Showing papers on "Powder metallurgy published in 1993"
TL;DR: The design of an aluminum alloy having good strength while maintaining a high resistance to fracture is discussed in this article, where the desired microstructure consists of a small volume fraction of an ultra-fine dispersion of hard particles.
Abstract: The design of an aluminum alloy having good strength while maintaining a high resistance to fracture is discussed. Theory suggests that the desired microstructure consists of a small volume fraction of an ultra-fine dispersion of hard particles. In addition to conventional heat treatments, dispersion hardened aluminum alloys have been recently produced by rapid solidification or mechanically alloying and powder metallurgy consolidation. Alloys which can serve as models for mechanistic studies of nucleation of non-coherent phases as well as the basis for a new class of engineering aluminum alloys are identified.
184 citations
TL;DR: In this paper, the phase selection in the final state during mechanical alloying of powder blends and milling of intermetallic compounds is mainly determined by the energetic destabilization of the competing phases caused by the milling process.
Abstract: Elemental Ti–Al powder blends were mechanically alloyed in order to study phase formation during the alloying process. In addition, the stability of intermetallic phases upon milling was investigated separately in order to determine the origins of phase selection during the milling process. It was found that by mechanical alloying of powder blends, as well as by ball milling of Ti-aluminides for long milling times, the same metastable phases were formed for corresponding compositions, i.e., the hep solid solution for Al concentrations up to 60 at. % and the fcc solid solution for 75 at. % Al. X-ray diffraction (XRD) analyses indicated that the process of mechanical alloying occurred via the diffusion of Al into Ti. By lowering the milling intensity, a two-phase mixture of the hcp solid solution and the amorphous phase was observed for Ti50Al50 and confirmed by transmission electron microscopy (TEM). The results show that phase selection in the final state during mechanical alloying of Ti–Al powder blends and milling of intermetallic compounds is mainly determined by the energetic destabilization of the competing phases caused by the milling process. The destabilization is most pronounced in the case of intermetallic compounds due to the decrease in long-range order upon milling. For the final milling stage, phase formation can be predicted by considering the relative stabilities of the respective phases calculated by the CALPHAD method using the available thermodynamic data for the Ti–Al system.
85 citations
TL;DR: In this paper, it has been demonstrated that using high-purity gas-atomized Cu powders blended with commercial high purity Fe powders and controlling the time/temperature processing conditions within specific limits, it is possible to produce Cu-Fe deformation-processed alloys with strength/conductivity properties matching those of Cu-Nb, Cu-Ta, and Cu-Cr alloys.
Abstract: Powder metallurgical techniques have been employed to prepare the precursor billets in the preparation of Cu-15 vol pct Fe alloys by deformation processing. It has been demonstrated that by (1) using high-purity gas-atomized Cu powders blended with commercial high-purity Fe powders and (2) controlling the time/temperature processing conditions within specific limits, it is possible to produce Cu-Fe deformation-processed alloys with strength/conductivity properties matching those of Cu-Nb, Cu-Ta, and Cu-Cr alloys. These properties are significantly superior to the best commercial alloys.
63 citations
TL;DR: In this paper, the reaction sintering of Ni3Al was performed with uniaxial compressive stresses ranging from 0 to 120 MPa, using elemental powders with the stoichiometric composition preheated to 620°C in a vacuum of 7 × 10−3 Pa.
Abstract: Reactive sintering of Ni3Al was performed with uniaxial compressive stresses ranging from 0 to 120 MPa, using elemental powders with the stoichiometric composition preheated to 620°C in a vacuum of 7 × 10−3 Pa. It was shown that both compressive stress and heat flow strongly affected the reaction process and, hence, the structure and density of reaction-sintered products. Without compression, reaction-sintered products had a relative density up to 98% and were mainly composed of Ni3Al with uniformly distributed fine pores and large shrinkage cavities located in the center. Green density has little effect on densification. There was no effect from a compressive stress when it was applied to specimens after the onset of the self-sustaining reaction. When a green compact self-ignited under a pre-loaded compressive stress (50 MPa), a highly densified product (relative density as much as 99.3%) was obtained. In addition, the product, which was composed of Ni3Al, NiAl and Ni, did not contain large shrinkage cavities. A detailed description of the entire reaction process is given on the basis of electron probe microanalyses and thermodynamic considerations.
59 citations
TL;DR: In this paper, the wear behavior of sintered high-speed steel-type particulate composites is investigated, which depends significantly on microstructural parameters such as those of the metallic matrix and primary carbides, but also on some additional parameters related to powder metallurgy processing.
