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Showing papers in "Metallurgical and Materials Transactions A-physical Metallurgy and Materials Science in 1984"


Journal ArticleDOI
TL;DR: In this article, a new method of modeling material behavior which accounts for the dynamic metallurgical processes occurring during hot deformation is presented, which considers the workpiece as a dissipator of power in the total processing system and evaluates the dissipated power co-contentJ = ∫o σ e ⋅dσ from the constitutive equation relating the strain rate (e) to the flow stress (σ).
Abstract: A new method of modeling material behavior which accounts for the dynamic metallurgical processes occurring during hot deformation is presented. The approach in this method is to consider the workpiece as a dissipator of power in the total processing system and to evaluate the dissipated power co-contentJ = ∫o σ e ⋅dσ from the constitutive equation relating the strain rate (e) to the flow stress (σ). The optimum processing conditions of temperature and strain rate are those corresponding to the maximum or peak inJ. It is shown thatJ is related to the strain-rate sensitivity (m) of the material and reaches a maximum value(J max) whenm = 1. The efficiency of the power dissipation(J/J max) through metallurgical processes is shown to be an index of the dynamic behavior of the material and is useful in obtaining a unique combination of temperature and strain rate for processing and also in delineating the regions of internal fracture. In this method of modeling, noa priori knowledge or evaluation of the atomistic mechanisms is required, and the method is effective even when more than one dissipation process occurs, which is particularly advantageous in the hot processing of commercial alloys having complex microstructures. This method has been applied to modeling of the behavior of Ti-6242 during hot forging. The behavior of α+ β andβ preform microstructures has been exam-ined, and the results show that the optimum condition for hot forging of these preforms is obtained at 927 °C (1200 K) and a strain rate of 1CT•3 s•1. Variations in the efficiency of dissipation with temperature and strain rate are correlated with the dynamic microstructural changes occurring in the material.

1,121 citations


Journal ArticleDOI
TL;DR: In this article, a theoretical model based on the Hunt model is developed, and it is found that the theory adequately explains the variation in primary spacing, λ 1, with the growth rate,V.
Abstract: The primary spacing data of Part I are compared to the existing theoretical models of Hunt and of Kurz and Fisher, and a significant disagreement is found. A theoretical model based on the Hunt model is developed, and it is found that the theory adequately explains the variation in primary spacing, λ1, with the growth rate,V. A maximum in λ1,vs V is predicted and the velocity at which the maximum occurs matches with the result obtained experimentally. It is shown that the maximum in λ1 corresponds to the dendrite-to-cell transition, and cellular structures are found to grow with much smaller spacings than dendritic structures under identical growth conditions.

355 citations


Journal ArticleDOI
TL;DR: In this paper, a two-dimensional transient model for convective heat transfer and surface tension driven fluid flow is developed, which describes the transient behavior of the heat transfer process of a stationary band source.
Abstract: A two-dimensional transient model for convective heat transfer and surface tension driven fluid flow is developed. The model describes the transient behavior of the heat transfer process of a stationary band source. Semi-quantitative understanding of scanning is obtained by a coordinate transformation. The non-dimensional forms of the equations are derived and four dimensionless parameters are identified, namely, Peclet number (Pe), Prandtl number (Pr), surface tension number(S), and dimensionless melting temperature(@#@ Tm * @#@). Their governing characteristics and their effects on pool shape, cooling rate, velocity field, and solute redistribution are discussed. A numerical solution is obtained and presented. Quantitative effects of Prandtl number and surface tension number on surface velocity, surface temperature, pool shape, and cooling rate are presented graphically.

