Deformation of metastable betaTi-15Mo-5Zr alloy single crystals
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IT has recently been shown, based on transmission electron microscopic observations, that mechanical twinning plays an important role in the deformation of many beta Ti alloys, e.g. Ti-V ~-4Ti-Mo 3 and Beta I I I ) ,6 These investigations indicated that { 112) and (332) twins are formed during deformation depending on the alloy system and composition. On the other hand, it has also been confirmed that dislocation core structure influences the flow stress of a beta Ti-V alloy in a manner similar to that observed in other bcc metals and alloys based on observations of an asymmetry of the operative slip plane and of the yield stress 7 as well as a strong temperature dependence of the yield stress, s Therefore, whether mechanical twinning occurs or not in a beta Ti alloy single crystal may depend on the deformation axis since the occurrence is expected to be affected by the ease of crystallographic slip. The purpose of the present investigation is to clarify the deformation characteristics of a beta Ti alloy single crystal in terms of slip and twinning modes. EXPERIMENTAL The material for this investigation, generously supplied by Kobe Steel, Ltd., has a nominal composition of Ti-15Mo-5Zr. The material was received in the form of 15 mm diam rod. The actual composition determined by chemical analysis is as follows (in wt pct): Mo 14.74
Zr 4.76
Fe 0.046
C 0.008
O 0.183
N 0.0058
In this alloy the bcc solid solution is retained on water quenching from temperatures above 1003 K. Coarse grained samples of Ti-15Mo-5Zr alloy were prepared by annealing premachined specimens in a vacuum of less than 1 x 10 -4 Pa at 1673 K for 36 ks. The shape of the premachined specimens was 2.8 mm diam • 30 m m gage length for testing in tension or 2.2
S. HANADA and O. IZUMI are Instructor and Professor, respectively, Research Institute for Iron, Steel and Other Metals, Tohoku University, Sendai 980, Japan. Manuscript submitted January 21, 1980.
• 2.2 • 45 mm for testing in compression. After the annealing treatment only the tensile specimens whose grain boundaries were normal to the deformation axis and extended completely across the gage section (bamboo structure) were selected. The specimens were solution treated at 1003 K for 3.6 ks and then quenched in water. The tensile specimens having two or three grain boundaries in the gage length of 30 mm were tested in this condition. Single crystal compression specimens of about 2.2 x 2.2 • 6.0 m m were sectioned transversely from the coarse grained 2.2 • 2.2 • 45 mm bar and were subjected to the same heat treatment as that of the tensile specimens. The orientation of the crystals was determined by the Laue back reflection method prior to testing. Testing was done at room temperature at a strain rate of about 1.4 • 10 .3 s-1. After mechanical deformation the crystals were examined metallographically and were cut into thin wafers by spark erosion. Thin foils for transmission electron microscopic (TEM) observations were prepared from these slices using standard electropolishing techniques. 9
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