Adiabatic shearband in WHA in high-strain-rate compression

w Using UCSD’s recovery Hopkinson technique Nemat-Nasser, S., Isaacs, J.B., Starrett, J.E., 1991. Hopkinson techniques x for dynamic recovery experiments. Proc. R. Soc. London, A135, 371 , enhanced for high-temperature compression w experiments Nemat-Nasser, S., Isaacs, J., 1997. Direct measurement of isothermal flow stress of metal at elevated x temperatures and high strain rates. Acta. Met., 45, 907 , two techniques are developed to create adiabatic shearbands in Ž . Ž . tungsten heavy alloy WHA samples at high strain rates 3000 to 5000rs , and the results are compared with those obtained Ž y3 . at low strain rates 10 rs . In the first technique, a cylindrical sample is subjected to a single compression pulse and is recovered without being subjected to any additional loading. The change in the surface temperature of the sample is measured during its high-strain-rate deformation, using an infrared technique. In the second scheme, a small circular cylindrical sample is constrained at both ends by thin confining rings and then subjected to axial compression in the recovery Hopkinson bars at various initial temperatures. Barreling of the sample takes place under somewhat controlled conditions and hence, this experiment is referred to as the ‘controlled barreling’ test. The confining rings promote shearband formation. Since the samples have been subjected to only a single compression pulse, the experiments allow correlating the resulting microstructural changes with the corresponding temperature and loading histories. SEM observations revealed that intergranular fracture occurs within the shearbands in the Fe–Ni matrix. Some transgranular fracture was also observed. The high-strain-rate controlled barreling test performed at the initial room temperature, invariably leads to adiabatic shearbandŽ . ing. However, at suitably high initial temperatures e.g., 500–7008C , the shearbanding gives way to a diffused yet highly localized deformation. In addition, the strain, strain-rate, and temperature dependency of the flow stress of this material is quantified, based on high-strain-rate isothermal and adiabatic, and quasi-static flow stress measurements. These and related features are discussed in this paper. q 1998 Elsevier Science Ltd. All rights reserved.