Structure of mechanically alloyed Ti-Al-Nb powders

Ti-Al-Nb ternary powder mixtures containing 24Al-11Nb, 25Al-25Nb, 37.5Al-12.5Nb, and 28.5Al-23.9Nb (at. pct) were mechanically alloyed in a SPEX 8000 mixer mill using a ball-to-powder weight ratio of 10:1. The structural evolution in these alloys was investigated by X-ray diffraction and transmission electron microscopy techniques. A solid solution of Al and Nb in Ti was formed at an early stage of milling, followed by the B2/body-centered cubic (bec) and amorphous phases at longer milling times. The stability of these phases and their transformation to other phases have been investigated by heat treating these powders at different temperatures. The B2/bcc phase transformed into an orthorhombic (O-Ti₂AlNb) or a mixture of the orthorhombic (O) and hexagonal close-packed (α₂-Ti₃Al) phases, the proportion of phases being dependent on the powder composition. Milling beyond the amorphous phase formation resulted in the formation of an fee phase in all the powders, which appears to be TiN, formed as a result of contamination of the powder. Formerly Graduate Student, University of Idaho     

Elevated-temperature stability of mechanically alloyed Cu-Nb powders

When two-phase mixtures of ductile metals are mechanically alloyed, they often assume a convoluted lamellar structure. Since these powders are consolidated at elevated temperatures, their structures (and, therefore, properties) are likely to be altered by consolidation processing. We have investigated microstructural changes that take place on heat-treating mechanically alloyed Cu −20 vol pct Nb alloys. The transition from a “platelike” to a spherical microstructure is described, and the kinetics of this process appear controlled by a type of boundary diffusion, even though the coarsening temperature was high in terms of the homologous temperature of Cu. Reasons for this behavior are suggested. Finally, during heat treatment (carried out in H), a Nb layer forms around the particles. The thickness of this layer (and the corresponding zone denuded of Nb within the particle) increases with continued elevated-temperature exposure, and at a rate consistent with the process being driven by curvature forces. Formerly Professor, Department of Materials Science and Engineering, University of Virginia, Charlottesville, VA     

Shock consolidation of mechanically alloyed amorphous Ti-Si powders

Mechanical alloying was used to synthesize amorphous 5Ti-3Si atomic ratio powders in a SPEX mill under Ar atmosphere. X-ray diffraction analysis revealed formation of a single-phase amorphous compound after about 24 hours of milling. High-resolution transmission electron microscopy (TEM) showed that the milled powder still contained nanocrystallites of Ti and Si among regions of generally amorphous compound. The mechanically alloyed amorphous powder was shock consolidated, using a plate impact assembly, to produce bulk compacts. The compaction resulted in a significant amount of crystallization, forming 30- to 40-nm crystals of TiSi₂ and Ti₅Si₃ intermetallic compounds. The compacts were subsequently annealed above the crystallization temperature, measured to be ∼640 °C using differential thermal analysis. The compacts annealed at 800 °C for 1 hour showed only limited grain growth to ∼50-nm crystallite size. Microhardness of the shocked amorphous alloy compacts was ∼1100 KHN, which increased to ∼1250 KHN upon subsequent annealing, with the formation of a more homogeneous nanocrystalline microstructure. Formerly Undergraduate Research Assistant, School of Materials Science and Engineering, Georgia Institute of Technology. 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.     

Effects ofcontaminationonproperties of W-15Cu preparedfrom mechanically alloyed powders

Abstract

To eliminate the contamination of activator elements, such as Fe and Ni, W-15Cu compacts were prepared from mechanically alloyed powders using an attritor with a zirconia tank, balls and agitator arms. Coarse tungsten and copper powders, 9·9 μm and 13·3 μm, respectively, were milled to 1·26 μm composite powders after 145h of milling. The milled powder contained little free copper and was highly combustible in air. After sintering, the 50 vol.-% dense green compacts attained a density of 15·8g cm^-3 or 96·2%. The microstructure consists of uniformly interdispersed tungsten and copper. When stainless steel grinding balls were used, the powder was heavily contaminated with Fe and Ni. The contamination improved the density slightly, but the grain size and the electrical resistivity increased significantly as well. The sintering behaviours of the two composite powders were similar. Most densification occurred during heating before reaching the melting point of copper.     

