Sintered Tungsten Heavy Alloy Research Articles

Crystallographic texture evolution in hot-press sintered tungsten heavy alloy with the effect of niobium (Nb) addition

This study examines the impact of niobium on the crystallographic texture evolution and grain boundary distribution of hot press sintered tungsten heavy alloy (W-4.9Ni-2.1Cu). EBSD analysis was used to examine the evolution of crystallographic texture and grain boundary character distribution in sintered compacts. The important findings of this study depict that cubic texture component {001} <111> were evolved in niobium unalloyed compacts due to the solidification of atoms aligned in a specified preferred orientation. Besides, it reveals the weakness in the cubic texture in niobium alloyed compacts due to the recrystallization followed by sub structural recovery occurrence during sintering. In addition, an increase in CSL fraction in niobium alloyed compacts illustrates the dominance of surface diffusion during sintering caused by the grain refinement attributed from the niobium enrichment at the tungsten-binder interface. Metallographic studies showed that the niobium concentration increment in WHAs has concomitant effect with sintered compacts' properties. A W-4.9Ni-2.1Cu-1 Nb alloy achieved the highest values for micro-hardness (432 HV0.5), electrical conductivity (23.74 % of IACS), and relative density (99.90 %).

Influence of Cobalt content on the microstructure and texture evolution of hot-press sintered Tungsten nickel copper Alloy (W-Ni-Cu)

This study examines the effect of cobalt on the crystallographic texture and grain boundary evolution of hot-press sintered tungsten heavy alloy (W-4.9Ni-2.1Cu). Traditional tungsten heavy alloy with a Ni:Cu ratio of 7:3 has cobalt added (i.e., cobalt content increased from 0 to 1 wt% with a 0.25 wt% interval). The samples were produced using the hot press sintering method with the following parameters: 50 MPa pressure, 10 °C/min heating rate, 1450 °C temperature, and a 20-min hold time. The unalloyed cobalt WHA compact has a fibre texture in the (001) plane that is aligned in the 〈111〉 orientation. In addition, the cobalt-alloyed WHAs compacts exhibit a lower proportion of grain boundaries with low angles. Enhanced solid solution formation between cobalt and WHAs during liquid phase sintering led to increased compacts densification and randomised weak fibre development in cobalt-alloyed compacts. Texture deterioration (001) <111> in cobalt-added WHAs is caused predominantly by substructural recovery during liquid phase sintering. This study indicates that the addition of cobalt to the stoichiometry of WHAs resulted in the homogenous distribution of binder matrix within the compact during sintering. As a result, the W-4.9Ni-2.1Cu-1Co alloy exhibited improved microhardness (429.5 HV0.5), electrical conductivity (23.34% of IACS), and relative density (98.10%).

Crystallographic texture and grain boundary character development of hot-press sintered tungsten heavy alloy: The effect of rhenium concentration

In this study, the effects of rhenium on the evolution of microstructure and texture in hot press sintered tungsten heavy alloy (93W-4.9Ni-2.1Cu) is examined. Rhenium is added (i.e., increased rhenium from 0 to 1 wt % with interval of 0.25 wt %) to a conventional tungsten heavy alloy while maintaining a constant Ni: Cu (7:3) ratio. The samples were prepared using a vacuum hot press sintering method with parameters including 50 MPa pressure, 10 °C/min heating rate, and 1450 °C temperature for a 20-min holding time. The impact of rhenium on densification, crystallographic texture development, and grain boundary features of Re alloyed WHA was studied using electron backscattered diffraction (EBSD). The development of grain boundary texture was the result of a Ʃ3 CSL increase in sintered compact. The base alloy exhibits fibre texture in the (001) plane with the <111> direction and a higher proportion of low-angle grain boundaries. Reduced kinetics of grain growth during sintering led to a higher degree of densification and randomised weak fibre texture in rhenium-alloyed samples. The deterioration of texture in the (001) <111> direction in rhenium-added alloys was caused by recrystallization following recovery during sintering. Metallographic studies reveal that an increase in rhenium in the WHA's stoichiometry resulted in particle refinement during sintering, thereby enhancing the WHA's properties. Maximum micro-hardness (432 HV0.5), electrical conductivity (23.74 % of IACS), and relative density (99.90 %) were observed in a 93W-4.9Ni-2.1Cu (Re 1 wt% alloy).