294 citations


Journal ArticleDOI
TL;DR: In this paper, the microstructure, texture, and whisker orientations in 6061 Al-20 wt pct SiC whisker composites have been examined using transmission electron microscopy and X-ray diffraction.
Abstract: The microstructure, texture, and whisker orientations in 6061 Al-20 wt pct SiC whisker composites have been examined using transmission electron microscopy and X-ray diffraction. Tension creep tests of the composite material have also been conducted in the temperature range 505 to 644 K (450 to 700 F). The steady state creep rate of the composite depends strongly on the temperature and applied stress. The stress exponent for the steady state creep rate of the composite is approximately 20.5 and remains essentially constant within the range of test temperatures. The activation energy is calculated to be 390 kJ/mol, nearly three times as high as the activation energy for self-diffusion of aluminum. No threshold stress was observed. Fracture surface examination using scanning electron microscopy shows that the composite fails by coalescence of voids in the aluminum matrix which originate at the aluminum-SiC interface. It is demonstrated that SiC paniculate composites are less creep resistant than SiC whisker composites.

270 citations


Journal ArticleDOI
TL;DR: In this article, a scaling law is shown to exist between the secondary dendrite arm spacing, λ2, near the tip and the Dendrite tip radius, p, which is λ 2/ρ = 2.2 ± 0.3.
Abstract: Directional solidification experiments have been carried out in a succinonitrile-5.5 mol pct acetone system to characterize dendrite tip radius and interdendrite spacings as functions of growth rate and temperature gradient in the liquid. A maximum in primary dendrite spacing as a function of growth rate is observed, and this maximum is found to occur at the dendrite-cellular transition velocity. A scaling law is shown to exist between the secondary dendrite arm spacing, λ2, near the tip and the dendrite tip radius, p, which is λ2/ρ = 2.2 ± 0.3. Experimental results on ρ have been found to agree with the theoretical model based on the marginal stability criterion.

252 citations



Journal ArticleDOI
TL;DR: In this paper, a series of Ag-Cu alloys between 1 wt pct Cu and the eutectic composition (28.1 wtpct Cu) at speeds between 1.5 and 400 cm per second were used for beam solidification.
Abstract: Electron beam solidification passes have been performed on a series of Ag-Cu alloys between 1 wt pct Cu and the eutectic composition (28.1 wt pct Cu) at speeds between 1.5 and 400 cm per second. At low growth rates conventional dendritic or eutectic structures are obtained. The maximum growth rate of eutectic structure is 2.5 cm per second. At high growth rates microsegregation-free single phase structures are obtained for all compositions. The velocity required to produce this structure increases with composition for dilute alloys and agrees with the theory of absolute stability of a planar liquid-solid interface with equilibrium partitioning. For alloys between 15 and 28 wt pct Cu, the velocity required to produce the microsegregation-free extended solid solution decreases with composition and is related to nonequilibrium trapping of solute at the liquid solid interface. At intermediate growth rates for alloys with 9 wt pct Cu or greater, a structure consisting of alternating bands of cellular and cell-free material is obtained. The bands form approximately parallel to the local interface.

187 citations


Journal ArticleDOI
TL;DR: In this paper, the authors examined conditions for the formation of macroscopic segregation channels in the ammonium chloride-water and lead-tin systems, using base-chilled molds and concluded that channels originate at the growth front, rather than within the dendritic array.
Abstract: Conditions for the formation of macroscopic segregation channels have been examined in the ammonium chloride-water and lead-tin systems, using base chilled molds. Such channels develop when the rejected solute is less dense than the solvent and are therefore a result of density inversion, but slow (≺5 rpm) rates of mold rotation, about axes inclined to the vertical by 20 deg to 30 deg, throughout the time of solidification, effectively prevent the formation or propagation of these channels. Artificially created channels or those momentarily blocked fail to continue and are overgrown, but channels can be initiated by drawing liquid upward from close to the growth front in fine capillaries. Examination of these effects leads to the conclusion that channels originate at the growth front, rather than within the dendritic array, and that their formation is necessarily preceded by a liquid perturbation from the less dense boundary layer into the supernatant, quiescent bulk liquid. Intermittent ‘solute fingers’ are then fed by dendritic entrainment to produce stable convective plumes and concomitant channels. It is considered that the effects of mold precession are primarily caused by translation of bulk liquid across the dendritic growth front, shearing off convective perturbations from the boundary layer before they have time to develop. The nature of the liquid movements is discussed and shown to be a function of the mold dimensions. The inclination of the gravitational vector within the solid-liquid, dendritic array is considered to be of secondary importance to the formation or prevention of channels.