Oxidation of mechanically alloyed powders during processing: origins and control

Abstract

Oxide dispersion strengthened (ODS) alloys prepared by mechanical alloying (MA) and subsequent consolidation are usually subjected to a series of heat treatments during production, typically comprising a degassing process at ?600°C and a preconsolidation high temperature 'soak' at ?1000°C, both under vacuum. In the current work, the oxidation behaviour of a prototype ODS Fe₃Al alloy and a commercial FeCrAl alloy has been studied during simulation of these temperature and pressure regimes. After the high temperature 'soak' simulation, oxidation had taken place on both alloys with a significantly thicker scale forming on the ODS Fe₃Al. This scale is believed to be the source of much of the high alumina content found in fully consolidated ODS Fe₃Al. Variation in the amount of particulate alumina found in different batches of commercially consolidated powder is discussed. Novel processes involving hydrogen purging and powder precompaction have been employed to decrease oxidation and thereby increase sintering efficiency.     

机械合金化纳米晶Cu-5% Cr粉末的热静液挤压研究

采用机械合金化制备了纳米晶Cu-5%(质量分数,下同)Cr粉末,然后对其进行了热压制坯和热静液挤压致密化研究。结果表明,经10h高能球磨,Cu-5%Cr粉末中Cu的晶粒尺寸细化到约50nm,成为纳米晶粉末。采用热压制坯和热静液挤压工艺可以使纳米晶Cu-5%Cr粉末接近完全致密化。球磨10h的Cu-5%Cr合金粉末经400℃热压制坯和600℃、挤压比为4的热静液挤压后相对密度达到99.3%。热静液挤压致密化后的Cu-5%Cr合金的晶粒有所长大,Cu基体的平均晶粒尺寸达到了500nm左右,变成了亚微米晶材料。该亚微米晶Cu-5%Cr合金具有较好的性能。     

Solid-state crystalline-glassy cyclic phase transformations of mechanically alloyed Cu33Zr67 powders

Cyclic crystalline-glassy-crystalline phase transformations have been investigated during high-energy ball milling of elemental Cu₃₃Zr₆₇ powders under an argon gas atmosphere. The results show that the metallic glass, which is obtained after 86 ks of milling time, transforms into a new metastable phase of nanocrystalline fcc CuZr₂ upon milling for 173 ks. It subsequently transforms to the same glassy phase after 259 ks of the milling time. Such cyclic phase transformations are observed several times during the milling. The present work shows that this phenomenon does not have any obvious analogues with the periodic redox reactions or with diffusive-reactive phenomena known in chemistry. On the basis of our results, the destabilizing effect of the defects created by the milling media (balls), which leads to the cyclic transformations, depends not only on the milling time but also on the milling rotation speed. he is also a Permanent Staff Member, Mining and Petroleum Engineering Department, Faculty of Engineering, Al-Azhar University, Nasr City 11371 Cairo, Egypt.     

Thermal transformations in mechanically alloyed Fe-Zn-Si materials

The ball milling of elemental powders corresponding to Γ (Fe₃Zn₁₀)+0.12 wt pct Si; Γ₁ (Fe₅Zn₂₁) + 0.12 wt pct Si; δ (FeZn₇)+0.12 wt pct Si; and ζ (FeZn₁₃)+0.12 wt pct Si composition ratios yields crystalline, mechanically alloyed phases. Differential scanning calorimetry (DSC) measurements of these materials show that they evolve differently, with well-defined characteristic stages. The activation energies for processes corresponding to these stages, based on kinetic analyses, are determined and correlated to microstructural evolvements. The processes occurring during the first stage below 250 °C, for all of the materials studied using X-ray diffraction (XRD) analysis, are associated with release of strain, recovery, and limited atomic diffusion. The activation energies for recovery processes are 120 kJ/mole for the Γ+0.12 wt pct Si, 131 kJ/mole for δ+0.12 wt pct Si, and 96 kJ/mole for ζ+0.12 wt pct Si alloys. At higher temperatures, recrystallization and other structural transformations occur with activation energies of 130 and 278 kJ/mole for Γ+0.12 wt % Si; of 161 kJ/mole for Γ₁+0.12 wt pct Si; of 167 and 244 kJ/mole for δ+0.12 wt pct Si; and of 641 kJ/mole for the ζ+0.12 wt pct Si. In addition, a eutectic reaction at 420 °C±3 °C, corresponding to the Zn-Si system, and a melting of Zn in Fe-Zn systems are observed for the ζ+0.12 wt pct Si material. The relation of FeSi formation in the Sandelin process is discussed.     