Evolution of crystallographic texture and grain boundary character in hot press sintered tungsten heavy alloy as a function of sintering temperature

Using the vacuum hot press sintering method, tungsten heavy alloys (93 W − 4.9Ni − 2.1Cu) was produced. The study explored the effect of sintering temperature on W-Ni-Cu heavy alloy. Studies in metallography reveals that an increase in sintering temperature led to the coalescence of tungsten grains and affected their mechanical, electrical properties. As a result, an alloy sintered at a high temperature (1450 °C) has the highest hardness (410 Hv0.5), electrical conductivity (19.72% of IACS), and relative density (96.6%).

Evolution of microstructure and crystallographic texture during cold rolling of liquid phase sintered tungsten heavy alloy

In this study, the evolution of microstructure and crystallographic texture of two-phase tungsten heavy alloy (almost pure tungsten embedded in nickel‑tungsten‑iron matrix) during cold rolling was investigated systematically. Electron back scatter diffraction studies reflected that the matrix phase accommodates more strain in the early stages of deformation and that as deformation progresses, the matrix phase exhausts its work hardening ability, resulting in the onset of micro-shear band formation. The tungsten phase on the other hand showed the presence of large orientation gradients accompanied with significant flattening and elongation of grains. At higher rolling reductions (effective true strain of −2.66), extensive shear bands spanning across several tungsten particles and the matrix were observed in the microstructure, which in turn had implications on the evolution of the crystallographic texture. It was observed that weak αb. c. c. (<110>∥RD) and γ fiber (<111>∥ND) emerge in tungsten at low to intermediate rolling reductions, and that αb. c. c. fiber strengthens much more than γ fiber at higher rolling reductions with prominent intensity at rotated cube position. For the matrix phase, a weak beta fiber (fiber connecting the brass {011}<211>, copper {112}<111>, and S {123}<634> orientations in the Euler space) was observed at low to intermediate rolling reductions, followed by strengthening of Goss and Brass orientations at higher rolling reductions.

Effects of Sintering Conditions on Structures and Properties of Sintered Tungsten Heavy Alloy.

Probably the most advantageous fabrication technology of tungsten heavy alloys enabling the achievement of required performance combines methods of powder metallurgy and processing by intensive plastic deformation. Since the selected processing conditions applied for each individual processing step affect the final structures and properties of the alloys, their optimization is of the utmost importance. This study deals with thorough investigations of the effects of sintering temperature, sintering time, and subsequent quenching in water on the structures and mechanical properties of a 93W6Ni1Co tungsten heavy alloy. The results showed that sintering at temperatures of or above 1525 °C leads to formation of structures featuring W agglomerates surrounded by the NiCo matrix. The sintering time has non-negligible effects on the microhardness of the sintered samples as it affects the diffusion and structure softening phenomena. Implementation of quenching to the processing technology results in excellent plasticity of the green sintered and quenched pieces of almost 20%, while maintaining the strength of more than 1000 MPa.

Study on the solid-phase sintering of the nano-structured heavy tungsten alloy powder

Recently, the high performance W–Ni–Fe–Co heavy tungsten alloy has become as the major core material of armor piercing ammunition. Since the melting temperature of tungsten element is too high to be fabricated by the melting process, that the W–Ni–Fe–Co alloy only can be synthesized by powder metallurgy process. In this study, two compositions of alloy powders, 93W–3Ni–2Fe–2Co and 93W–3.5Ni–1.5Fe–2Co, were selected for investigating their microstructure and mechanical properties after solid-phase sintering. These pre-alloyed powders with crystal cell size about 16 nm were synthesized by mechanical alloying (MA) the mixture of appropriate composition of pure elements in the Spex mill for 8 h. Then, the MA powders were compressed by cold isostatic pressing (CIP) and vacuum sintered at various temperature below 1400 °C for different time. Microstructure characterization of the sintered tungsten heavy alloys was conducted by means of SEM with EDS capability, X-ray diffraction (XRD), and TEM techniques. The result reveals that the microstructure of these sintered alloys was found to consist of the tungsten matrix phase and the Fe–Ni solid solution phase. The hardness of these sintered tungsten heavy alloy presents a trend with increasing sintering temperature and sintering time.

