Characterization of a marginal glass former alloy solidified in gas atomized powders

Al₉₀Sm₁₀, a marginal glass former, was rapidly solidified using high pressure gas atomization (HPGA). Rapid solidification is a non-equilibrium process, with different degrees of departure from full equilibrium constituting a microstructural hierarchy that correlates with increasing solidification rate. In accordance with this the resultant HPGA powders show a variety of microstructures according to their particle diameters, corresponding to degree of undercooling, with an amorphous structure appearing at high cooling rates. Five distinct phases and microstructures have been identified at different undercoolings; Al solid solution, tetragonal Al₁₁Sm₃, two different orthorhombic phases, and an amorphous phase which exists in company with a high number density of Al nanocrystals. The product phases of the rapid solidification were identified and analyzed using high energy transmission X-ray diffraction (HEXRD), high resolution transmission electron microscopy (HRTEM), scanning electron microscopy (SEM), Energy Dispersive X-ray Spectroscopy (EDS) and thermal analysis (DSC). The results of the study will be helpful in identifying metastable phase hierarchy and glass formation during vitrification of marginal glass formers.     

Internal oxidation of rapidly solidified silver-tin-indium alloy powders

The internal oxidation behaviour of rapidly solidified silver-tin-indium (Ag-Sn-In) alloy powders is described. Internal oxidation of Ag-Sn-ln alloy has gained importance in view of the possible replacement of toxic silver-cadmium oxide (Ag-CdO) material by a better property silver-tin oxide (Ag-SnO₂) electrical contact material. Rapidly solidified Ag-Sn-In alloy powders of composition Ag-6.0Sn-3.0In were prepared by gas atomization. The important characteristics of alloy and powders, from the point of view of internal oxidation, were determined. The powders were internally oxidized and subsequently processed by conventional powder metallurgy techniques. The important physical properties, such as electrical conductivity, hardness, density and microstructure, were determined. The physical properties, especially the microstructure and rate of internal oxidation, were compared with the materials prepared by the conventional internal oxidation route.     

Microstructural characteristics and gas content of rapidly solidified powders

The gas content and microstructural characteristics of rapidly solidified powders manufactured by different techniques were studied by Bureau of Mines researchers. Powders were screened and classified into size fractions. Powder characteristics including gas content and porosity were measured and related to powder particle size. Three different atomizing gases, argon, helium, and nitrogen were used in manufacturing the powders. In one series of experiments one gas was used to atomize the melt while a different gas was used in the melting and powder collection chambers. The gas content of the powders was shown to consist of three separate components: (1) solid solution, (2) physical entrapment associated with macroporosity, and (3) surface reaction such as surface oxide. The various components of gas content could be identified by the shape of the curve plotting gas content versus particle size. The identification of the presence of entrained gas as porosity from these curves is important because after consolidation, high-pressure bubbles of inert gas can result. This porosity can cause problems during subsequent heating or joining operations, seriously degrading mechanical properties. Analysis of gas content versus particle size represents a sensitive, technique to detect the presence of porosity.     

Production of rapidly solidified magnesium powders by gas atomisation

Abstract

Rapidly solidified powders of magnesium were produced in a pilot plant gas atomiser. Argon gas at 1·85 MPa was used as the atomising agent in a ‘confined design’ nozzle operating vertically upwards. Oxygen was introduced into the plant at a level of 1% to cause controlled oxidation to passivate the particles. Powders were sized using dry sieving down to 32 μm and wet sieving for smaller sizes. Sauter mean diameter varied between 18·45 and 21·40 μm depending on the rate of production. The distribution of sizes was typically bimodal with the separation point of the peaks at ~34 μm. This supported the particle formation theory developed earlier which predicted that atomised metal powders consisted of two families of particles. The fine range of the particles was spherical up to ~30 μm in diameter. In the coarse range, which was virtually free of satellites, the overall round nature of the particles was preserved, but some oblong shapes were also observed. Comparison with the atomisation of aluminium shows that, despite having a lower surface tension, magnesium powders are marginally coarser than those of aluminium produced under the same conditions. This is explained in terms of the faster acceleration of magnesium droplets (by virtue of lower density) in flight during secondary breakup.

