Fe Heavy Alloy Research Articles

Effect of Heating Rate on Densification and Grain Growth During Spark Plasma Sintering of 93W-5.6Ni-1.4Fe Heavy Alloys

Blended 93W-5.6Ni-1.4Fe powders were sintered via the spark plasma sintering (SPS) technique using heating rates from 10 K min−1 to 380 K min−1 (10 °C min−1 to 380 °C min−1). The kinetics of densification and grain growth were analyzed to identify heating rate effects during the SPS of 93W-5.6Ni-1.4Fe powders. The activation energies for densification were calculated and compared with the experimental values for diffusion and other mass transport phenomena. The results show that for the slowly heated specimens [heating rate <100 K min−1 (100 °C min−1)], densification occurs mainly through dissolution–precipitation of W through the matrix phase and W grain boundary diffusion. The concurrent grain growth is dominated by surface diffusion at a low sintering temperature and by solution–reprecipitation and Ni-enhanced W grain boundary diffusion at a higher temperature. For the specimens sintered with heating rates higher than 100 K min−1 (100 °C min−1), the apparent activation energy value for the mechanism controlling densification is a strong function of the relative density, and fast densification controlled by multiple diffusion mechanisms and intensive viscous flow dominates over the grain growth. High SPS heating rate is favorable to obtain high density and fine-grained tungsten heavy alloys.

Fine-grained 93W–5.6Ni–1.4Fe heavy alloys with enhanced performance prepared by spark plasma sintering

Blended 93W–5.6Ni–1.4Fe powders were sintered via the spark plasma sintering (SPS) technique. Fine-grained 93W–5.6Ni–1.4Fe heavy alloys (W grain size is about 6µm) with liquid-phase sintered microstructure were obtained by control of the SPS process. After SPS, the alloys show relative density of 0.95 and tungsten–tungsten contiguity of 0.53. The alloys exhibit improved bending strength (about 1580MPa) and yield strength (about 1050MPa at room temperature and about 640MPa at 800°C), due to their fine-grained structure. The fracture morphology after bending test is mainly characterized as tungsten–tungsten intergranular rupture. A decreased tungsten–matrix interface decohesion is presented due to the improved binding strength of tungsten–matrix by “SPS effect”. The mechanical properties of SPSed 93W–5.6Ni–1.4Fe heavy alloys are dependent on the microstructural parameters such as tungsten grain size, matrix volume fraction and tungsten–tungsten contiguity.

Spark-Plasma Sintering of W-5.6Ni-1.4Fe Heavy Alloys: Densification and Grain Growth

W-5.6Ni-1.4Fe heavy alloys were prepared by the method of spark-plasma sintering, and the densification and grain growth kinetics were analyzed as a function of various parameters such as sintering temperature and dwell duration. It is found that the local temperature gradient at the vicinity of the pores can cause the matrix phase melting or softening, resulting in a viscous layer coating the W particles and an improved solubility of W into the matrix phase. In the initial stage, particle rearrangement and neck formation and growth take place, and γ-(Ni, Fe) matrix phase has formed. Dissolution-precipitation and Ni-enhanced W grain boundary diffusion together with viscous process contribute to the simultaneous densification and grain growth in the intermediate stage. During the final stage, fast grain growth, controlled by both gas-phase diffusion and dissolution-precipitation mechanisms, dominates over the densification.

Production of high-purity ammonium perrhenate for W–Re–Ni–Fe heavy alloys

The paper presents the developed method for purification of ammonium perrhenate solutions by ion-exchange method on strongly acidic cation-exchange resin C160. The developed method is characterized by high, over 95% efficiency of sorption of potassium, sodium, calcium, iron, magnesium and lead ions, and high efficiency of regeneration of the resin with 32% solution of nitric(V) acid. In that way ammonium perrhenate was produced of the following composition: 69.4% Re, <0.0005% Ca, <0.001% K, <0.0005% Mg, <0.0005% Cu, <0.0005% Na, <0.0005% Mo, <0.0005% Ni, <0.0005% Pb, <0.0005% Fe. Some selected results of the experiments into possibilities for application of that specifically purified ammonium perrhenate in production of W–Re–Ni–Fe heavy alloys are presented as well as physical and mechanical properties of the heavy alloys. It was established that the purified ammonium perrhenate is a proper material for production of heavy alloys, and the generated sinters can be used in manufacture of armour-piercing discarding sabots.