Characteristics of densification and distortion of Ni–Cu liquid-phase sintered tungsten heavy alloy

Abstract The characteristics of densification and distortion of a 80W–14Ni–6Cu alloy during liquid-phase sintering (LPS) were investigated in relation to the liquid-phase formation. By means of differential thermal analysis (DTA), the melting process of the Ni–Cu matrix in a die-pressed compact was examined and compared with that in a loose powder mixture. It was revealed that enhanced inter-diffusion and in situ alloying occur between the tightly compacted elemental Cu and Ni powders during heating, leading to an extended temperature range for Ni–Cu liquid formation. Dilatometry and furnace sintering tests showed that the early melting of the Ni–Cu matrix results in an onset of liquid-phase sintering at a lower temperature with die compaction. Full densification of the compact, together with satisfactory microstructure, can be achieved prior to full melting of the Ni–Cu matrix. Distortion is delayed until the end of liquid-phase formation.

Tungsten heavy alloy: deformation texture and recrystallization of tungsten particles

Sintered tungsten heavy alloy (90W, 7Ni, 3Fe) has been deformed in different ways and heat treated at 800–1100 °C. The texture and dislocation density of the tungsten particles were determined as well as the effect of the heat treatment on recovery and recrystallization. A 〈1 1 0〉 deformation texture of the tungsten particles was found using X-ray diffraction. Formation of dislocation cells and subgrains was found to occur during the heat treatment as well as recrystallization by grain boundary migration. A model for the recrystallization of particles by grain boundary motion in a deformed heavy alloy is presented.

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

Abstract 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–5.6Ni–1.4Fe 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.

Effects of sintering conditions on mechanical properties of mechanically alloyed tungsten heavy alloys

The effects of sintering conditions on the microstructural evolution and mechanical properties of mechanically alloyed tungsten heavy alloys were investigated. W, Ni and Fe powders were mechanically alloyed in a tumbler ball mill at a milling speed of 75 rpm, ball-to-powder ratio of 20∶1 and ball filling ratio of 15%. The mechanically alloyed powders were compacted and solid-state sintered at a temperature of 1300°C for 1 hour in a hydrogen atmosphere. The solid-state sintered tungsten heavy alloy was subsequently liquid-phase sintered at 1470°C with varying sintering times from 4 min to 90 min. The solid-state sintered tungsten heavy alloy showed fine tungsten particles of 3 μm in diameter and high relative density above 99%. The volume fraction of the W-Ni-Fe matrix phase was measured, as 11% and tungsten/tungsten contiguity was 0.74 in solid-state sintered tungsten heavy alloys. Mechanically alloyed and two-step sintered tungsten heavy alloys showed tungsten particles of 6–15 μm and a volume fraction of the W-Ni-Fe matrix phase of 16% and tungsten/tungsten contiguity of 0.40. The solid-state sintered tungsten heavy alloy exhibited a yield strength of about 1100 MPa due to its finer tungsten particles, while it showed low elongation and impact energy due to its large tungsten/tungsten contiguity. The yield strength of two-step sintered tungsten heavy alloys increased with the decreasing of tungsten particle size and volume fraction of the W-Ni-Fe matrix.

Effects of Sintering Conditions on Mechanical Properties of Mechanically Alloyed Tungsten Heavy Alloys

The effects of sintering conditions on the microstructural evolution and mechanical properties of mechanically alloyed tungsten heavy alloys were investigated. W, Ni and Fe powders were mechanically alloyed in a tumbler ball mill at a milling speed of 75 rpm, ball-to-powder ratio of 20∶1 and ball filling ratio of 15%. The mechanically alloyed powders were compacted and solid-state sintered at a temperature of 1300°C for 1 hour in a hydrogen atmosphere. The solid-state sintered tungsten heavy alloy was subsequently liquid-phase sintered at 1470°C with varying sintering times from 4 min to 90 min. The solid-state sintered tungsten heavy alloy showed fine tungsten particles of 3 μm in diameter and high relative density above 99%. The volume fraction of the W-Ni-Fe matrix phase was measured, as 11% and tungsten/tungsten contiguity was 0.74 in solid-state sintered tungsten heavy alloys. Mechanically alloyed and two-step sintered tungsten heavy alloys showed tungsten particles of 6–15 μm and a volume fraction of the W-Ni-Fe matrix phase of 16% and tungsten/tungsten contiguity of 0.40. The solid-state sintered tungsten heavy alloy exhibited a yield strength of about 1100 MPa due to its finer tungsten particles, while it showed low elongation and impact energy due to its large tungsten/tungsten contiguity. The yield strength of two-step sintered tungsten heavy alloys increased with the decreasing of tungsten particle size and volume fraction of the W-Ni-Fe matrix.

