Fractographical Analyses of Crack Initiation Site in High-cycle Fatigue for Ti–Fe–O Alloy at Low Temperature

• Published in: ISIJ International 2018/07/15, Vol.58(7), pp.1332-1340
• Main Authors: Umezawa, Osamu, Yuasa, Takayuki, Li, Weibo
• Format: Article
• Language: English
• Subjects: fatigue; low temperature; macrozone; subsurface crack initiation; texture; titanium alloys; transgranular crack
• ISSN: 0915-1559; 1347-5460
• DOI: 10.2355/isijinternational.ISIJINT-2017-673

High-cycle fatigue properties of a Ti–Fe–O alloy with different processed products such as rolled plate (L and T), cross-rolled plate (CR) and groove-rolled bar (CS) were evaluated at 77 K and 293 K. Fine equiaxed α grains randomly oriented with [0001] perpendicular to tensile axis were produced in the CS. No significant difference of 107 cycles fatigue strength was recognized among the test materials at each temperature, although the CS exhibited an improved fatigue strength in long-life regime at 293 K. The subsurface crack initiation was dominant in lower stress level and at 77 K. The subsurface crack initiation sites consisted of facet or facets. The facets were identified as (0001) in the L, T and CR. In the CS, the (0001) facet provided an origin of subsurface crack initiation site, but the {10 1 0} facets mainly covered the sites at 77 K. The combination of shear stress and opening stress on {10 1 0} may be responsible for forming a facet and its growth in the neighboring grain. The dependence of subsurface crack initiation site size on the maximum stress range was evaluated, where the maximum stress intensity factor range, ΔKImax, revealed the temperature and stress dependences.