MST/915     

Dynamic consolidation of rapidly solidified titanium alloy powders by explosives

Consolidation of rapidly solidified titanium alloy powders employing explosively generated shock pressures was carried out successfully. The cylindrical explosive consolidation technique was utilized, and compacts with densities in the range 97 to 100% were produced. Better consolidation (with more interparticle melting regions and less cracking) was achieved by using a double tube design in which the outer tube (flyer tube) was explosively accelerated, impacting the powder container. Optical and transmission electron microscopy observations were carried out to establish microstructural properties of the products. It was observed that consolidation is achieved by interparticle melting occurring during the process. The interior of the particles in Ti-17 alloy exhibited planar arrays of dislocations and twin-like features characteristic of shock loading. Two dominant types of microstructures (lath and equiaxed) were observed both in Ti-662 and Ti-6242 + 1% Er compacts, and very fine erbia (Er₂O₃) particles were seen in the latter alloy. The micro-indentation hardness of the consolidated products was found to be higher than that of the as-received powder material; and the yield and ultimate tensile strengths were found to be approximately the same as in the as-cast and forged conditions. The ductilities (as measured by the total elongation) of the shockcompacted materials were much lower than those of the cast or forged alloys. Hot isostatic pressing of the shock-consolidated alloys increased their ductility. This enhancement in ductility is thought to be due to the closure of existing cracks. These excellent mechanical properties are a consequence of strong interparticle bonding between individual powder particles. It was also established that scaling up the powder compacts in size is possible and compacts with 50, 75, and 100 mm diameter were successfully produced.     

Degassing and surface analysis of gas atomized aluminium alloy powders

The degassing behaviour and surface characterization of Al-Mg base alloys has been investigated using quadrupole mass spectrometry (QMS) and X-ray photoelectron spectroscopy (XPS). The alloy composition, particle size and the nature of the atomizing gas have been studied in terms of gas evolution and surface composition. XPS has been used both to measure oxide thicknesses and magnesium enrichment ratios. XPS results show that magnesium segregation increases for larger particle sizes and this is supported by QMS, with a correspondingly higher hydrogen evolution on heating being observed for the larger size fractions. High-resolution XPS of the carbon 1s photoelectron peak (C1s) indicates the presence of carbonate component on the as-received magnesium-containing powders. This component is less pronounced on degassed powders indicating the evolution of CO₂ on heating. This observation is supported by thermodynamic calculations.     

Microstructures of explosively consolidated rapidly solidified aluminium and Al-Li alloy powders

The microstructures and the characteristics of water-atomized, nitrogen gas-atomized Al powders and ultrasonic argon gas-atomized Al-Li alloy powder were investigated by means of metallography, SEM, Auger electron spectroscopy and X-ray diffraction techniques. Rapidly solidified powders were explosively consolidated into different sized cylinders under various explosive parameters. The explosively consolidated compacts have been tested and analysed for density microhardness, retention of rapidly solidified microstructures, interparticle bonding, fractography and lattice distortion. It is shown that the explosive consolidation technique is an effective method for compacting rapidly solidified powders. The characteristics of surface layers play a very important role in determining the effectiveness of the joints sintered, and the Al-Li alloy explosive compacts present an abnormal softening appearance compared to the original powder.     

Metastable microstructures in rapidly solidified zirconium alloys

Zirconium alloys exhibit a wide variety of phase transformations. The present review outlines some aspects of phase transformations, which are encountered in rapidly solidified zirconium alloys. The possibility of solidification of unalloyed zirconium directly into the low temperature phase is examined and the role that solidification microstructure plays in modifying the to martensitic transformation in rapidly solidified material is discussed. Zirconium alloys undergo two types of displacive transformations, namely, the martensitic transformation and the transformation. The influence of rapid solidification on the transition from the martensitic to the transformation is also discussed. Addition of transition metals are known to depress the melting point of zirconium alloys very drastically. As a consequence, glass forming abilities of a number of binary and ternary zirconium alloys are quite strong. A number of zirconium based metal-metal amorphous alloys have been synthesized using rapid solidification. In recent years, this work has been extended to bulk metallic glasses, which usually contain a larger number of alloying elements. Crystallization of these glasses and quasi-crystalline phase formation in these systems is also discussed.     