180 citations


Journal ArticleDOI
TL;DR: The effect of retrogression and reaging on the microstructure of Al-7075 in the T651 temper, both in the matrix and on grain boundaries, was studied using transmission electron microscopy as discussed by the authors.
Abstract: The effect of the retrogression and reaging treatments (RRA) on the microstructure of Al-7075 in the T651 temper, both in the matrix and on grain boundaries, was studied using transmission electron microscopy. The processes occurring in the matrix during the retrogression treatment are principally the dissolution of small particles of the η’ transition phase, transformation to η of the larger particles of η’, coarsening of the three commonly observed variants of the η phase precipitates (η1, η2, and η4), and precipitation of new η phase particles, particularly the η1 variant. The main process occurring during the reaging treatment is either growth of partially dissolved η’ particles or precipitation of the η’ phase. These lead to a microstructure containing many fine η’ precipitates and some larger η1 and η2 particles with a smaller amount of coarse η4 particles, resulting in a broad particle size distribution. The high strength of the 7075 alloy in the RRA temper is believed to arise from the relatively high overall concentration of particles in this dispersion. The retrogression treatment produces rapid initial coarsening of the grain boundary particles, which are primarily η phase precipitates, resulting in an increase in their volume per unit grain boundary area,V A . The beneficial effect of the RRA treatment on the susceptibility of 7075-T651 to SCC is believed to be due, at least partially, to the increased value ofVA produced by the RRA treatment.

178 citations


Journal ArticleDOI
TL;DR: In this paper, the analytical electron microscope (AEM) was used to examine the microstructure of type 316LN stainless steel alloys which had been annealed for 50 to 300 hours in the temperature range 600 to 700 °C.
Abstract: The analytical electron microscope (AEM) was used to examine the microstructure of type 316LN stainless steel alloys which had been annealed for 50 to 300 hours in the temperature range 600 to 700 °C. The M23C6 carbide chemistry and distribution are described as a function of heat treatment.X-ray spectroscopy in the AEM revealed significant chromium depletion at grain boundaries in the vicinity of carbides for samples aged at 50 and 100 hours at 650 °C and 100 and 300 hours at 700 °C, with lower grain boundary chromium values observed at 650 °C than at 700 °C. The width of the chromium depleted zone normal to the grain boundaries increased with increasing annealing time and/or temperature. Measurements of chromium concentration along the grain boundaries away from a carbide were made after aging at 700 °C for 100 hours, and the chromium level rose steadily until the bulk value was reached at a distance of ~3μm from the carbide. The width of the chromium depleted zone normal to the boundaries in the same sample was an order of magnitude less. Some molybdenum depletion was also found at the grain boundaries, and the Mo-depletion profiles were in form and extent similar to the chromium results. Simple thermodynamic models were used to calculate the equilibrium value of chromium at the carbide-matrix interface, and the chromium distribution along and normal to the grain boundaries. The results of these models agreed well with the AEM results, and the agreement can be improved by considering the effect of electron probe configuration on the AEM measurements. The calculated thermodynamic data and the AEM results were related to the corrosion behavior of the alloys. The occurrence of severe asymmetries in some concentration profiles normal to the grain boundaries, which increased with increasing annealing temperature or time, was shown to be due to boundary movement during the discontinuous precipitation of M23C6 carbides.

