alloys from 10Cr to 20Cr were negative temperature dependence between 77 K and about 300 K. From change of the temperature-resistivity curves with Cr content, it is expected starting temperature of athermal ω formation decreases with increase of Cr content. Trend of changes in HV of Ti-Cr, Ti-Fe and Ti-V alloys as a function of e/a was the almost same, though absolute these values of the three alloys are not same.

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Microstructural control in surface layers using continuous cyclic bending (CCB) and its bendability in a commercial purity titanium sheet have been investigated. The sheet of the titanium worked by CCB with little dimensional change showed 1.2 times higher hardness than that before CCB on the surface. The microstructure was controlled by CCB and the subsequent annealing for grain refinement leading to better formability in microscopic level. There was a remarkable difference in softening behavior after annealing between the samples continuous-cyclic-bent in rolling and transverse directions. This is attributable to formation of the microstructure where recrystallization does not occur easily after CCB in transverse direction. The fine grained samples showed smooth surfaces after a bending formability test, while the coarse grained one had a lot of microscopic cracks on the surface, especially for the CCB in transverse direction.

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This research has been performed in order to develop a titanium alloy which had high temperature oxidation resistance more than ASTM Gr.37 (Ti-1.5Al), which was the existing titanium alloy for exhaust systems. Specifically, high temperature oxidation resistances of Ti-Al-Si alloys and Ti-Al-Si-Nb alloys were evaluated from viewpoints of weight gain, formation of oxygen diffusion zone, thickness reduction, grain growth and embrittlement. It was found that small amount additions of Al, Si and Nb were effective in improvement of oxidation resistance of titanium. On the basis of this result, Ti-0.5Al-0.45Si-0.2Nb alloy was developed. This alloy has excellent oxidation resistance and suitable strength property for exhaust systems used at high temperature. This alloy is consequently considered to be applied to not only motorcycle exhaust systems but also automobile exhaust systems.

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Beta-titanium alloy bolts were developed with cold forging process. It was formerly difficult to manufacture beta-titanium bolts by cold forging, because beta-titanium alloy is easy to wear with dies. Now the lubricant coating of beta-titanium material and new cold forging processes are developed, resulting in manufacturing cold-forged beta-titanium bolts. It is confirmed to be able to mass-produce beta-titanium bolts. The beta-titanium bolts could be strengthened up to 800∼1200 MPa by heat treatments. It is, however, essential to control both the tensile strength and ductility, depending on uses.

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The creep behavior at ambient temperature of typical h.c.p., b.c.c. and f.c.c. metals and alloys of annealed state were surveyed. Cubic metals and alloys demonstrated negligible creep strain under all stress ranges. In contrast, h.c.p. metals and alloys demonstrated significant primary creep behavior. In particular, Ti-6Al-4V alloy and CP-Ti metal showed significant creep behavior and accumulated large creep strain more than 1% in 90 s and 1600 s, respectively under 0.9 of 0.2% proof stress. After being cold-rolled, Ti-6Al-4V alloy and CP-Ti metal showed less significant creep behavior.

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