A sintering study on the β-spodumene-based glass ceramics prepared from gel-derived precursor powders with LiF additive

Beta-spodumene (Li₂O·Al₂O₃·4SiO₂, LAS) powders were prepared by a sol-gel process using Si(OC₂H₅)₄, Al(OC₄H₉)₃, and LiNO₃ as precursors and LiF as a sintering aid agent. Dilatometry, X-ray diffraction (XRD), scanning electron microscopy (SEM), scanning transmission electron microscopy (STEM), and electron diffraction (ED) were utilized to study the sintering, phase transformation, microstructure, and properties of the β-spodumene glass-ceramics prepared from the gel-derived precursor powders with and without LiF additives. For the LAS precursor powders containing no LiF, the only crystalline phase obtained was β-spodumene. For the pellets containing less than 4 wt pct LiF and sintered at 1050 °C for 5 hours the crystalline phases were β-spodumene and β-eucryptite (Li₂O·Al₂O₃·2SiO₂). When the LiF content was 5 wt pct and the sintering process was carried out at 1050 °C for 5 hours, the crystalline phases were β-spodumene, β-eucryptite (triclinic), and eucryptite (rhombohedral (hex.)) phases. With the LiF additive increased from 0.5 to 4 wt pct and sintering at 1050 °C for 5 hours, the open porosity of the sintered bodies decrease from 30 to 2.1 pct. The grains size is about to 4 to 5 μm when pellect LAS compact contains LiF 3 wt pct as sintered at 1050 °C for 5 hours. The grains size grew to 8 to 25 μm with a remarkable discontinuous grain growth for pellet LAS compact contain LiF 5 wt pct sintered at 1050 °C for 5 hours. Relative densities greater than 90 pct could be obtained for the LAS precursor powders with LiF > 2 wt pct when sintered at 1050 °C for 5 hours. The coefficient of thermal expansion of the sintered bodies decreased from 8.3 × 10⁻⁷ to 5.2 × 10⁻⁷/°C (25 °C to 900 °C) as the LiF addition increased from 0 to 5 wt pct.     

Nanocrystalline iron sintering behavior and microstructural development

Nanocrystalline (20 nm) iron powder was closed-die sintered in a hydrogen atmosphere at a stress of 10.1 MPa and at temperatures between 670 and 1270 K. The maximum densification rate was approximately 6 × 10⁻⁴ s⁻¹. Density greater than 90 pct was obtained at sintering temperatures greater than 990 K. Densification was marked microstructurally by local gradients which appeared after initial cold compaction. Oxygen content in the starting powder was high but was effectively a monolayer of surface adsorbed oxygen. Despite the reducing sintering atmosphere, oxide was present in dense specimens as a fine dispersion of order 0.1 to 1μm. The extent of oxide formation can be controlled by closed-die sintering to a stable structure of interconnected porosity followed by open-die resintering in the reducing atmosphere. Final grain size in material sintered 1 hour at 1080 K was generally less than 200 nm, although scattered coarsening to approximately 5 μm was observed.     

Thermal diffusivity of sintered stainless steel-alumina composites

Thermal diffusivity measurements were carried out as a function of temperature on sintered AISI 304 stainless steel-alumina composites having various compositions (0.001, 0.01, 0, 1, 1, 2, 3, 5, 7, 8, and 10 wt pct Al₂O₃). The measurements were carried out between room temperature and 1473 K. The thermal diffusivity and the thermal conductivity were found to increase with temperature in all the composite specimens. The thermal diffusivity was found to decrease with increasing weight fraction of alumina. This tendency can clearly be seen at temperatures above 755 K. The experimental results are in good agreement with the simple rule of mixture, the Eucken equation, and the Ohm’s law model developed by Hayashi et al. at weight fractions of alumina below 5 wt pct. Beyond this composition, the thermal diffusivity/conductivity shows a large discrepancy from the models. This could probably be attributed to the accumulation of alumina particles during cooled pressing and sintering.     