177 citations


Journal ArticleDOI
TL;DR: The characteristic fatigue behavior of carburized and quenched steel with internal oxides and non-martensitic microstructure near the surface was presented through rotating bending fatigue test as discussed by the authors.
Abstract: The characteristic fatigue behavior of carburized and quenched steel with internal oxides and non-martensitic microstructure near the surface was presented through rotating bending fatigue test. The S-N diagram revealed two knees and the specimens continued to fail over 107 stress cycles. The fatigue limit could not be obtained even at 108 stress cycles. The test results were compared to those for the specimens without surface structure anomalies to show the effect of structure anomalies on the fatigue behavior of carburized steel.

Journal ArticleDOI
TL;DR: The tensile strength of electrodeposited layered composites of the nominal overall composition 90 pct Ni-10 pct Cu is shown to increase sharply to the 1300 MPa range as the thickness of the Cu layers is decreased below ~0.4 ώm.
Abstract: The tensile strength of electrodeposited layered composites of the nominal overall composition 90 pct Ni-10 pct Cu is shown to increase sharply to the 1300 MPa range as the thickness of the Cu layers is decreased below ~0.4 ώm. This tensile strength value is almost a factor of three greater than that measured for Ni itself, and more than a factor of two greater than the handbook value for Monel 400.

Journal ArticleDOI
TL;DR: In this paper, a fast algorithm for solving the multiparticle diffusion problem (MDP) is described, permitting simulation of coarsening dynamics by cyclic time-stepping and updating the diffusion solution for large random particle arrays.
Abstract: Phase coarsening, also termed Ostwald ripening, is generally thought to be a slow, diffusion-controlled process which occurs subsequent to phase separation under extremely small under- or over-saturation levels. The theory due to Lifshitz, Slyozov, and Wagner (LSW), which predicts the coarsening kinetics and the particle distribution function, are applicable todilute systems only, in which particle-particle interactions are unimportant. Most liquid phase sintered systems, however, have large enough volume fractions of the dispersed phase to violate the essential assumptions of LSW theory. Recent progress will be described on simulating Ostwald ripening in randomly dispersed, high volume fraction systems. A fast algorithm for solving the multiparticle diffusion problem (MDP) will be described, permitting simulation of coarsening dynamics by cyclic time-stepping and updating the diffusion solution for large random particle arrays. The rate constants, controlling the growth of the average particle, and the particle distribution functions were obtained by numerical simulations up to a volume fraction of 0.55. A new statistical mean field theory has now been developed which reproduces the MDP simulation data accurately, and finally makes clear how the linear mean-field approximations employed by LSW theory must be modified to describe real systems. The predictions of the mean field are found to compare favorably with experimental measurements made over a wide range of volume fraction solid of the kinetics of Ostwald ripening in liquid phase sintered Fe-Cu alloys. The new theory provides a comprehensive approach to understanding microstructural coarsening in liquid phase sintered systems.

Journal ArticleDOI
TL;DR: In this paper, the Gibbs energies for the binary alloy systems Ag-Cu and Cd-Zn have been modeled for the area of rapid solidification, and the phase equilibria and chemical spinodals have been calculated.
Abstract: Gibbs energies have been modeled for the binary alloy systems Ag-Cu and Cd-Zn, simple eutectic systems of interest in the area of rapid solidification. Parameters of the thermodynamic functions are derived from phase diagram and thermochemical data. Metastable phase equilibria have then been calculated, as well as the chemical spinodals and the locus of compositions and temperatures where liquid and solid have equal Gibbs energies (T 0).

Journal ArticleDOI
TL;DR: In this article, the authors investigated the character and mechanism of two-way shape memory in Cu-Zn-Al alloys by means of closely controlled thermomechanical cycling and careful measurement of the progressive effect of the particular training routine, as well as by correlary studies of submicrostructural evolution as training proceeds.
Abstract: The character and mechanism of two-way shape memory in Cu-Zn-Al alloys is investigated by means of closely controlled thermomechanical cycling and careful measurement of the progressive effect of the particular “training” routine, as well as by correlary studies of submicrostructural evolution as training proceeds. The results establish the quantitative relationship between the cyclic training routine and the ability of the sample to exhibit two-way shape memory. The variation of numerous training parameters with cycling is presented and interpreted. Microscopic studies indicate that as two-way shape memory training proceeds, specific physical features develop in the parent phase submicrostructure, particularly dislocation tangles and “vestigial” martensite markings; these assist in the nucleation and growth of a preferred martensite plate arrangement during cooling.