SPS densification behavior of W-5.6Ni-1.4Fe heavy alloy powders

Spark plasma sintering (SPS) method was used to sinter two different types of 93W-5.6Ni-1.4Fe (wt pct) heavy alloy powders which were respectively prepared by simple mixing and high-energy ball milling (HEBM) for 40 h. In SPS processing, the powder compacts were heated at 100 °C/min to the desired sintering temperature with 5 min holding. The SPS densification behavior of the two types of powders was investigated. For the simple mixed powders, the compacts start to shrink around 850 °C, and the densification is enhanced by diffusion and plastic deformation with the increase of temperature. Consequently the compacts shrink effectively and obtain nearly full density around 1230 °C. While for the as-milled powders, recovery and recrystallization occur between 700–850 °C due to the crystal lattice distortion and defects resulted from HEBM, causing the soften of particles and a slight shrink of compacts. When increasing temperature, nanosintering of W crystallines in the matrix occurs in the inside of the composite powders, and the densification rate accelerates and maximizes at about 1050 °C. Owing to the improved activity of powders from HEBM, the tungsten grains coarsen rapidly in the sintering, simultaneously, the harmful intermetallic phases Ni2W4C and Fe6W6C are generated.

Influence of mixing and effect of agglomerates on the green and sintered properties of 97W–2.1Ni–0.9Fe heavy alloys

Abstract Feedstock of 97 wt.% tungsten heavy alloy with different particle characteristics was injection molded as tensile bars and investigated for green density, solvent debinding kinetics, sintered density, and the tensile properties. Feedstock with higher agglomerate content resulted in injection molded bars with lower green density. Contrary to the expectations, the debinding kinetics, sintered density, and sintered properties of the heavy alloys were not influenced by the agglomerate content. Microstructural and fractographic analysis of the sintered samples supported the above observations, with similarities in microstructural parameters such as contiguity, porosity, grain size, and fracture morphology of the sintered alloys. It is concluded that favorable wetting, spreading and solubility characteristics of tungsten and the liquid phase consisting of W–Fe–Ni eliminate the inhomogeneities in the microstructure resulting in densification and uniform sintered properties.

Investigation of deformation and heat treatment effects on properties of PM 92.5W–5Ni–2.5Fe heavy alloys

This paper presents the effects of vacuum heat treatment under different cooling conditions on mechanical and structural properties of forged heavy alloys, such as 92.5W–5Ni–2.5Fe and 92.5W–5Ni–25Fe microalloyed with Co. The tungsten composition in the c phase has proved to be higher and more homogenous in the rapidly cooled alloys than in the slowly cooled ones. The effects of chemical composition inhomogeneity on mechanical and structural properties of alloys were also analysed and discussed. The results of tensile and toughness testing have shown an increase in ductility and toughness, while the strength of heat treated alloys decreased in comparison with the strength of forged alloys. The fracture analysis has shown that in the sintered and rotary forged alloys, intergranular fracture of the tungsten phase and transgranular fracture of the γ phase occurred, respectively. The fracture of these phases after heat treatment was characterised by transgranular morphology.

Heating rate effects on microstructural properties of liquid phase sintered tungsten heavy alloys

Dimensional precision is one of the prime concerns in forming net-shape bodies by liquid phase sintering. The factors that lead to densification can also cause distortion, leaving a narrow processing window in which to achieve densification without distortion. This paper reports the effect of heating rate on liquid phase sintering of W–Ni–Fe heavy alloys. Heavy alloys with compositions of 83, 88, and 93 wt.% W, balance Ni–Fe in the ratio 7:3, were sintered at heating rates of 1, 5, 10, and 15 °C/min. Over this range, heating rate did not have a significant effect on densification nor distortion. Critical values of contiguity and connectivity were identified that separate distortion from shape preservation. The grain size distributions produced by different heating rates are self-similar. A previously proposed cumulative intercept grain size distribution function gives a good fit to the grain size data. The mean grain growth rate constant is 2.8 μm3/s. The grain growth rate constant varies inversely with the volume fraction of liquid to the 2/3 power.