58 citations
TL;DR: In this paper, the as-milled condition of a mixture of elemental Al and Ti powders was characterized in the hot consolidation process, and the existence of an amorphous phase in the powders considerably facilitates their hot consolidation.
Abstract: Mechanically alloyed mixtures of elemental Al and Ti powders have been characterized in the as-milled condition, as well as after hot consolidation. Ball-milling in an argon atmosphere first induces alloying of the elements, followed by the formation of a certain amount of an amorphous-like phase. This amount increases as the equiatomic composition is approached. However, milling in nonsealed mills usually leads to the production of some form of titanium nitride, particularly in Ti-rich mixtures. In the consolidated products obtained from long-milled powders, intermetallic compounds were found to be the predominant phases. The existence of an amorphous phase in the as-milled powders considerably facilitates their hot consolidation.
58 citations
55 citations
TL;DR: In this paper, a mechanical alloying process for a mixture of elemental Fe and Cr powders with the Cr compositions 20-70% was investigated through the measurements of x-ray diffraction, magnetization, and 57Fe Mossbauer spectrum.
Abstract: A mechanical alloying process for a mixture of elemental Fe and Cr powders with the Cr compositions 20–70 at. % was investigated through the measurements of x‐ray diffraction, magnetization, and 57Fe Mossbauer spectrum. We show that magnetic studies provide more detailed information about the alloying process occurring during ball milling than the conventional diffraction techniques in this particular system. A final product after ball milling was identified as a high‐temperature phase of the α solid solution, regardless of compositions studied. Powders subjected to milling in Ar gas atmosphere for 200 h were further ball milled in N2 atmosphere. The presence of N2 gas has caused a partial amorphization. The amorphous phase thus produced is found to be paramagnetic at room temperature.
53 citations
TL;DR: In this article, a closed processing system for consolidation of amorphous alloy powders has been developed in order to suppress the formation of oxide layers in the interparticle boundaries and to avoid the dangers of explosions.
Abstract: Recently, Al-based and Mg-based amorphous alloys have attracted much attention as light weight structural materials because of their high tensile strength and good ductility. The development of a process for consolidating amorphous alloy powders to amorphous alloy bulks is essential to the structural application of these new alloys. The consolidation of Fe-based and Co-based amorphous alloy powders has been carried out by a number of techniques, such as explosive compaction, dynamic compaction, static high pressure compaction, extrusion and rolling. More recently, it has been reported that mechanical properties of amorphous alloy compacts deteriorate in the presence of oxide films in interparticle boundaries. In the present study, a closed processing system for consolidation of amorphous alloy powders has been developed in order to suppress the formation of oxide layers in the interparticle boundaries and to avoid the dangers of explosions. In this system, a series of processing from the powder production to the extrusion of them can be performed in a vacuum or an inert gas atmosphere. The compressive strength of Al[sub 85]Ni[sub 10]Mm[sub 5] (Mm: mischmetal) amorphous compacts prepared by this process is examined and compared with that prepared by the ordinary one.
52 citations
TL;DR: In this article, the homogenization process that occurs during liquid-phase sintering of WC-Co carbide structures has been investigated and a post-hot isostatic pressing was used to improve the final density of some of these samples.
Abstract: WC-Co cemented carbides are widely used as machining tools, mining tools, and wear-resistant parts. Multilayer graded structures with Co content ranging from 10 to 30 wt.% from one side of the structure to the other have been prepared by using either solid-state or liquid-phase sintering. Post-hot isostatic pressing was used to improve the final density of some of these samples. Special attention was given to the homogenization process that occurs during liquid-phase sintering of the graded structures.
TL;DR: In this article, the interaction between SiC and Ti powder at 1073 −1523 K was investigated employing a combination of x-ray diffraction, scanning electron microscopy with EDS, Auger spectroscopy, and transmission electron microscope.
Abstract: The interaction between SiC and Ti powder at 1073–1523 K was investigated employing a combination of x-ray diffraction, scanning electron microscopy with EDS, Auger spectroscopy, and transmission electron microscopy. As a result of the interaction, a triple-layer reaction zone was formed. The most important part of the reaction zone was a mixed TiC–Ti5Si3(C) layer. Thin TiC sublayers were formed on both the inner and the outer sides of the mixed reaction layer. The reaction zone was found to grow by a parabolic law with the kinetic constant, k = 1.3 × 10−3 exp (-21800/t) cm2/s. The growth process of the SiC/Ti reaction zone was assumed to be controlled by diffusion of all three components of the system: Ti, Si, and C. Thin reaction layers (<5 μm) obtained after short exposures at relatively low temperatures formed coatings on the SiC surface; thicker reaction layers spalled off the ceramic surface. Experiments with the samples partially immersed into the metal powder showed that interaction between SiC and Ti was very sensitive to the environment.