Microstructures and their stability in rapidly solidified Al-Fe-(V, Si) alloy powders

Microstructures and their stability in as-atomised Al-6.5Fe-1.5V and Al-6.5Fe-1.5V-1.7Si powders have been investigated using transmission electron microscopy (TEM) equipped with energy dispersive X-ray spectroscopy (EDXS), scanning electron microscopy (SEM), X-ray diffraction (XRD) and differential scanning calorimetry (DSC) techniques. It was observed that microstructures of the as-atomised powder particles showed a close relationship with powder particle sizes. The as-atomised powders exhibited three types of microstructures, namely 'zone A', 'zone B' and 'zone C'. The 'zone A' type microstructure consisted of very fine and homogeneous distributed precipitates in the -Al matrix. The 'zone B' microstructure represented the regions consisting of microcellular structures whereas the 'zone C' microstructure represented the regions consisting of coarse cellular structures and globular quasi-crystalline phase particles. Fine powder particles exhibited both 'zone A' and 'zone B' microstructures. The size of 'zone A' decreased with increasing powder particle sizes. The intercellular phases in 'zone B' of both Al-Fe-V and Al-Fe-V-Si were very fine, randomly oriented microquasi-crystalline icosahedral particles. Microstructures of coarse powder particles exhibited both 'zone B' and 'zone C'. The intercellular phases in 'zone C' of Al-Fe-V powders could be Al₆Fe, whereas in Al-Fe-V-Si powders they were probably silicide phase. Formation of powder microstructures may be explained by the interactions between the growing -Al fronts with the freely dispersed, primary phase particles or the solute micro-segregation. Studies using DSC techniques have revealed the microstructural stability of as-atomised powders. There were three DSC exotherms observed in the as-atomised Al-Fe-V powders. The 'zone A' was stable at elevated temperatures and the exotherm peak corresponding to the transformation reactions occurring in 'zone A' was at 360°C. The exotherm peak, which might correspond to the transformation of the globular clusters of microquasi-crystalline icosahedral phase to single-phase icosahedral particles, was at 450°C. The exotherm peak, which may correspond to the formation of Al₁₃Fe₄ and Al₄₅(V, Fe)₇ phases, was at 500°C. In the as-atomised Al-Fe-V-Si powders, only one exotherm was observed with a peak at 400°C. This exotherm may correspond to precipitation of silicide phase particles.     

Chemical segregation of solute elements in ultrasonically gas atomized aluminium alloy powders

A new experimental approach to the evaluation of chemical segregation of solute elements in ultrasonically gas atomized aluminium-alloy powders using X-ray spectral data of scanning electron microprobe analyser is described. The experimentally obtained chemical segregation data is compared with the conventional method of quantitative analysis and with theoretical predictions as determined from Scheil’s approach to the evaluation of elemental segregation during the solidification process. A comparison of experimental and theoretical predictions confirms the validity of the experimental approach in the estimation of solute segregation levels and also suggests that the solidification conditions considered for estimation of microchemical segregation can appropriately be applied to ultrasonically gas atomized powders.     

Electron-beam weld microstructures and properties of aluminium-lithium alloy 8090

Lithium-containing aluminium alloys are of considerable current interest in the aerospace and aircraft industries because lithium additions to aluminium improve the modulus and decrease the density compared to conventional aluminium alloys. Few commercial aluminium-lithium alloys have emerged for use in the aerospace industry. One such candidate is 8090, a precipitation-hardenable Al-Li-Cu-Mg alloy. The influence of electron-beam welding on the microstructure and mechanical properties of alloy 8090 material has been evaluated through microscopical observations and mechanical tests. Microscopic observations of the electronbeam welds revealed an absence of microporosity and hot cracking, but revealed presence of microporosity in the transverse section of the weld. Mechanical tests revealed the electronbeam weld to have lower strength, elongation and joint efficiency. A change in microscopic fracture mode was observed for the welded material when compared to the unwelded counterpart. An attempt is made to rationalize the behaviour in terms of competing mechanistic effects involving the grain structure of the material, the role of matrix deformation characteristics, grain-boundary chemistry and grain-boundary failure.     

Characterization of rapidly solidified aluminium-silicon alloy

A metallographic investigation of as-cast LM-13 aluminium-silicon alloy, solidified at different cooling rates (using permanent moulds or a single-roll melt spinner), is presented with special reference to the modification of eutectic silicon. and the refinement of primary aluminium. The refinement of microstructure with the increase in cooling rate is mainly attributed to the limited growth kinetics of the nucleated phase during solidification. The rapidly solidified ribbon was heat-treated in order to determine the microstructural stability at high temperature. The tensile strength, percentage elongation and hardness values of as-solidified and heat-treated samples correlate well with the changes in the microstructure observed.     

