The brittle-to-ductile transition in cold rolled tungsten: On the decrease of the brittle-to-ductile transition by 600 K to-65 °C

This paper attempts to elucidate fundamental mechanisms of the brittle-to-ductile transition (BDT) in textured, pre-deformed, polycrystalline body-centred cubic metals. For this purpose, five tungsten sheets have been rolled out from one and the same sintered ingot representing degrees of deformation of 1.8, 2.5, 3.0, 3.4, and 4.1 (logarithmic notation). Electron backscatter diffraction measurements display the evolution of the microstructure with increasing degree of deformation. The mean grain size of the five sheets in the normal direction is 1.1, 0.59, 0.48, 0.37, and 0.3 mu m. Toughness tests based on the K-concept have been performed and the brittle-to-ductile transition temperature of the five sheets has been determined. The crack system used was the L-T crack system. The results have been benchmarked against the EDT temperature of a sintered ingot. The results show a decrease of the BDT temperature with increasing degree of deformation starting from 600 degrees C (873 K) for the sintered ingot via 115 degrees C (388 K), 85 degrees C (358 K), 75 degrees C (348 K), 60 degrees C (333K) down to 65 degrees C (208 K) for the most heavily deformed tungsten plate. The results are discussed against the background of (i) rolling induced lattice defects such as (i-a) vacancies, (i-b) dislocations (i-c) grain boundaries, (ii) crystallographic texture, (iii) the state of stress, (iv) impurities, (v) sinter pores, and (vi) the geometrical arrangement of areas with low and high fracture toughness. The authors postulate that the BDT is still controlled by the kink-pair nucleation process of a screw dislocation and that the BDT temperature scales with the spacing of dislocation sources (e.g. grain boundary ledges, low angle boundaries, debris loops) along the crack front.