The thermoviscoplastic response of polycrystalline tungsten in compression

The thermomechanical response of commercially pure polycrystalline tungsten was investigated over a wide range of strain rates and temperatures. The material was examined in two forms: one an equiaxed recrystallized microstructure and the other a heavily deformed extruded microstructure that was loaded in compression along the extrusion axis. Low strain rate (10(-3)-10(0) s(-1)) compression experiments were conducted on an MTS servohydraulic load frame equipped with an infra-red furnace capable of sustaining specimen temperatures in excess of 600 degrees C. High strain rate (10(3)-10(4) s(-1)) experiments were performed on a compression Kolsky bar equipped with an infra-red heating system capable of developing specimen temperatures as high as 800 degrees C. Pressure-shear plate impact experiments were used to obtain shear stress versus shear strain curves at very high rates (similar to 10(4)-10(5) s(-1)). The recrystallized material was able to sustain very substantial plastic deformations in compression (at room temperature), with a flow stress that appears to be rate-dependent. Intergranular microcracks were developed during the compressive deformations. Under quasi-static loadings a few relatively large axial splitting cracks were formed, while under dynamic loadings a very large number of small, uniformly distributed microcracks (that did not link up to form macrocracks) were developed. The rate of nucleation of microcracks increased dramatically with strain rate. The extruded tungsten is also able to sustain large plastic deformations in compression, with a flow stress that increases with the rate of deformation. The strain hardening of the extruded material is lower than that of the recrystallized material, and is relatively insensitive to the strain rate. High-temperature experiments at low and high strain rates show that the strain hardening is also insensitive to the temperature over this temperature range. The flow stress is shown to be strongly temperature-dependent at low homologous temperatures. (C) 2000 Published by Elsevier Science S.A. All rights reserved.