TL;DR: In this paper, the adiabatic chemical reaction behavior of shock-compressed Ni-Al powder mixtures of varying morphology and different volumetric distributions has been investigated by microstructural and differential thermal analysis (DTA) to understand the mechanistic changes responsible for chemical reactions occurring during shock treatment.
Abstract: The adiabatic chemical reaction behaviour of shock-compressed Ni-Al powder mixtures of varying morphology and different volumetric distributions has been investigated by microstructural and differential thermal analysis (DTA) to understand the mechanistic changes responsible for chemical reactions occurring during shock treatment. Mechanically mixed Ni-Al powders undergo exothermic chemical reactions at temperatures close to the melt-temperature of AI. In contrast, shock-treated Ni-Al powder mixtures exhibit a “pre-initiation” exothermic event, before the main exothermic reaction. Different forms (reaction start and peak temperatures) of the preinitiation exotherm are observed depending on the degree of macroscopic mixing, contact intimacy and activation, accomplished during shock compression of the powder mixtures of different morphology and volumetric distribution, all shock-treated under the same conditions. Mixtures containing equimolar volumetric distribution of powders of more irregular (flaky) morphologies undergo a significant extent of configuration change during shock-compression, resulting in the formation of an activated, intimately mixed and close-packed state. In such a state, chemical reaction is readily initiated by external thermal stimulation, such as heating during DTA. In fact, a greater degree of configuration change, activation and more intense mixing occurring during shock-compression can even lead to reaction initiation and completion in the shock duration itself.
TL;DR: In this paper, the role of particle/matrix interface reinforcement and the matrix on the crack initiation and propagation behavior of SiC composite composites was evaluated using tensile and instrumented impact tests supplemented by optical and scanning-electron fractography and oblique section metallography.
Abstract: Magnesium and magnesium-alloy composites reinforced with SiC particles have been fabricated by a powder metallurgy route involving low-energy mechanical alloying as well as dry mixing, and by the ingot metallurgy route based on a fluxless melting technology. Tensile and instrumented impact tests supplemented by optical and scanning-electron fractography and oblique-section metallography have been used to assess the role of particle/matrix interface reinforcement and the matrix on the crack initiation and propagation behavior. Of the routes employed, low-energy mechanical alloying provided the best property combination.
28 Feb 1993-Materials Science and Engineering A-structural Materials Properties Microstructure and Processing
TL;DR: In this article, microstructural differences (particle size and chemistry, grain and subgrain sizes, dislocation density) have been found which affect the observed mechanical properties of the material.
Abstract: In two different commercial Mo-base alloys (TZM), produced by vacuum melting and by powder metallurgy respectively, microstructural differences (particle size and chemistry, grain and subgrain sizes, dislocation density) have been found which affect the observed mechanical properties of the material. The compression-creep properties at 1423 K show a negligibly small creep rate at a stress of approximately 200 MPa. Trapping of dislocations by particles is proposed to be the controlling deformation mechanism during creep. The microstructure and fatigue properties of TZM welds were also investigated. Friction welds showed the best mechanical properties. Fatigue measurements in load control at room temperature and 1123 K show that the endurance limit of the vacuum-melted alloy is higher than that of the powder-metallurgically processed alloy.
TL;DR: In this article, the authors combine short-time high-temperature exposure with either high-pressure application or plasma discharge to minimize the microstructural changes during compaction of metastable powder materials.
Abstract: Nonconventional consolidation methods have been developed to minimize the microstructural changes during compaction of metastable powder materials. The methods presented in this article combine short-time high-temperature exposure with either high-pressure application or plasma discharge. The current understanding of pressure sintering was used to identify the densification mechanisms for each process. When high pressure is applied, the contribution of plastic yielding to densification is augmented such that lower temperatures and shorter times than in conventional processes are conceivable for the overall compaction process. Plasma discharge results in particle surface activation that enhances particle sinterability and reduces high-temperature exposure. To illustrate the specifics of these new nonconventional consolidation methods, examples of short-time densification of difficult-to-sinter materials are presented.