The role of particle size on the laser sintering of iron powder

The effects of powder particle size on the densification and microstructure of iron powder in the direct laser sintering process were investigated. Iron powders with particle sizes ranging from 10 to 200 μm were used. It was found that the sintered density increases as the laser energy input is increased. There is, however, a saturation level at which higher density cannot be obtained even at very intensive energy input. This saturation density increases as the size of the iron particles decreases. Meanwhile fine powders with narrow particle size distributions have a tendency toward agglomeration, and coarse powders with broad particle size distributions have a tendency toward segregation, both of them resulting in lower attainable density. In order to investigate the role of particle size, a “densification coefficient (K)” was defined and used. This coefficient depends on the particle size and the oxygen content of iron powder. The results of this investigation demonstrate that the presence of oxygen significantly influences the densification and pore morphology of laser-sintered iron. At higher oxygen concentrations, the iron melt pool is solidified to agglomerates, and formation of pores with orientation toward the building direction is more likely to occur. When the oxygen concentration is kept constant, the densification coefficient decreases with decreasing the particle size, meaning the densification kinetics enhances. This article presents the role of powder characteristics and the processing parameters in the laser sintering of iron powder as a model material. The mechanism of particle bonding and microstructural features of laser-sintered parts are addressed.     

Effect of conventional and microwave sintering on the properties of yttria alumina garnet-dispersed austenitic stainless steel

The current study examines the effect of heating mode, temperature, and varying yttria alumina garnet (YAG) addition (5 and 10 wt pct) on the densification and properties of austenitic (316L) stainless steel. The straight 316L stainless steel and 316L-YAG composites were heated in a radiatively heated (conventional) and 2.45 GHz microwave sintering furnace. The compacts were consolidated through solid state as well as supersolidus sintering at 1200 °C and 1400 °C, respectively. Both 316L and 316L-YAG compacts couple with microwaves and heat to the sintering temperature rapidly (∼45 °C/min). The overall processing time was reduced by about 90 pct through microwave sintering. As compared to conventional sintering, compacts sintered in microwaves exhibit higher densification and finer microstructure but no corresponding improvement in mechanical properties and wear resistance. This has been correlated to elongated, irregular pore structure in microwave-sintered compacts.     

Synthesis of iron aluminides from elemental powders: Reaction mechanisms and densification behavior

Pressureless sintering and hot pressing experiments were conducted on elemental powder compacts of Fe-15.8 wt pct Al and Fe-32 wt pct Al, corresponding approximately to the compositions of stoichiometric Fe₃Al and FeAl, respectively. Upon heating near the melting point of aluminum, an exothermic reaction was initiated in the compacts, resulting in synthesis of the desired compounds with reaction times on the order of seconds. Thermal analysis and microstructural observations indicate the formation of a transient liquid phase during rapid exothermic compact heating. The mechanisms shown to be responsible for microstructural development include initial compound formation in the solid state, appearance of an aluminum-rich liquid at the aluminum particle sites, iron dissolution accompanied by outward spreading of the liquid, and subsequent precipitation of the iron-rich compounds. Apparent enthalpies of formation,ΔH _f ^°(298), estimated from reaction temperature measurements were −18 and −31.8 kJ/mol for Fe₃Al and FeAl, respectively. The influences of heating rate, green density, and aluminum particle size on sintered density were studied for pressureless reaction sintering in vacuum. The effects of processing variables on densification were explained as the net result of swelling during heating and subsequent shrinkage due to the transient liquid phase. Near full density Fe₃Al and FeAl compounds were obtained through the application of external pressures near 70 MPa during reaction in a hot press. These alloys were partially ordered, chemically homogeneous, and exhibited an equiaxed grain structure with an average grain size below 10μm. The Fe₃Al material exhibited significantly higher fracture strength and somewhat lower ductility than coarse-grained wrought material of the same composition.     

Influence of phosphorus element on direct laser sintering of multicomponent Cu-based metal powder

This article presents a detailed investigation on the influence of the phosphorus element upon the laser sintering of a multicomponent Cu-based metal powder system consisting of Cu, Cu-10Sn, and Cu-8.4P. Powder systems containing 0, 10, 15, and 20 wt pct CuP were sintered in atmosphere at room temperature using the following optimal processing parameters: laser power of 350 W, scan speed of 0.04 m/s, scan line spacing of 0.15 mm, and layer thickness of 0.25 mm. It was found that the relative density of the sintered sample with 15 wt pct CuP increased by 24,4 pct as compared with the sample without phosphorus addition. A further increase in the CuP content (≥20 wt pct), however, resulted in a poor densification with a serious delamination. The exact metallurgical roles of the phosphorus element in the laser sintering process were addressed as follows. First, the phosphorus could prevent the sintering system from oxidation by forming CuPO₃, thereby improving the wetting characteristics and the sintering kinetics. Second, the phosphorus could decrease the surface tension of molten materials, leading to a successive transition from highly discontinuous sintered tracks to fairly coherent ones with increasing the phosphorus content. Third, the phosphorus could lower the melt viscosity, thereby improving the microstructural homogeneity of the laser-sintered samples.     

