Flow Instabilities and Fracture in Ti-6AI-4V Deformed in Compression at 298 K to 673 K SATISH V. KAILAS, Y.V.R.K. PRASAD, and S.K. BISWAS Uniaxial compression tests were conducted on Ti-6A1-4V specimens in the strain-rate range of 0.001 to 1 s -~ and temperature range of 298 to 673 K. The stress-strain curves exhibited a peak flow stress followed by flow softening. Up to 523 K, the specimens cracked catastrophically after the flow softening started. Adiabatic shear banding was observed in this regime. The frac- ture surface exhibited both mode I and II fracture features. The state of stress existing in a compression test specimen when bulging occurs is responsible for this fracture. The instabilities observed in the present tests are classified as "geometric" in nature and are state-of-stress de- pendant, unlike the "intrinsic" instabilities, which are dependant on the dynamic constitutive behavior of the material. I. INTRODUCTION TITANIUM alloys undergo flow instabilities, during deformation processing t~'zj at elevated temperatures (>973 K). Instabilities and inhomogeneous flow are ob- served in Ti-6A1-4V alloy (henceforth called Ti64) at room temperature, t3-7] and these lead to fracture of the mate- rial. 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 al., t41 Timothy and Hutchings, tSj and Makel and Wilsdorf. tTl Two types of features were recorded: (1) ductile dimples and (2) relatively smooth, flat fea- tures. In addition, localized surface melting was ob- served 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 en- hanced by the presence of oxygen.tgl Further, the pres- ence of aluminum in the alloy causes dynamic strain aging at lower strain rates and temperatures higher than 523 K./~~ The present investigation studies the manifes- tations of flow instabilities and fracture characteristics of Ti64 deformed in uniaxial compression in the tempera- ture range of 298 to 673 K. In a similar study on com- mercial 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. 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 acic- ular 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 deformation. A central hole, perpendicular and up to the cylinder axis, of 0.5-mm diameter was provided to insert a thermocouple to measure the temperature rise during compression. The tests were carried out at constant true- strain rates of 0.001, 0.01, 0.1, and I s -j to a reduction of 50 pct in height at 298, 373, 523, and 673 K. Tests were performed in a Dartec (Stourbridge, United Kingdom) servohydraulic testing machine, which has the facility of exponential decay of actuator speed with stroke during the test. The deformed specimens were sectioned diametrically and were examined microstructurally using standard metallographic techniques. The load-stroke curves obtained from the compression tests were converted into true-stress-true-plastic strain curves by subtracting the elastic portion of strain and using standard equations for true-stress and true-strain calculations. III. RESULTS Typical true-stress-true-plastic-strain curves recorded at a strain rate of 0.01 s -~ and at different temperatures in the range of 298 to 673 K are shown in Figure l(a). Figure l(b) shows the stress-strain curves in the same temperature range at 1 s -~. The temperature rise was sig- nificant (10 to 80 K), possibly the result of the cata- strophic cracking observed during compression. A significant temperature rise was observed only after the peak stress was reached. The general features of the curves are similar at other strain rates of 0.001 and 0.1 s -~. The material exhibits work hardening in the initial stages of METALLURGICAL AND MATERIALS TRANSACTIONS A VOLUME 25A, OCTOBER 1994--2173