Microstructural control of and mechanical properties of mechanically alloyed tungsten heavy alloys

93W-5.6Ni-l.4Fe tungsten heavy alloys with controlled microstructures were fabricated by mechanically alloying of elemental powders of tungsten, nickel and iron by two different process routes. One was the full mechanical alloying of blended powders with a composition of 93W-5.6Ni-l.4Fe, and the other was the partial mechanical alloying of blended powders with a composition of 30W-56Ni-14Fe followed by blending with tungsten powders to form a final composition of 93W-5.6Ni-l.4Fe. The raw powders were consolidated by die compaction followed by solid state sintering at 1300°C for 1 hour in a hydrogen atmosphere. The solid state sintered tungsten heavy alloys were subsequently liquid phase sintered at 1445∼1485°C for 4-90 min. The two-step sintered tungsten heavy alloy using mechanically alloyed 93W-5.6Ni-l.4Fe powders showed tungsten particles of about 6-15 μm much finer than those of 40 um in a conventional liquid phase sintered tungsten heavy alloy. An inhomogeneous distribution of the solid solution matrix phase was obtained in the two-step sintered tungsten heavy alloy using partially mechanically alloyed powders. The two-step sintered tungsten heavy alloy using mechanically alloyed 93W-5.6Ni-l.4Fe powders showed larger elongation of 16% than that of 1% in the solid state sintered tungsten heavy alloy due to the increase in matrix volume fraction and decrease in W/W contiguity. Dynamic torsional tests of the two-step sintered tungsten heavy alloys showed reduced shear strain at maximum shear stress than did the sintered tungsten heavy alloys using the conventional liquid phase sintering.

Mechanical alloying process of 93W-5.6Ni-1.4Fe tungsten heavy alloy

The mechanical alloying process of 93W-5.6Ni-1.4Fe tungsten heavy alloy from the elemental powders of W, Ni and Fe by a high energy ball mill in argon atmosphere was investigated. The mechanical alloying process parameters such as milling speed, milling time, ball-to-powder ratio and ball filling ratio were varied in order to investigate their influence on the microstructural evolution of mechanically alloyed powders. The mechanical alloying process proceeded following five distinct stages such as flattening stage, welding dominant stage, equiaxed particle fprming stage, random lamellar forming stage and steady state stage with increasing the milling time. The steady state stage of mechanical alloying was reached after milling for 48 hours with milling speed of 75 rpm, ball-to-powder ratio of 20:1 and ball filling ratio of 15%. Nanocrystalline grain size of 16 nm was obtained at the steady state stage of mechanical alloying. Mechanically alloyed powders were consolidated by cold isostatic pressing and followed by sintering at temperature ranged 1300–1485°C for 1 hour in hydrogen atmosphere. When liquid phase sintered at 1485°C, tungsten heavy alloy from mechanically alloyed powders showed finer tungsten particles about 27μm than that from conventionally blended powders. The density of liquid phase sintered tungsten heavy alloy decreased with increasing the milling time due to the swelling during sintering. When solid state sintered below 1430°C, tungsten heavy alloy from mechanically alloyed powders showed ultra-fine tungsten particles about 3μm and showed high relative density above 97% insensitive to the milling time.

Test temperature and strain rate effects on the properties of a tungsten heavy alloy

Liquid phase sintered tungsten heavy alloy specimens with a 90W-7Ni-3Fe composition were tested for temperature and strain rate effects on mechanical behavior. Both fracture stress and strain were measured for samples tested at 20, 300, or 600 °C, with crosshead speeds ranging from 0.004 to 400 mmJs in an argon atmosphere. Fracture surface examinations showed a dramatic increase in tungsten cleavage as the ductility increased. The effect of an increasing strain rate is a slight strength increase with a concomitant ductility decrease. Alternatively, higher test temperatures degrade strength with a nonsystematic effect on ductility; maximum ductility occurs at 300 °C and a slow strain rate. Surface oxidation at 600 °C greatly degrades ductility. The results are mathematically modeled using classic strain rate dependent equations.