Dynamic response of conventional and hot isostatically pressed Ti-6Al-4V alloys: experiments and modeling

This paper presents the results of a systematic comparative study of the dynamic thermomechanical response of Ti-6Al-4V alloys with three different microstructures. Two of the alloys are produced by the hot isostatically pressed technique using rapidly solidified granules, with one alloy milled prior to hot pressing. Experiments are performed over a broad range of strain rates, 10(-3)-7000 s(-1), and initial temperatures, 77-1000 K. Depending on the test temperature, compressive strains of 10-60% are achieved. The microstructure of the undeformed and deformed specimens is investigated, using optical microscopy. The dependence of the flow stress on the temperature and the strain rate is examined for various strains and it is related to the corresponding material microstructure. The results show that adiabatic shearbands develop at high strain rates, as well as at low strain rates and high temperatures. Depending on the test temperature, shearbands initiate once a sample is deformed to suitably large strains. The flow stress is more sensitive to temperature than to the strain rate. Based on these results and other published work, the thermally activated mechanisms associated with the dislocation motion are identified. The physically based model proposed by Nemat-Nasser and Li (1997) for OFHC copper, is suitably modified and applied to this class of titanium alloys, In the absence of dynamic strain aging, the model predictions are in good accord with the experimental results. Comparing the results for the three considered Ti-6Al-4V alloys, with different microstructures, it is found that the initial microstructural features affect only the magnitude of the threshold stress and the athermal part of the flow stress, but not the functional dependence of the thermally activated part of the flow stress on the temperature and the strain rate. (C) 2001 Elsevier Science Ltd. All rights reserved.