Heating rate effects on microstructural properties of liquid phase sintered tungsten heavy alloys

Dimensional precision is one of the prime concerns in forming net-shape bodies by liquid phase sintering. The factors that lead to densification can also cause distortion, leaving a narrow processing window in which to achieve densification without distortion. This paper reports the effect of heating rate on liquid phase sintering of W–Ni–Fe heavy alloys. Heavy alloys with compositions of 83, 88, and 93 wt.% W, balance Ni–Fe in the ratio 7:3, were sintered at heating rates of 1, 5, 10, and 15 °C/min. Over this range, heating rate did not have a significant effect on densification nor distortion. Critical values of contiguity and connectivity were identified that separate distortion from shape preservation. The grain size distributions produced by different heating rates are self-similar. A previously proposed cumulative intercept grain size distribution function gives a good fit to the grain size data. The mean grain growth rate constant is 2.8 μm 3/s. The grain growth rate constant varies inversely with the volume fraction of liquid to the 2/3 power.

Structure analysis of the constitutional phases in liquid phase sintered W–Mo–Ni–Fe heavy alloys

The crystal structures of constitutional phases in two different tungsten heavy alloys processed via different conditions were examined. The compositions of these two alloys were W–11.9Mo–17.0Ni–7.7Fe and W–29.6Mo–17.0Ni–7.7Fe (at.%), which were liquid phase sintered at 1500 °C for 5 or 240 min, and followed by either furnace-cooling or water-quenching. Increase in isothermal hold caused increased concentration of Mo but decreased concentration of W in the matrix phase, which did not affect the lattice parameter of the matrix phase to a significant extent. Quenching the specimen in water caused increase in the concentrations of both W and Mo in the matrix phase, and, consequently, increases in the lattice parameter of the matrix phase. A tungsten heavy alloy with a high alloying concentration of Mo was prone to induce the precipitation of an intermetallic phase during cooling, which was enhanced by increasing the isothermal hold at the liquid phase sintering temperature and decreasing the cooling rate. The structure of this intermetallic phase is analogous to that of MoNi, and can be designated as (W x Mo 1− x )(Fe y Ni 1− y ). The composition of this intermetallic phase varied with the composition of the alloy and its cooling rate subsequent to sintering. For a furnace-cooling condition, the atomic ratio of W to Mo ( x/1− x) in this intermetallic phase was about 0.47 times the atomic ratio of W to Mo of the original alloy composition. Such a proportional constant decreased to about 0.30 when the specimen was water-quenched.

Precipitation of an Intermetallic Phase in Mo-alloyed Tungsten Heavy Alloys

Mo–Ni–Fe heavy alloys essentially is constant. 2,9,20) Accordingly, alloying by Mo reduces the solubility of tungsten in the liquid phase during sintering, which retards the growth of W grains, promotes refinement, and exhibits better mechanical properties. Kemp 2) analyzed the microstructural evolution and the kinetic process of grain growth of W–Mo–Ni–Fe alloys, and noted that during furnace cooling a new intermetallic phase precipitated in the interfaces between the W–Mo grains and the matrix phase, and modified the mechanical properties. However, its exact composition and structure were not identified. It was also noted that the intermetallic phase could not be eliminated through conventional heat treatment. The above-mentioned reports mentioned some phenomena in the liquid-phase-sintered W–Ni–Fe alloys. However, the detail of microstructural evolution of W–Mo–Ni–Fe heavy alloys during liquid phase sintering remains unclear, and the effect of the Mo addition has not been fully discussed. Furthermore, the precipitation mechanism of the intermetallic phase, and the relationship between the intermetallic phase and cooling rate has still not been clear yet. Therefore, this study aims to provide further analysis of the issues mentioned above. A transmission electron microscopy (TEM) with an attached Energy Dispersive X-ray Spectrometer (EDS) was used to perform more accurate analysis, because the electron beam of TEM was only 10 nm in diameter and the effective excitation volume was approximately 10 5 mm 3 . 21) As only few studies have used TEM with EDS to analyze the microstructures of the matrix phase in the sintered W–Mo– Ni–Fe heavy alloys, TEM with EDS were used as the primary tools to examine the above-mentioned questions.