Journal ArticleDOI
TL;DR: In this article, the available information on the phase diagram is reviewed and theoretical calculations are made to establish the ternary Al-Ti-B phase diagram, which cannot explain the important effect of boron.
Abstract: A number of researchers have suggested that there may be phase relationships in the Al-Ti-B system, which can be used to explain why boron improves the grain refining ability of aluminum-titanium master alloys. In this paper, the available information on the phase diagram is reviewed and theoretical calculations are made to establish the ternary Al-Ti-B phase diagram. The phase diagram cannot explain the important effect of boron. It appears to be necessary to seek another explanation.

Journal ArticleDOI
TL;DR: In this article, the role of microstructure and environment in influencing ultra-low fatigue crack propagation rates has been investigated in 7075 aluminum alloy heat-treated to underaged, peak-aged, and overaged conditions and tested over a range of load ratios.
Abstract: The role of microstructure and environment in influencing ultra-low fatigue crack propagation rates has been investigated in 7075 aluminum alloy heat-treated to underaged, peak-aged, and overaged conditions and tested over a range of load ratios. Threshold stress intensity range, ΔK0, values were found to decrease monotonically with increasing load ratio for all three heat treatments fatigue tested in 95 pct relative humidity air, with ΔK 0 decreasing at all load ratios with increased extent of aging. Comparison of the near-threshold fatigue behavior obtained in humid air with the data forvacuo, however, showed that the presence of moisture leads to a larger reduction in ΔK0 for the underaged microstructure than the overaged condition, at all load ratios. An examination of the nature of crack morphology and scanning Auger/SIMS analyses of near-threshold fracture surfaces revealed that although the crack path in the underaged structure was highly serrated and nonlinear, crack face oxidation products were much thicker in the overaged condition. The apparent differences in slow fatigue crack growth resistance of the three aging conditions are ascribed to a complex interaction among three mechanisms: the embrittling effect of moisture resulting in conventional corrosion fatigue processes, the role of microstructure and slip mode in inducing crack deflection, and crack closure arising from a combination of environmental and microstructural contributions.

Journal ArticleDOI
TL;DR: In this article, the authors characterized the dislocation structures in the ferrite of a C-Mn-Si dual-phase steel intercritically annealed at 810°C by transmission electron microscopy.
Abstract: Dislocation structures in the ferrite of a C-Mn-Si dual-phase steel intercritically annealed at 810°C were characterized at various tensile strains by transmission electron microscopy At strains which corresponded to the second stage on a Jaoul-Crussard plot of strain hardening behavior, the dislocation density in the ferrite is inhomogeneous, with a higher density near the martensite. The third stage on a Jaoul-Crussard plot corresponds to the presence of a well-developed dislocation cell structure in the ferrite. The average cell size during this stage is smaller than the minimum size reported for deformed iron, and the cell size was inhomogeneous, with a smaller cell size near the martensite.

