Crystallographic Orientation Analysis of Fatigue Crack Initiation Site Formed at Cryogenic Temperatures in Ti-5Al-2.5Sn ELI Forged Alloy

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  • Ｔｉ‐５Ａｌ‐２．５Ｓｎ　ＥＬＩ合金鍛造材における極低温疲労破壊起点部の結晶方位解析
  • Ti 5 Al 2 5 Sn ELI ゴウキン タンゾウザイ ニ オケル キョクテイオン ヒロウ ハカイ キテンブ ノ ケッショウ ホウイ カイセキ

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Tensile and high-cycle fatigue properties were investigated at cryogenic temperatures of 4K, 20 K and 77K for Ti-5%Al-2.5%Sn extra low interstitial (ELI) forged alloy. High-cycle fatigue strength is relatively low compared to that at ambient temperature in spite of the high tensile strength. In the specimens fatigue-tested at cryogenic temperatures, a facet was observed at a fatigue crack initiation site. The crystallographic orientation of the facet was determined by electron backscatter diffraction (EBSD) method in a scanning electron microscope (SEM). The SEM-EBSD analyses revealed that the facet plane was {1121}, which is one of twin planes in alpha-type titanium. There were some terraces on the facet and their orientation had {1121} twin relationship to that of the other part (matrix). These results show that the {1121} twins were formed during cyclic loading, leading to the fatigue crack initiation at {1121} twin/matrix interface. Probably, this twin-assisted crack initiation degrades the high-cycle fatigue properties of the present alloy.

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