Variables on the precipitation of an intermetallic phase for liquid phase sintered W–Mo–Ni–Fe heavy alloys

The concentration of Mo in the liquid phase of W–Mo–Ni–Fe heavy alloys during isothermal hold dictates the precipitation of an intermetallic phase during cooling. If the concentration of Mo atoms in the matrix phase exceeds an equilibrium value (between 8 and 9 at.%), Mo atoms have a strong tendency to precipitate along with W, Ni and Fe from the matrix phase onto the solid matrix phase interface during cooling. The coprecipitation of W, Mo, Ni, and Fe results in the formation of an intermetallic phase in the interface between the solid grains and the matrix phase, (W 4Mo 6)(Ni 7Fe 3), which possibly has the same structure as MoNi. This intermetallic phase is difficult to eliminate by a fast water-quench practice.

Intensive grinding of metal powders in electro-magneto-mechanical mill

Bulk 93W–5.6Ni–1.4Fe heavy alloys were prepared by an optimum sintering schedule and post-heat treatment using spark plasma sintering (SPS) technique. The microstructural parameters of the alloys, including tungsten grain morphology, tungsten grain size and W–W contiguity, are highly related to the heat treatment conditions. After cyclic heat treatment at 1400 °C with one cycle, the alloy shows fine near-spherical tungsten grains embedded in the matrix phase with decreased W–W contiguity. The alloy exhibits improved bending strength (about 1700 MPa) and yield strength (about 1100 MPa at room temperature and about 700 MPa at 800 °C), due to the microstructural modification. The bending fracture morphology is characterized as tungsten–tungsten intergranular separation, tungsten transgranular cleavage, matrix ductile rupture and local matrix rupture on the separated W–W interfaces. The room temperature yield strength of the spark plasma sintered 93W–5.6Ni–1.4Fe heavy alloys are dependent on the microstructural factors such as tungsten grain size, matrix volume fraction and tungsten–tungsten contiguity.

Influence of the addition of Lanthanum on a W–Mo–Ni–Fe heavy alloy

In order to improve the mechanical properties of a W–Mo–Ni–Fe heavy alloy, the influence of adding rare earth element La on heavy alloy and its action mechanism have been studied in this paper. The results showed that the mechanical properties of this alloy can remarkably be improved when a proper amount of La is added. Scanning electron microscopy, transmission electron microscopy, X-ray diffraction pattern and auger electron spectroscopy experiments revealed that the improvement of mechanical properties was primarily due to the formation of stable LaMnO 3 and a small amount of Mn 3O 4, which changed the existence form and distribution state of the impurity element oxygen, reduced the interfacial segregation of oxygen and thus increased the bonding strength of W-matrix interfaces. Consequently, the mechanical properties of the alloy were improved.

Udulation of W/matrix interface by resintering of cyclically heat-treated W [sbnd]Ni [sbnd]Fe heavy alloys

When liquid-phase sintered W Ni Fe alloys were cyclically heat-treated at 1100 °C adn resintered at 1485 °C, undulation of W/matrix interface resulted. The irregularity of the interface increased with the number of heat-treatment cycles. The residual thermal stress of W grains measured by X-ray diffraction increased with the number of heat-treatment cycles and exceeded the yield stress of W single crystal in certain crystallographic directions. A calculation by the finite element method also showed non-uniform distribution of thermal stress on W grains. Local yielding of W grains is believed to occur during the cyclic heat treatment. The observed undulation of W/matrix interface appears therefore to result from preferential dissolution of material from regions with higher strain energy and precipitation of material at regions with lower strain energy at the resintering temperature. The undulation disappeared with the grain growth during prolonged resintering.

A comparison of the sintering characteristics of ball-milled and attritor-milled W–Ni–Fe heavy alloy

Blended elemental W–7 wt.%Ni–3 wt. %Fe powders were mechanically alloyed in a planetary ball mill and a heavy duty attritor. The structural and morphological characteristics of the as-milled powders were characterized by x-ray diffraction and electron microscopy techniques. A ternary solid solution of Fe and Ni in W formed in both the cases; the amount of Ni and Fe dissolved is higher in the attritor-milled powder than in the ball-milled powder. Morphologically, the W-rich particles in the attritor-milled powders were spheroidal, smaller in size, and showed a narrower size distribution than in the ball-milled condition. These differences led to increased density in the attritor-milled powder compact and significant differences in chemical composition of the matrix region and W-rich grains. Transmission electron microscopy investigations of the matrix region in the compacts revealed the presence of Fe2W precipitates uniformly distributed in an Ni(Fe, W) matrix.