Journal ArticleDOI
TL;DR: In this article, the relationship between interlamellar spacing and temperature for isothermal growth conditions and between translation velocity and spacing for forced-velocity growth conditions is reviewed for a range of steels and nonferrous alloys.
Abstract: After defining interlamellar spacing the various optical and electron optical methods for measuring spacing are outlined. It is clear for both isothermal and forced velocity transformation conditions that pearlite can grow at a constant velocity with a range of true spacings. The minimum true spacing and mean true spacing are not related by a constant factor, but this may vary from system to system and with temperature in a given system. The relationship between interlamellar spacing and temperature for isothermal growth conditions and between translation velocity and spacing for forced-velocity growth conditions is reviewed for a range of steels and nonferrous alloys. It is seen that the velocity-spacing relationship for the two modes of transformation is the same. For isothermal transformation a linear relationship between reciprocal spacing and temperature is generally observed, but for steels containing alloy additions there is little evidence of the predicted inflexion corresponding to a temperature at which alloy partitioning at the transformation front ceases. The lack of precise interfacial energy data makes it difficult to determine reliably the relationship between measured and critical spacings, although it seems likely to be in accord with the maximum growth rate or maximum rate of entropy production optimization criteria.

Journal ArticleDOI
TL;DR: In this article, the authors investigated the influence of a preferred crystallographic orientation on mechanical properties of textured Ti-6Al-4V material and found that high cycle fatigue and fatigue crack growth were performed in vacuum, laboratory air, and a 3.5 pct NaCl solution.
Abstract: Tensile properties, high cycle fatigue strength, and fatigue crack propagation behavior were evaluated on highly textured Ti-6Al-4V material to investigate the influence of a preferred crystallographic orientation on mechanical properties. Thermomechanical treatments were used to develop three different textures: a basal, basal/transverse, and transverse type, all of which exhibited the same homogeneously equiaxed microstructure. The Young’s modulus was found to vary between 107 and 126 GNm-2, and yield strength changed from 1055 to 1170 MNm-2. Ductility was only slightly affected by texture. High cycle fatigue and fatigue crack growth measurements were performed in vacuum, laboratory air, and a 3.5 pct NaCl solution. It is shown that laboratory air can be regarded as a quite corrosive environment. In vacuum the highest fatigue strength values were measured whenever loads were perpendicular to basal planes. However, these conditions had the highest susceptibilities to air and 3.5 pct NaCl solution environments. Nearly no influence of texture on fatigue crack propagation was found in vacuum, but in a corrosive environment crack growth parallel to (0002)-planes was much faster than perpendicular to these planes. To explain the corrosive effect on the fatigue properties of the textured material hydrogen is thought to play a key role.

Journal ArticleDOI
TL;DR: In this paper, the effects of slip character and grain size on the intrinsic material and extrinsic closure contributions to fatigue crack growth resistance have been studied for a 7475 aluminum alloy.
Abstract: The effects of slip character and grain size on the intrinsic material and extrinsic closure contributions to fatigue crack growth resistance have been studied for a 7475 aluminum alloy. The alloy was tested in the underaged and overaged conditions with grain sizes of 18 μm and 80 μm. The fracture surface exhibited increased irregularity and planar facet formation with increased grain size, underaging, and tests in vacuum. These changes were accompanied by an increased resistance to fatigue crack growth. In air the 18 μm grain size overaged material exhibited relatively poor resistance to fatigue crack growth compared with other microstructural variants, and this was associated with a lower stress intensity for closure. All materials exhibited a marked improvement in fatigue crack growth resistance when tested in vacuum, with the most significant difference being ˜1000× at a ΔK of 10 MPa m1/2 for the 80 μm grain size underaged alloy. This improvement could not be accounted for by either an increase in closure or increased crack deflection and is most likely due to increased slip reversibility in the vacuum environment. The intrinsic resistance of the alloy to fatigue crack growth was microstructurally dependent in vacuum, with large grains and planar slip providing the better fatigue performance.

Journal ArticleDOI
TL;DR: In this paper, the stable and metastable equilibria of the Fe-Cu system were derived using the thermodynamic equations derived from equilibrium data, and the calculated metastable miscibility gap of the liquid phase also agrees with the experimental data.
Abstract: Thermodynamic and phase diagram data in the Fe-Cu system are evaluated. For the liquid and fcc phases, the Margules-type of equations is used. For the bcc phase, the same type of equation is used to describe the non-magnetic contribution to the Gibbs energy. In addition, a magnetic term is included. Using the thermodynamic equations derived from equilibrium data, the stable and metastable equilibria of this system are calculated. Agreement between the calculated and experimental phase diagram is good except for temperatures higher than 1720 K. For these temperatures, the calculated liquidus tends to be higher. The possibility of supercooling which may account for some of the lower temperatures measured should not be excluded. The calculated metastable miscibility gap of the liquid phase also agrees with the experimental data.