Densification behavior of dynamically shock compacted AI2O3/ZrO2 powders synthesized through rapid solidification

The rapidly solidified alumina-zirconia eutectic contains high volume fractions of nanocrystallinet- ZrO₂, which makes the material a promising precursor for the manufacturer of fracture-resistant ceramic specimens. Unfortunately, conventional powder processing and sintering techniques are inadequate for the fabrication of dense specimens using this material. We have used dynamic shock compaction to facilitate the achievement of high density specimens which retain the unique microstructure of the precursor material. In an attempt to quantify the dynamics of the microstructural evolution which occurs during the compaction process, we have investigated the effect of various particle size distributions on the densification behavior of the material during the shock compaction and postcompaction sintering cycles. The shock compaction process produced high densities (∼73 to 78 pct of single-crystal theoretical) by inducing a highly efficient packing of the particles. A bimodal powder distribution was also compacted and this specimen exhibited a relative density of 86.2 pct, approximately 10 pct higher that those of the unimodal compositions. In this compact, the small particles efficiently filled the interstices between the larger particles. The high density of the bimodal compact did not translate to a high sintered density, however. This article is based on a presentation made in the symposium “Dynamic Behavior of Materials,” presented at the 1994 Fall Meeting of TMS/ASM in Rosemont, Illinois, October 3-5, 1994, under the auspices of the TMS-SMD Mechanical Metallurgy Committee and the ASM-MSD Flow and Fracture Committee.     

An examination of the interparticle contact area during sintering of W-0.3 Wt Pct Co

As a powder compact sinters, its microstructure evolves. One way to quantify the scale of the microstructure is to consider the interparticle contact area. This study examines two known models for calculating the interparticle contact area: the classic two-sphere model and the Voronoi cell model. Both models have particular assumptions about the microstructure that make them not applicable for treating densification to near full density with concurrent grain growth. The classic two-sphere model assumes a regular packing of particles and a perfectly spherical particle geometry and neglects an increasing particle coordination number with sintering. The Voronoi cell model assumes that the scale of the microstructure remains constant; i.e., as long as the compact is densifying, grain growth does not occur. We propose a modified Voronoi cell that accounts for an increasing grain size, making it applicable to a general case where grain growth occurs during sintering. The three models are compared to the interparticle contact area data, obtained by stereology techniques, for W-0.3 wt pct Co sintered from green state to near full density. The original Voronoi cell model fits the data only at low temperatures, before the onset of grain growth. Below approximately 90 pct relative density, the two-sphere model with an assumed coordination number of six (coordination number in a green compact) and the modified Voronoi cell model provide a good fit to the data. At higher densities, both models overestimate the interparticle contact area.     

Sintering behavior of nanocrystalline γ-Ni-Fe powders

Solid state sintering behavior of nanocrystalline (35 nm) γ-Ni-Fe powders, which were sintered under hydrogen atmospheres at temperatures up to 1300 K, was investigated by laser-photo-dilatometry. The sinterability of the γ-Ni-Fe powders was found to depend crucially on the state of agglomeration and was generally limited to about 80 pct theoretical density. Metallographic observations revealed that the nanocrystalline powders showed a high degree of agglomeration which resulted in a difficult-to-sinter bimodal pore distribution. Although nanoscale intraagglomerate porosity was easily eliminated in the course of sintering, microscale interagglomerate porosity largely proved to resist densification. Experimentally observed shrinkage rates exhibit two densification maxima at the onset and the end of densification. No significant grain growth was observed during sintering.     