TL;DR: In this paper, the Supersolidus Sintering of high speed steels is described as a special case of Molybdenum-based alloys, and the authors present a detailed discussion.
Abstract: (1993). Supersolidus Sintering of High Speed Steels: Part 1: Sintering of Molybdenum Based Alloys. Powder Metallurgy: Vol. 36, No. 3, pp. 213-219.
TL;DR: In this article, a computational cell is constructed with interlinked phases, consisting of a core of low thermal expansion material (tungsten) and a edge network of high thermal conductivity phase (copper).
Abstract: Thermal properties are important to several applications for powder metallurgy products. For example, liquid-phase sintered tungsten-copper composites are used in microelectronic packaging to obtain a high thermal conductivity in a low thermal expansion material. This article addresses modeling the thermal properties for composites fabricated by liquid-phase sintering. A computational cell is constructed with interlinked phases, consisting of a core of low thermal expansion material (tungsten) and a edge network of high thermal conductivity phase (copper). This structure is used to calculate the composition effects on the coefficients of thermal expansion and thermal conductivity. The results are applied to prior reports on W-Cu and used as a basis to identify several candidate high thermal conductivity systems for future development.
TL;DR: Corrosion tests were performed on solid 316 stainless steel, solid titanium, as well as porous titanium specimens of various porosities as discussed by the authors, and the results showed that titanium was more susceptible to corrosion than solid titanium.
TL;DR: In this article, a glass-metal nanocomposites involving copper and nickel, respectively, have been synthesized in bulk form by hot pressing sol-gel derived silica-metal nanoparticle composite powders.
Abstract: Glass-metal nanocomposites involving copper and nickel, respectively, have been synthesized in bulk form by hot pressing sol-gel derived silica-metal nanoparticle composite powders. The particle diameters range from 9 to 17.5 nm. The specimens exhibit the characteristic behavior of the metallic species in their nanocrystalline forms.
TL;DR: In this article, a preform of nickel powder or nickel/alumina powder blends were infiltrated with molten aluminum to produce nickel aluminides, and it was found that reducing the initial preform temperature below the melting point of aluminum results in a reduction of the rate and extent of aluminide formation, allowing, in turn, reduction of macrosegregation within the resulting infiltrated material.
Abstract: Preforms of nickel powder or nickel/alumina powder blends were infiltrated with molten aluminum to produce nickel aluminides. Application of a pressure of 3.6 and 6.9 MPa on the melt allowed infiltration of preforms with nickel powder particles between 5 and 15 μm in diameter, which could not be infiltrated under the sole action of capillary forces. By varying the initial preform temperature from 705 °C to 280 °C, the diameter of nickel powder particles from 15 to 150 μm, and the volume fraction of alumina from 0 to 34 vol pct, pressure-infiltrated samples with large variations in the extent of reaction between nickel and aluminum were pro- duced. The range of microstructure extended from fully reacted nickel aluminide to essentially unreacted nickel/aluminum samples containing low levels of final porosity. In particular, it was found that reducing the initial preform temperature below the melting point of aluminum results in a reduction of the rate and extent of aluminide formation, allowing, in turn, reduction of macrosegregation within the resulting infiltrated material.
TL;DR: In this article, a closed processing system for preparation and consolidation of amorphous alloy powders has been developed, which can be carried out under a well-controlled atmosphere in which the oxygen and moisture components less than 1 ppm.
Abstract: A closed processing system for preparation and consolidation of amorphous alloy powders has been developed. The sequent steps can be carried out under a well-controlled atmosphere in which the oxygen and moisture components less than 1 ppm. For an Al 85 Ni 5 Y 8 CO 2 (at%) amorphous alloy which has the highest tensile strength (1250 MPa) in the Al-based amorphous alloys, the crystallized bulks have been produced by extrusion of the amorphous powder. Their mechanical properties have been examined and compared with those produced by the ordinary processing
Abstract: The kinetics of the initial stage of uniaxial vacuum-hot-pressing of commercially pure spherical titanium powders are investigated for temperatures between 730 and 870-degrees-C and applied pressures from 25 to 50 MPa. Densification kinetics are found to agree with models of Arzt, Ashby and coworkers for hot isostatic pressing by power law creep. The best fit with all data is for a creep exponent of 4.9 and a creep activation energy of 208 kJ mol-1. With the addition to the titanium powder of 10 vol.% TiC powder of same size, the densification rates remain in overall agreement with the same models but become from 5 to 20 times slower. Addition of the inert ceramic particles slows hot pressing kinetics in two ways: (i) by decreasing the initial density of the compact and (ii) by reducing the densification rate at interparticle contacts. It is shown that this latter effect decreases the densification rate by a factor of at most 2 in the present experiments, hence, the former accounts for the significantly larger reductions in densification kinetics observed experimentally.
