Flow Instabilities and fracture in Ti-6Al-4V deformed in compression at 298 K to 673 K
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I.
INTRODUCTION
T I T A N I U M alloys undergo flow instabilities, during deformation processing t~'zj at elevated temperatures (>973 K). Instabilities and inhomogeneous flow are observed in Ti-6A1-4V alloy (henceforth called Ti64) at room temperature, t3-7] and these lead to fracture of the material. The instabilities manifest as adiabatic shear bands, which result in local melting and fracture. The fracture surfaces were examined by Makel and Eylon, tr] Grebe et a l . , t41 Timothy and Hutchings, tSj and Makel and Wilsdorf. tTl Two types of features were recorded: (1) ductile dimples and (2) relatively smooth, flat features. In addition, localized surface melting was observed by Makel and Eylon. t6J The surface features that have been observed in conjunction with local areas of melting on the surfaces of tensile samples include open surface shear zones, localized shear band dimples, and transition dimples.ITJ These are formed by a combination of void nucleation and growth with rapid localized shear. Microstructurally, the lower temperature deformation conditions in Ti64 cause planar slip and twinning, tS] the occurrence of planar slip in the alpha grains being enhanced by the presence of oxygen.tgl Further, the presence of aluminum in the alloy causes dynamic strain aging at lower strain rates and temperatures higher than 523 K./~~ The present investigation studies the manifestations of flow instabilities and fracture characteristics of Ti64 deformed in uniaxial compression in the temperature range of 298 to 673 K. In a similar study on commercial titanium, it has been observed l~j that adiabatic shear bands occur at higher strain rates (-> 1 s-~). These instabilities are termed "geometric" to distinguish them from the microstructural flow inhomogeneities observed at lower strain rates and higher temperatures. Similar classifications were also reported in a duplex steel that exhibited local shear band formation during necking in plane-strain extension, t~2,~31 SATISH V. KAILAS, formerly Graduate Student, is Research Associate, Department of Mechanical Engineering, Indian Institute of Science. Y.V,R.K. PRASAD, Professor, Department of Metallurgy, and S.K. BISWAS, Professor, Department of Mechanical Engineering, are with the Indian Institute of Science, Bangalore 560 012, India. Manuscript submitted May 25, 1993. METALLURGICAL AND MATERIALS TRANSACTIONS A
II.
EXPERIMENTAL DETAILS
Hot-rolled commercial Ti64 rods (A1 8.02 pct, V 3.78 pct, Fe 0.08 pct, C 0.007 pct, 0 986 ppm, H 826 ppm. and N 74ppm) of 15-mm diameter were used in this investigation. The initial microstructure is acicular and has Widmannstatten morphology. Compression tests were carried out on cylinders (6 mm in diameter and 9-mm high) machined from the rod in such a way that the compression axis was along the rolling direction. The specimens had grooves of 0.1-mm depth on their two parallel faces to facilitate lubrication during compression. Comers of the cylinders were chamfered by 0.5 mm to avoid the effect of foldover during initial deformatio
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