The effect of Mo addition on the liquid-phase sintering of W heavy alloy

The morphological and compositional changes of grains have been investigated in the initial stage of liquid-phase sintering of W-Mo-Ni-Fe powder compacts. Both large (5.4-μm) and small (1.3-μm) W powders have been used to vary their time of dissolution in the liquid matrix. When 8OW-10M0-7Ni-3Fe (wt pct) compacts of fine (about 1- to 2-μm) Mo, Ni, and Fe and coarse (5.4-μm) W powders are liquid-phase sintered at 1500 °C, the Mo powder and a fraction of the W powder rapidly dissolve in the Ni-Fe liquid matrix. The W-Mo grains (containing small amounts of Ni and Fe) nucleate in the matrix and grow while the W particles slowly dissolve. In this transient initial stage of the liquid-phase sintering, duplex structures of coarse W-Mo grains and fine W particles are obtained. As the W particles dissolve in the liquid matrix during the sintering, the W content in the precipitated solid phase also increases. The dissolution of the small W particles is assessed to be driven partially by the coherency strain produced by Mo diffusion at the surface. During sintering, the W particles continuously dissolve while the W-Mo grains grow. When the compacts are prepared from a fine (1.3-μm) W powder, the W grains dissolve more rapidly, in about 1 hour, and only W-Mo grains remain. These observations show that the morphological evolution of grains during liquid-phase sintering can be strongly influenced by the chemical equilibrium process. formerly Graduate Student, Department of Materials Science and Engineering, Korea Advanced Institute of Science and Technology     

Effect of cobalt addition on the liquid-phase sintering of W-Cu prepared by the fluidized bed reduction method

A new process, fluidized bed reduction (FBR) method, was applied for fabrication of uniform W-Cu sintered material. Liquid-phase sintering was carried out to obtain fully densified W-Cu composite, and the effect of cobalt addition on the sintering behavior was investigated. It was found that fully densified material could not be obtained even after sintering at 1200 °C for 4 hours in the case of 75W-25Cu, while more than 96 pct density could be obtained as soon as the sintering temperature reached 1200 °C when 0.5 wt pct cobalt was added prior to the sintering. It has been found that the wetting angle of the liquid copper is reduced significantly by the addition of cobalt, and the formation reaction of Co₇W₆ intermetallic compound at the surface of the tungsten powder is mainly responsible for the enhancement of the densification process.     

Preparation of high porosity metal foams

Metal foams with porosities greater than 90 pct were prepared by a novel powder metallurgy route using a polymeric vehicle. Coarse titanium powder and fine carbonyl iron powder were tested. The powders were blended with each component of a two-part polyol-isocyanate foaming system, and the resulting suspensions were mixed and allowed to expand. Although the resulting polymer-metal foam was closed cell, particles were not retained in the windows. Upon pyrolysis to remove the resin, the windows opened and the final sintered metal foam was reticulated. Such foams present very low sintered density and are correspondingly weak after sintering but offer a fine reticulated structure with cell diameters in the region of 100 to 200 μm. They may have applications in the areas of catalysis, biomaterials, and composites.     

Rapid sintering of iron powders under action of electric field

Abstract

A new rapid sintering technique for iron powders compacted under the action of an electric field with high current density has been advanced. The results show that the sintering densification of iron powder could be finished in less than 6 min at a temperature of 800^u C reached at a heating rate of 600 K s^?1, and the relative density of the sintered compact was over 95%. Moreover, the sintering densification was almost finished in the heating stage of the compact.     

Processing and Characterization of Cu-Al-Ni Shape Memory Alloy Strips Prepared from Prealloyed Powder by Hot Densification Rolling of Powder Preforms