13 Apr 1993
Journal Article•
TL;DR: In this paper, the effect of various processing parameters and steps on the microstructure and mechanical properties of 2124 Al/SiC p is considered; it is shown that secondary processing such as extrusion or section rolling and a homogenization treatment, is essential if optimum properties are to be achieved.
Abstract: The achievement of uniform and reproducible properties in silicon carbide particulate-reinforced 2124 aluminum alloy matrix composites produced by the powder metallurgy route requires proper selection of the various processing parameters and steps. The effect of various processing parameters and steps on the microstructure and mechanical properties of 2124 Al/SiC p is considered; it is shown that secondary processing, such as extrusion or section rolling and a homogenization treatment, is essential if optimum properties are to be achieved
TL;DR: In this paper, the influence of the extrusion ratio on sintering behavior of cold-extruded powder mixture Ti-48A1 has been investigated and it was found that both porosity and pore size in reactively sintered specimens largely decrease with increaseing extrusion ratios.
Abstract: The influence of the extrusion ratio on sintering behavior of cold-extruded powder mixture Ti-48A1 has been investigated. Both pressureless reaction sintering and hot isostatic pressing (HIP) without encapsulation were carried out. Moreover, two-step sintering,i.e., combination of pressureless sintering and HIP, was conducted. It was found that both porosity and pore size in reactively sintered specimens largely decrease with increaseing extrusion ratio. For a given extrusion ratio, the porosity after pressureless sintering decreases with increasing temperature. Although a reduction of porosity can be reached by directly HIP specimens, the effect of applied pressure in case of combined treatments is strongly dependent on extrusion ratio. By applying an extremely high extrusion ratio of 350, material with a porosity of only 0.7 pct has been prepared by pressureless sintering and subsequent HIP without encapsulation while a reverse treatment route led to a porosity of5%. On the contrary, lower porosities were obtained for low extrusion ratios of 17 and 25 by HIP and following pressureless sintering. The effect of extrusion ratio, as well as sintering temperature, was discussed. In addition, pore coalescing, gas penetration, and swelling were considered in order to understand the effect of applying pressure.
TL;DR: In this paper, hot isostatic pressing of powder mixtures (containing differently sized components) and of composite powders is analyzed and a simple scheme for estimating radial distribution functions has been developed to make modeling of these systems practical.
Abstract: Hot isostatic pressing (HIP) of powder mixtures (containing differently sized components) and of composite powders is analyzed. Recent progress, including development of a simple scheme for estimating radial distribution functions, has made modeling of these systems practical. Experimentally, powders containing bimodal or continuous size distributions are observed to hot isostatically press to a higher density under identical processing conditions and to show large differences in the densification rate as a function of density when compared with the “monosize” powders usually assumed for modeling purposes. Modeling correctly predicts these trends and suggests that they can be partially, but not entirely, attributed to initial packing density differences. Modeling also predicts increased deformation in the smaller particles within a mixture. This effect has also been observed experimentally and is associated with microstructural changes, such as preferential recrystallization of small particles. Finally, consolidation of a composite mixture containing hard, but deformable, inclusions has been modeled for comparison with existing experimental data. Modeling results match both the densification and microstructural observations reported experimentally. Densification is retarded due to contacts between the reinforcing particles which support a significant portion of the applied pressure. In addition, “partitioning” of deformation between soft matrix and hard inclusion powders results in increased deformation of the softer material.
TL;DR: In this paper, a micromechanical model for sintering of powders is described and macroscopic constitutive equations are derived from them, based on the assumption that the surface of the open pore space is in equilibrium and that densification and creep occur by grain boundary diffusion.
Abstract: Micromechanical models for the pressing and sintering of powders are described and macroscopic constitutive equations are derived from them. The effects of cohesion and friction between powder particles on the macroscopic behaviour during compaction are explored. The micromechanical model for sintering is based on the assumption that the surface of the open pore space is in equilibrium and that densification and creep occur by grain boundary diffusion. The thus developed constitutive equations are implemented in finite element codes. A simple example is shown, in which the pressing and sintering of a hard-metal cutting tool with a wedge-type profile is simulated. The simulation correctly predicts large gradients in green density and, as a consequence, large shape distortions during sintering.