Journal ArticleDOI
TL;DR: In this paper, it was shown that the structural modification in furnace cooled ingots, as in directionally grown samples, is primarily caused by modified growth of silicon, and the mechanism(s) for such modification are briefly discussed.
Abstract: Alloys in the range 0 to 24 wt pct Si have been examined by careful thermal analysis, macroand microscopical study with modification by a mixed alkali fluoride flux, and by separate additions of sodium, potassium, lithium, and strontium. Sodium and strontium exert similar effects and potassium and lithium differing and minor effects; with a mixed alkali flux the influence of sodium is dominant. The normal and modified eutectic arrests in the presence of primary aluminum both show comparable supercoolings and recalescence behavior relative to the horizontal growth temperatures, but this is not observed in hypereutectic alloys containing primary silicon. Both normal and modified eutectics grow radially inward from crucible walls, but the details of the growth fronts are very different: there is no nucleation from the bulk liquid in modified alloys and it is uncertain if this occurs in normal alloys. It is concluded that the structural modification in furnace cooled ingots, as in directionally grown samples, is primarily caused by modified growth of silicon. The mechanism(s) for such modification are briefly discussed.

Journal ArticleDOI
TL;DR: In this paper, the sintering atmosphere role in stabilizing detrimental residual pore structures was examined on alloys containing 93, 95, or 97 wt pct W with Ni:Fe ratios of 7:3.
Abstract: Residual porosity has a strong negative effect on the ductility of tungsten-nickel-iron heavy metals. This investigation examines the sintering atmosphere role in stabilizing detrimental residual pore structures. Two types of experiments are reported on alloys containing 93, 95, or 97 wt pct W with Ni:Fe ratios of 7:3. The negative effect of prolonged sintering is attributed to pore coarsening involving trapped gas in the pores. Calculated pore growth rates for hydrogen filled pores suggest that pore coarsening involves both ripening and coalescence driven by tungsten grain growth. The effect of the sintering atmosphere is analyzed for final stage pore elimination. It is demonstrated that a change in sintering atmosphere from hydrogen to argon midway through the sintering cycle can aid pore degassing and increase the sintered ductility and strength.

Journal ArticleDOI
TL;DR: The effect of nitrogen implantation on the wear behavior of a range of metals has been studied in this paper, showing that the best improvements in wear performance occur only when implantation produces a change in the dominant mode of wear and the formation of surface oxide layers during the wear process is often associated with these changes in wear behavior.
Abstract: The effect of nitrogen implantation on the wear behavior of a range of metals has been studied. Hardness measurements and pin-on-disc wear tests which characterize specifically the properties of the thin implanted layer have been carried out. It is shown that large improvements in wear resistance can be obtained by nitrogen implantation and that a loose correlation exists between reduced wear and increased surface hardness. However, it is demonstrated, on the basis of detailed examination of the wear tracks, that the best improvements in wear performance occur only when implantation produces a change in the dominant mode of wear. It is further shown that the formation of surface oxide layers during the wear process is often associated with these changes in wear behavior. The wear mechanisms, and the changes thereof, are described and discussed in detail for Ti-6Al-4V, hard chromium electroplate, and a range of ferrous alloys.