Influence of pre-alloyed Ni-Fe-Mo binder metal on properties and microstructure of tungsten heavy alloys

The effect of a pre-alloyed Ni-Fe-Mo binder metal on the properties and microstructure of tungsten heavy alloys has been investigated. The results show that the 93W + 7(Ni-Fe-Mo) heavy alloy in which the pre-alloyed Ni-Fe-Mo powder was used for binder metal has a higher tensile strength and a better ductility than the 93W + 4.9Ni + 2.1Fe heavy alloy. Because of the very fast diffusion of molybdenum in tungsten, the molybdenum is well distributed in tungsten grains and matrix in the 93W + 7(Ni-Fe-Mo) alloy after sintering, though molybdenum has been pre-alloyed in the binder metal before sintering. The tungsten content in the matrix of the 93W + 7(Ni-Fe-Mo) alloy is clearly reduced. However, the growth of tungsten grains during sintering was not found to be restrained. The pre-alloyed Ni-Fe-Mo binder metal makes the 93W + 7(Ni-Fe-Mo) alloy have a liquid of higher molybdenum content in the initial stage of sintering, which may be favourable for reducing the interfacial energy of tungsten (solid)-matrix (liquid) and for obtaining a lower contiguity.

Matrix Penetration of W/W Grain Boundaries and Its Effect on Mechanical Properties of 93W-5.6Ni-1.4Fe Heavy Alloy

The matrix phase of 93W-5.6Ni-l.4Fe heavy alloy has penetrated into W/W grain boundaries during a newly developed heat treatment which consists of repeated isothermal holdings at 1150 °C and water quenching between them. With the matrix penetration, the impact energy increases from 57 to 170 J, whereas the ultimate tensile strength and elongation remain un- changed. The tensile fracture surface of specimens appears similar regardless of the matrix penetration: predominant cleavage of W grains and dimple fracture of matrix. The fracture sur- face by impact, on the other hand, reveals a change of W/W interface failure toward ductile failure of penetrated matrix with many dimples by increasing the matrix penetration. The in- crease in the fraction of ductile failure and observed crack blunting is attributed to the increase in impact energy with the matrix penetration.

The reduction of the interfacial segregation of phosphorus and its embrittlement effect by lanthanum addition in a W-Ni-Fe heavy alloy

The addition of La to sintered 93W-4.9Ni-2.1Fe (weight percent) heavy alloy substantially reduces the embrittlement effect of P, because it forms a stable compound with P of possibly the form LaPO4. When 150 ppm of P is added to the powder mixture with about 19 ppm of P and 1 ppm of S as trace elements, the impact energy is very low, even after the heat treatment at 1150°C in N2 and water quenching due to the segregation of P at the interfaces. With the addition of La to this alloy (with 150 ppm of P added), its impact energy increases until, at the La content of 0.3 wt pct, it becomes equal to that of the alloy without any P added. When 0.2 wt pct of La is added to the alloy (without doping with P), the impact energy does not vary with the cooling rate after the heat treatment at 1150°C and is unaffected by the additional heat treatments at 500°C or 600°C. The structure of the fracture surfaces and the concentration of P measured at the interfaces also show that the La addition of a sufficient amount almost eliminates the P segregation at the grain-matrix interface. It appears that a stable compound of La, P, and O is formed at relatively high temperatures, above the heat-treatment temperature of 1150°C. The results show that by adding La, the variation of the impact property of this alloy with the amount of P in raw material powder or with the heat-treatment condition after sintering can be substantially reduced.

Impact strength of notched specimen for sintered W-Ni-Fe alloy.

Instrumented Charpy tests were carried out for sintered W-Ni-Fe alloys in order to clarify the effects of notch radius on impact strength. Sintered 93.0W-4.9Ni-2.1Fe and 97.2W-2.0Ni-0.8Fe (in wt%)heavy alloy specimens were prepared. Notch radii of the specimens used were 1.0, 0.2 and 0.08 mm. The maximum load and the absorbed energy decreased with reducing notch radius. SEM fractography of impact specimens revealed a large amount of cleavage failure of tungsten grains. However, the absorbed energy corresponded well with the fraction of ductile fracture surface formed by the failure of matrix. Plastic strains near the fracture surface were measured by a grid method. The results indicated that the plastic zone size induced by an impact test was reduced as the notch radius decreased. On the basis of the observed plastic zone and the Rice-Rosengren-Hutchinson model, the estimated plastic works for the impact test specimens were close to the experimental absorbed energies.