Microstructure and mechanical properties of tungsten heavy alloys

Research highlights: → This paper describes the transition of tensile fracture mode from transgranular to intergranular at elevated temperature in case of tungsten heavy alloys → This paper, first time, describes the fact that the richer is the W content in the matrix, the higher the tensile properties of tungsten heavy alloys. → Also this paper explains the reason for the wavy-tensile-stress-strain-curve which is observed in tungsten heavy alloys during tensile testing at room temperature with a very slow strain rate. - Abstract: Two tungsten (W)-nickel (Ni)-copper (Cu) alloys (WNCs) and one W-Ni-iron (Fe) alloy (WNF) were prepared by liquid phase sintering at 1783 K and 1733 K, respectively. The average W-grain size in the sintered WNCs (60-70 μm) was coarser than that in the WNF alloy (30 μm) possibly due to the higher sintering temperature (1783 K) necessary for the former alloys. The volume of the matrix phase in the WNF (25-30 vol.%) was higher than that in WNCs (10-15 vol.%). The tensile properties and hardness of WNF specimens at room temperature were significantly superior to those of WNC specimens apparently due to the finer W-grain size, lesser contiguity and porosity in the former. WNF specimens, in contrast to WNCs, failed under tension by W-grain cleavage fracture, possibly due to relatively stronger matrix phase and W/matrix bonding. At very low strain rate (0.0001/s) the tensile curve of WNF was wavy in nature, but these were absent at higher strain rates (0.001 to 1/s). The tensile strength and elongation of WNF alloy remarkably deteriorated at higher temperatures (773 and 973 K), and the fracture changed to matrix failure mode apparently due to weakening of the matrix phase.