Tungsten heavy alloys (THAs) are two-phase composites consisting of tungsten particles/agglomerates surrounded by a ductile matrix [1]. Due to their superb mechanical properties and high specific mass, tungsten heavy alloys are used in demanding applications, such as kinetic penetrators, gyroscope rotors, or radiation shielding. Effective structure refinement can be introduced via methods of severe plastic deformation. However, their structure, consisting of hard tungsten particles embedded in a soft matrix, makes the deformation processing a challenging task.
This study focused on the characterization of deformation behaviour during thermomechanical processing of a W-Ni-Co tungsten heavy alloy (THA) via the method of rotary swaging at various temperatures. Rotary swaging is an intensive plastic deformation method advantageously used in the industry to gradually reduce cross-sections and increase lengths of axisymmetric workpieces [2].
The primary aim was to determine microstrain and characterize the dislocations and active slip system in the original sintered THA as well as in the rotary swaged bars, in order to characterize the effects of thermomechanical treatment on the microstructure. Emphasis was given on the investigation by neutron diffraction. Characterization of advanced materials by neutron powder diffraction provides information not accessible by other techniques. Thanks to the low absorption of neutrons, the bulk of the material and large-grain samples can be investigated, moreover - in many cases - in situ at elevated temperatures.
The sintered bars were processed by rotary swaging either at room temperature or at 900°C into circular swaged bars with a diameter of 10 mm. The neutron diffraction patterns for structure and microstrain determination were collected at ambient temperature on the MEREDIT diffractometer of CANAM infrastructure at NPI Řež near Prague [3].
Two phases were identified in all samples of W-Ni-Co alloy. The main phase (denoted W-B2) had α-W (B2) structure and was formed by the original tungsten powder grains. The second phase (denoted NiCo2W in what follows) with a weight fraction of 6%–7% and Ni-like structure (fcc) was present as well.
The analysis [4, 5] showed that the grains of the NiCo2W matrix refined significantly after the deformation treatments. The microstrain was higher in the cold swaged sample (44.2 × 10−4) than in the hot swaged sample (41.2×10−4). The evaluation of the modified Williamson-Hall plots showed that both the samples swaged at 20°C and 900°C exhibited the activation of edge dislocations with <111> {110} or <110> {111} slip systems, and/or screw dislocations with <110> slip system in the NiCo2W matrix.
Dislocation densities in NiCo2W phase were estimated from the diffraction peaks broadening. It was seen that the dislocation densities increased approximately 5 times after rotary swaging, and that it is 15% higher for the sample swaged at room temperature than for the sample deformed at 900°C. It was concluded from the comparison with the stress-strain test that the increased dislocation density due to swaging is responsible for the observed substantial mechanical strengthening (larger for the cold swaged bar).
The
study was supported by the Czech Science Foundation (project no. 19-15479S).
Measurements were carried out at the CANAM infrastructure of the NPI CAS Řež, and at the infrastructure Reactors LVR-15 and LR-0.