Making alloyed titanium diboride powders

Moscow Fine-Chemical Technology Institute. Translated from Poroshkovaya Metallurgiya, No. 6(366), pp. 20–24, June, 1993.     

Neutron diffraction and phase evolution of the mechanically alloyed intermetallic compound ζ-FeZn13

High-energy ball milling with subsequent annealing is used to synthesize the intermetallic compound ζ-FeZn₁₃. The mechanically alloyed phase in the as-milled state is determined to be nonequilibrium, or metastable. Transmission electron microscopy (TEM) studies show a highly defective microstructure with undefined grain areas, and the alloy can be described as a mechanical mixture of elemental Fe and Zn, based on neutron diffraction measurements. Characteristic stages associated with its transformation to the equilibrium state are identified based on differential scanning calorimetry (DSC) measurements. The activation energies corresponding to these stages are 128, 202, and 737 kJ/mole, respectively, with increasing transformation temperatures. The first stage is related to limited atomic diffusion or rearrangements, such as recovery, during thermal treatment, while the second stage depicts continued recrystallization and long-range atomic diffusion leading to a stable phase formation. The third and final stage marks structural decomposition of the equilibrium structure due to phase transition. Neutron diffraction of the equilibrium alloy confirmed that the structure is C2/m, with lattice parameters of a=13.40995 Å, b=7.60586 Å, c=5.07629 Å, and β=127 deg 18 minutes. The atomic positions of Fe and Zn compared well to reported values.     

Recrystallization in oxide-dispersion strengthened mechanically alloyed sheet steel

Systematic annealing at temperatures between 1300 °C and 1380 °C was applied to sheets of INCOLOY MA-956, an oxide-dispersion strengthened (ODS), mechanically alloyed, iron-base steel containing (in mass percent) 20.8Cr, 5.0A1, 0.5Y₂O₃, and 0.5Ti. The billets, comprised of hot isostatically pressed (“hipped”), mechanically alloyed powder, were hot- and cold-rolled to produce a 0.5-mm-thick sheet with a strong (100)«110» deformation texture. Light and transmission electron microscopy established that recrystallization initiated by nucleation at the sheet centerline. Initial rapid growth of the centerline-nucleated grains, designated stage I, resulted in plate-shaped grains oriented parallel to the rolling plane at the sheet centerline. Subsequent growth, designated stage II, was developed by planar growth fronts through the sheet thickness at a slower rate. The final product was a very coarse grain structure, sometimes with only a single grain through the sheet thickness. The recrystallization kinetics were typified by an incubation time, a temperature dependance characterized by an activation energy of 506 kJ/mole, and a decreasing rate of boundary migration with increasing time at temperature. The microstructural evolution is discussed in terms of the influences of deformation texture, residual stress, dislocation substructure, and oxide dispersion on the recrystallization process.     

Nonuniform recrystallization in a mechanically alloyed nickel-base superalloy

The recrystallization behavior of mechanically alloyed and extruded MA6000 oxide dispersion strengthened (ODS) nickel-base superalloy has been studied as a function of the position of the sample in the extruded bar. It was found that there are profound differences across the cross section of the bar, the edge regions recrystallizing more easily relative to the core. Furthermore, the recrystallized grains tend to be much more anisotropic at the edges. It appears that these differences are caused by inhomogeneous deformation. The oxide particles are more aligned along the extrusion direction at the regions near the surface, presumably because of the more extensive deformation in those regions. This explains the greater anisotropy in the recrystallized grains and the relative case of recrystallization (since the boundary mobility along the extrusion direction is high for aligned particles). Stored energy measurements confirm that the observed effects cannot be explained by differences in the driving force for recrystallization. Formerly Senior Researcher, Research Development Corporation of Japan