Microstructural properties and hot pressing of rapidly solidified hypoeutectic low alloy white cast iron powders

Abstract

Hypoeutectic low alloy white cast iron powders were produced using a rapid solidification technique. The morphology and microstructural properties of these powders were investigated with respect to cooling rate and particle size. The density of hot pressed compacts as a function of parameters such as hot pressing time and pressure is described. It was found that retained austenite in the form of cells or dendrites was the main constituent of the powders. At 720°C the powder particles can be hot pressed into high density compacts that have a fine cementite–ferrite microstructure. These ultrafine grained compacts exhibited good superplasticity at elevated temperatures. An elongation to failure of 300% was observed.

MST/1682     

Microstructural characterization of a rapidly solidified Al-Sr-Ti alloy

The microstructure of the melt-spun Al-7Sr-3Ti alloy has been characterized using X-ray diffraction, transmission electron microscopy and differential scanning calorimetry. The results show the microstructure of the melt-spun Al-7Sr-3Ti alloy is composed of the equilibrium α-Al, Al₄Sr, Al₃Ti and metastable Al₂₃Ti₉, different from that of the ingot-like alloy comprising α-Al, Al₄Sr and Al₃Ti. Moreover, the amount of Al₃Ti is much less than that of Al₄Sr and metastable Al₂₃Ti₉ in the melt-spun alloy. The melting temperature of primary phases and enthalpies of fusion for the melt-spun Al-7Sr-3Ti alloy are lower than those for the ingot-like alloy. The formation mechanism of the microstructure of the melt-spun alloy has also been discussed.     

The structure of a rapidly solidified Al-Fe-Ti-C alloy

The microstructures of melt-spun Al-2.03 Fe-0.46 Ti-0.35 C (at %) superheated to 1523 K (ribbon I), and 1673 K (ribbon II), respectively, before quenching, have been characterized using analytical electron microscopy and X-ray diffraction. A duplex microstructure has been observed for ribbon I, consisting of a microcellular region, across a sharp transition, followed by a coarser cellular or dendritic structure. The intercellular phases consisted mostly of Al₆Fe (few of Al₃Fe) and the dispersed TiC particles distributed in the -Al matrix with an exact orientation relationship. However, the microstructure of ribbon II comprised uniform, fine-scale dispersions of Al₆Fe phase in -Al grains, and larger size, elongated amorphous phase particles located along the grain boundaries, and approximately 0.46 at % Ti and 0.35 at % C dissolved in the -Al matrix. During the annealing of ribbon II, the amorphous phase transformed to _T-AlFeSi phase, the Al₆Fe dispersoids grew upwards and Al₃Fe, TiC particles precipitated in the -Al matrix. TiC phase formed both in ribbon I and in annealed ribbon II all had an atomic composition of TiC_0.79 (the nominal atomic percent ratio for the alloy was 0.74) and a lattice parameter of 0.424 nm. Moreover, there is a cube-cube orientation relationship between TiC and -Al matrix with a disregistry =0.049. In addition, the solidification characteristics of rapid solidification processing (RSP) Al-Fe-Ti-C alloy and mechanism of TiC formation have been discussed.     

Strengthening mechanisms in a rapidly solidified and aged Cu-Cr alloy

A single-roller melt spinning method was used to produce Cu-Cr microcrystal alloy ribbons. Upon proper aging treatment, the strength and hardness of the alloy were remarkably enhanced while the conductivity only had a minimal decrease. Grain refinement and coherent dispersion strengthening were proved to be the major factors contributing to the improvement of strength and hardness of the alloy after aging. The degree of coherent strengthening was almost identical with that calculated by the Gerold equation. Compared with the solid solution quenched Cu-Cr alloy, the peak hardness was increased 2.6 times, in which about 27% was attributed to the grain refinement and 73%, in turn, provided by coherent strengthening due to aging precipitation. Neither the solid solution strengthening nor vacancy strengthening had detectable effect on the strength and hardness of the rapidly solidified Cu-Cr alloy.