Journal ArticleDOI
Abstract: In the past two decades, researchers have repeatedly shown that Wagner's classical parabolic scaling theory has a limited temperature regime of applicability. In particular, at intermediate temperatures-generally the highest permitted for long-lived engineering systems-parabolic oxidation rates are higher and activation energies are lower than those values extrapolated from higher temperatures where lattice-diffusion-limited growth occurs. Short-circuit cation diffusion via scale grain boundaries and dislocations supports oxidation in the intermediate temperature regime, and scale growth at the scale/gas interface takes place at ledges provided by screw dislocations intersecting the metal. In situ observations of scale growth in a hot-stage environmental scanning electron microscope have provided insight into scale growth mechanisms and the formation of nonplanar oxidation products: whiskers, pyramids, and pits. Specific reference is made to the evolution of oxidation product morphologies for copper, nickel, iron, and chromium.

Journal ArticleDOI
TL;DR: In this article, the effect of grain refinement on the mechanical and the strain-memory properties of β-CuAlNi alloys was studied and it was found that the strain memory and pseudoelastic recovery properties were not affected significantly by decreasing grain size and the presence of second phase particles.
Abstract: A study has been made of the effect of grain refinement on the mechanical and the strain-memory properties of β-CuAlNi alloys. Addition of 0.5 pct Ti to CuAINi decreased the grain growth rate of the beta phase significantly. This appeared to be due mainly to the small fraction of the titanium in solid solution in the β-CuAlNi. By controlled annealing, a grain size as small as 15 μrn could be obtained, though some second phase γ2 was present due to incomplete precipitate dissolution. Stress-strain curves for most specimens in both the strain-memory and pseudoelastic states showed a three-stage characteristic with a region of lower slope between two regions of higher modulus. It was found that σ1, (the transition stress between stages 1 and 2) and (dσ/de@#@) (the slope of stage 2) increased with grain size according to a (g.s.)-1/2 relationship. The ultimate tensile strength and strain to fracture also followed a similar Hall-Petch relationship. The alloys showed higher strength in the martensitic state than in the pseudoelastic one. The presence of second-phase particles had no significant effect on the mechanical properties and martensite deformation behavior. Fracture strains as high as 7 pct were obtained at the finest grain sizes. It was found that the strain-memory and pseudoelastic recovery properties were not affected significantly by decreasing grain size and the presence of second phase particles. Maximum recovery strains of 6.5 pct were obtained in fine grain samples.

Journal ArticleDOI
TL;DR: In this paper, the available data on the solubility of Niobium carbide and niobium carbonitride in plain carbon and alloyed austenite has been analyzed via dilute solution thermodynamics with a view to establishing a consistent set of interaction parameters.
Abstract: The available data on the solubility of niobium carbide and niobium carbonitride in plain carbon and alloyed austenite has been analyzedvia dilute solution thermodynamics with a view to establishing a consistent set of interaction parameters for predicting austenite + niobium carbonitride equilibria. The computation algorithm includes the prediction of phase mass fractions as a function of alloy composition and temperature between 900° and 1300 °C (tie lines). Analogous ferrite equilibria are included.

Journal ArticleDOI
TL;DR: In this article, individual powder particles of a droplet-processed and rapidly solidified 303 stainless steel are characterized in terms of microstructure and composition variations within the solidification structure using scanning transmission electron microscopy (STEM).
Abstract: Individual powder particles of a droplet-processed and rapidly solidified 303 stainless steel are characterized in terms of microstructure and composition variations within the solidification structure using scanning transmission electron microscopy (STEM). Fcc is found to be the crystallization phase in powder particles larger than about 70 micron diameter, and bcc is the crystallization phase in the smaller powder particles. An important difference in partitioning behavior between these two crystal structures of this alloy is found in that solute elements are more completely trapped in the bcc structures. Massive solidification of bcc structures is found to produce supersaturated solid solutions which are retained to ambient temperatures in the smallest powder particles. Calculated liquid-to-crystal nucleation temperatures for fcc and bcc show a tendency for bcc nucleation at the large liquid supercoolings which are likely to occur in smaller droplets. The importance of small droplet sizes in rapid solidification processes is stressed.