Processing strategy for consolidating tungsten heavy alloys for ordnance applications

The microstructural evolution and mechanical properties of a mechanically alloyed and two-stage sintered tungsten heavy alloy were investigated Elemental powders of tungsten, nickel and iron of a composition corresponding to 93W–56Ni–14Fe were mechanically alloyed in a tumbler ball mill for 72 h Mechanically alloyed powders were solid-state sintered at 1300 °C for 1 hr in a hydrogen atmosphere followed by secondary sintering at 1445–1485 °C for a sintering time ranging from 4 to 90 min Solid-state sintered tungsten heavy alloys exhibited full densification (above 99% in relative density) due to the enhanced sintering resulting from mechanical alloying Secondary sintering with a rapid heating rate changed the microstructures of the solid-state sintered alloy with contiguous tungsten phases into a dispersion alloy with spherical tungsten particles embedded in the W–Ni–Fe matrix, maintaining fine tungsten particle due to the combination of a mechanical alloying and a short sintering time The two-stage sintered tungsten heavy alloy from mechanically alloyed powders showed finer tungsten particle (about 6 μm in diameter) than in conventional liquid-phase sintered tungsten heavy alloys The mechanical properties of a tungsten heavy alloy were found to be dependent on the microstructural parameters such as tungsten particle size, matrix volume fraction and tungsten/tungsten contiguity which are controllable through the two-stage sintering process

Combination of mechanical alloying and two-stage sintering of a 93W /5.6Ni /1.4Fe tungsten heavy alloy

The mechanical properties of tungsten heavy alloys are sensitive to the processing cycle and are adversely affected by residual porosity. Sintering times greater than 2 hours usually result in pore growth with degraded properties. The development of an optimized sintering atmosphere has allowed exploration of long sintering times without significant property degradation due to pore growth. The optimal cycle was used to sinter two heavy alloy compositions (88 and 95 wt pct W) for times up to 600 minutes at 1480 °C. The 88 pct W samples slumped, but the 95 pct W samples were fully densified and suitable for tensile testing. At long sintering times, the tungsten grains flattened and the tungsten contiguity decreased, indicating a transition to low-energy configurations for the solid-liquid interfaces. The cube of the mean grain size varied linearly with the isothermal sintering time. This allowed determination of grain size effects on mechanical properties, showing a decreasing yield strength with increasing time in agreement with the Hall-Petch behavior. The tensile strength and elongation were highest for sintering times from 30 to 90 minutes, reflecting a minimum in the residual porosity.

Sintering time and atmosphere influences on the microstructure and mechanical properties of tungsten heavy alloys

Tungsten heavy alloys (WHAs) are metal–metal composites consisting of nearly pure spherical tungsten particles embedded in a Ni–Fe–W or Ni–Co–W or Ni–Cu–W ductile matrix. In this dual phase alloy, there are several complicated relations between the ductile matrix and hard tungsten particles. The aim of this research was to examine the effect of varying tungsten content on the microstructure and mechanical properties of tungsten heavy alloys. The microstructural parameters (grain size, connectivity, contiguity and solid volume fraction) were measured and were found to have a significant effect on the mechanical properties of tungsten-based heavy alloys. The result shows that the binding strength between the W and the matrix phase has a major influence on the ductility of tungsten-based alloys. The larger this binding force is, the better the ductility is.

Tensile behavior change depending on the varying tungsten content of w-ni-fe alloys

Tungsten heavy alloys (WHAs) belong to a group of two-phase composites, based on W-Ni-Cu and W-Ni-Fe alloys. Due to their combinations of high density, strength, and ductility, WHAs are used as radiation shields, vibration dampers, kinetic energy penetrators and heavy-duty electrical contacts. This paper presents recent progresses in processing, microstructure, and mechanical properties of WHAs. Various processing techniques for the fabrication of WHAs such as conventional powder metallurgy (PM), advent of powder injection molding (PIM), high-energy ball milling (MA), microwave sintering (MW), and spark-plasma sintering (SPS) are reviewed for alloys. This review reveals that key factors affecting the performance of WHAs are the microstructural factors such as tungsten and matrix composition, chemistry, shape, size and distributions of tungsten particles in matrix, and interface-bonding strength between the tungsten particle and matrix in addition to processing factors. SPS approach has a better performance than those of others, followed by extrusion process. Moreover, deformation behaviors of WHA penetrator and depleted uranium (DU) Ti alloy impacting at normal incidence both rigid and thick mild steel target are studied and modelled as elastic thermoviscoplastic. Height of the mushroomed region is smaller for and it forms sooner in each penetrator as compared to that for .

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Recent Progress in Processing of Tungsten Heavy Alloys

Controlled processing of heavy alloys containing 88 to 97 pct W resulted in high sintered densities and excellent bonding between the tungsten grains and matrix. For these alloys, deformation and fracture behavior were studiedvia slow strain rate tensile testing at room temperature. The flow stress increased and the fracture strain decreased with increasing tungsten content. The tradeoff between strength and ductility resulted in a maximum in the ultimate tensile strength at 93 pct W. Microstructure variations, notably grain size, explain sintering temperature and time effects on the properties. During tensile testing, cracks formed on the surface of the specimens at tungsten-tungsten grain boundaries. The crack density increased with plastic strain and tungsten content. The surface cracks, though initially blunted by the matrix, eventually increased in density until catastrophic failure occurred. An empirical failure criterion was developed relating fracture to a critical value of the surface crack tip separation distance. Application of the model explains the effects of microstructural variables on tensile properties.

B. H. Rabin

Papers

Microstructure effects on tensile properties of tungsten-Nickel-Iron composites