The present work deals with the preparation of near-full density Cu-Al-Ni shape memory alloy (SMA) strips from argon-atomized prealloyed powder via a powder metallurgy (PM) route comprising cold die compaction to prepare powder preforms, sintering, and hot densification rolling of unsheathed sintered powder preforms under protective atmosphere at 1273 K (1000 °C). It has been shown that argon-atomized spherical Cu-Al-Ni SMA powder consisted of very fine equiaxed grains and no appreciable grain growth occurred during sintering at 1273 K (1000 °C). It also has been shown that no appreciable densification occurred during sintering, and densification was primarily achieved by hot rolling. The densification behavior of the sintered powder preforms during hot rolling was discussed. The hot-rolled Cu-Al-Ni strips were heat-treated at 1223 K (950 °C) for 60 minutes and water quenched. The heat-treated strips consisted of equiaxed grains with average size approximately 90 μm. The heat-treated Cu-Al-Ni SMA strips consisted of self-accommodated b₁^￠ \beta_{1}^{'} martensite primarily, and showed smooth b₁ T b₁^￠ \beta_{1} \Rightarrow \beta_{1}^{'} transformation behavior coupled with a very low hysteresis (≈25 K (25 °C)). The heat-treated strips exhibited an extremely good combination of mechanical properties with fracture strength of 530 MPa and 12.3 pct fracture strain. The mode of fracture in the finished strip was primarily void-coalescence-type ductile together with some brittle transgranular type. The shape memory tests showed almost 100 pct one-way shape recovery after 100 bending-unconstrained heating cycles at 4 pct applied prestrain, exhibiting good stability of Cu-Al-Ni strips under thermomechanical actuation cycling. The two-way shape memory strain was found approximately 0.45 pct after 15 training cycles at 4 pct training strain.     

The non-cubic lattice of rapidly quenched packet martensite

The positions of 200, 020 and 002 peaks of ferrous martensite have been measured individually by a precision X-ray diffractometer technique. Martensite with 18 wt pct Ni and 0.1, 0.2, 0.3, and 0.4 wt pct C was formed by salt-water quenching from preoriented austenite single-crystal slices about 0.5 mm thick. The variations in X-ray peak positions interpreted as changes in lattice parameter yield to the first approximation:Δa/a ₀ = −0.005 [wt pct C],Δb/a ₀ = −0.016 [wt pct C],Δc/a ₀ = −0.037 [wt pct C] for carbon contents between 0.1 and 0.4. For such carbon contents the martensite has packet or mixed packet-lenticular morphology; consequently packet martensite is not cubic as sometimes claimed. Instead its lattice is nearly like that of lenticular martensite with the"a " and"c " parameters varying with carbon content as previously observed for tetragonal martensite, and the" b " parameter unequal to the" a" . The significance of the inequality of the" a " and" b " parameters is unclear, but that inequality appears to be characteristic of some plate and lenticular martensite.     

Deoxidation of molybdenum during vacuum sintering

Molybdenum is usually fabricated through the powder metallurgy (P/M) process, using fine powders with a relatively high oxygen content. Oxygen, however, is one of the main elements causing embrittlement during the deformation processing of molybdenum, such as rolling, extrusion, and forging. Thus, how to deoxidize the compact as completely as possible is critical in the P/M process. This study shows that, as an alternative to hydrogen reduction, molybdenum oxides can be reduced by adding organic lubricants to the compact and by sintering the compact under high vacuum with long sintering times. After 10 hours of sintering at 1750 °C and a 0.03 torr vacuum, the oxygen content decreased from 0.927 wt pct of the green compact to 0.017 wt pct. The ductility also improved significantly compared to compacts sintered for 5 hours, which contained 0.218 wt pct oxygen. The morphology evolution, weight changes, and the X-ray analysis indicated that the oxide was first present in the form of MoO₃. It was then transformed into MoO₂ before deoxidation was completed. Two deoxidation mechanisms were identified: evaporation and decomposition of MoO₃ and MoO₂, with evaporation being dominant in the early-stage sintering and decomposition being dominant in the later stage.     

The properties of tungsten processed by chemically activated sintering

Two tungsten powders have been treated with small concentrations of sintering activators to provide for enhanced low temperature sintering. The experimental study focused on the determination of the processing effects on properties such as sintered density, grain size, hardness, and strength. Variables in the plan included tungsten particle size, type of activator, amount of activator, compaction pressure, and sintering temperature. The sintered density is found to have a dominant effect on strength and hardness. The various processing variables are analyzed in terms of their effects on density. At high sintered densities, grain growth acts to degrade the strength. Additionally, the nature of the sintering activator influences the fracture strength. In this study optimal strength occurred with a 0.7 μm tungsten powder treated with 0.29 wt pct Ni, sintered at 1200 °C for one hour. The resulting density was 18.21 g/cm³, with aR _A hardness of 69 and a transverse rupture strength of 460 MPa.