Formation of non-equilibrium alloys by high pressure melt quenching

Melt quenching under high pressure can promote the formation of metastable materials. High pressure accelerates the amorphization of Cu₆₀Ti₄₀ and Cd₄₃Sb₅₇. For an alloy systems having volume expansion after solidification, the higher the applied pressure, the lower the melting point and the higher the amorphization temperature, which promotes the formation of metallic glass. High pressure also enhances nucleation and suppresses grain growth, so solidification under high pressure can refine the crystal grains to form nanocrystalline alloys, such as Ti₆₀Cu₄₀, Cu₇₀Si₃₀ and Pd₇₈Si₁₆Cu₆ alloys.     

Laser plasma spectroscopy of Al-Cu-Fe quasicrystal an d alloy

Electron number density and temperature were determined from laser-induced plasmas produced by irradiating Al-Cu-Fe targets of a quasicrystal and of an alloy of similar composition. The Al(I) atomic emission spectra of the two systems were measured as a function of the distance from the target and of the time delay after laser irradiation. Differences of plasma characteristics were observed for laser ablation of quasicrystal and alloy targets, and the results were interpreted on the basis of different plasma formation mechanisms for the two systems.     

Formation of AI-Cr-Si quasicrystal with high silicon concentration by rapid quenching and its thermal and electrical properties

A quasicrystal revealing five-fold symmetry has been found to be formed in a rapidly quenched Al₆₂Cr₁₉Si₁₉ alloy containing a large amount of metalloid silicon. From analysis by the TEM/EDX method, the quasicrystalline single phase was determined to have a composition of Al_62.5Cr_17.6Si_19.9. The quasicrystal is composed of randomly-oriented equiaxed grains with an average size of 0.5 μm. The quasicrystal transforms to a stable Al₁₃Cr₄Si₄ compound with a complex cubic structure in the temperature range of 710 to 800 K. The activation energy and heat for the transformation are 85 kJ mol^?1 and 2.57 kJ mol^?1, respectively. The electrical resistivities (?) at 4.2 and 250 K are 2.83 and 3.65 μ?m, respectively, and its temperature coefficient at 250 K is 9.33 × 10^?4K^?1. The formation of the quasicrystal in the vicinity of Ai₁₃Cr₄Si₄ was inferred to be due to the combination effect of a great supercooling ability caused by the low melting temperature for AI-Si and Cr-Si eutectic type alloys and the difficulty of diffusivity of the constituent atoms in the ternary compound with a large unit cell and a strong bonding nature between chromium and aluminium or silicon.     

Two-dimensional static deformation of an anisotropic medium

The problem of two-dimensional static deformation of a monoclinic elastic medium has been studied using the eigenvalue method, following a Fourier transform. We have obtained expressions for displacements and stresses for the medium in the transformed domain. As an application of the above theory, the particular case of a normal line-load acting inside an orthotropic elastic half-space has been considered in detail and closed form expressions for the displacements and stresses are obtained. Further, the results for the displacements for a transversely isotropic as well as for an isotropic medium have also been derived in the closed form. The use of matrix notation is straightforward and avoids unwieldy mathematical expressions. To examine the effect of anisotropy, variations of dimensionless displacements for an orthotropic, transversely isotropic and isotropic elastic medium have been compared numerically and it is found that anisotropy affects the deformation significantly.     

Effect of high pressure on interdiffusion in an Al-Mg alloy

Interdiffusion in the Al?4.06 at % Mg alloy has been investigated under high pressure in the temperature range 690 to 877 K. The diffusion coefficients decreased with pressure. The activation volume and energy of interdiffusion were determined.     

Microstructure evolution of Al-Mg alloy during solidification under high pressure

The microstructure of Al-21.6Mg alloy solidified under high pressure was investigated. The results show that the amount of β-Al₃Mg₂ phase decreases with increasing pressure and a supersaturated Al(Mg) solid solution is formed under 2 GPa. The distribution of Mg in the solid solution is inhomogeneous, causing peak asymmetry of the XRD patterns under high pressures. The Mg concentration in the interdendritic region extends up to about 30 at.%, which is higher than that in the dendrite. Besides solution treatment, mechanical alloying, mechanical deformation of pre-alloy ingots and rapid solidification, solidification under high pressure is proved to be a new way to prepare supersaturated Al(Mg) solid solution.     

Synthesis of cubic boron nitride from rhombohedral form under high static pressure

The cubic, zincblende-type boron nitride (z-BN) has been synthesized from the rhombohedral form (r-BN) under high static pressures greater than 6 GPa without any planned addition of catalysts. The process of forming z-BN has been delineated from isobaric and isothermal series of data. At 6GPa, r-BN begins conversion to the graphite-type form (g-BN) upon heating to 600 °C. This conversion terminates at 1200 °C forming single-phase g-BN, which in turn transforms into z-BN at temperatures higher than 1300 °C. The appearance of z-BN occurs at lower temperatures when the pressure is raised to 7 or 8 GPa. At pressures beyond 10 GPa the wurtzite-type form (w-BN) is observed between 400 and 1200 °C, whereas z-BN is formed above 1000 °C. The boundary of pressure-temperature conditions for synthesizing z-BN from r-BN runs through 6GPa and 1300 °C, and is located near to the lowest bound hitherto known for non-catalytic z-BN synthesis from g-BN.     

Crack kinking under high pressure in an elastic-plastic material

Directional crack growth criteria in compressed elastic–plastic materials are considered. The conditions at the crack tip are evaluated for a straight stationary crack. Remote load is a combined hydrostatic stress and pure shear, applied via a boundary layer assuming small scale yielding. Strains and deformations are assumed to be small. Different candidates for crack path criteria are examined. Maximum non-negative hoop stress to judge the risk of mode I and maximum shear stress for mode II extension of the crack are examined in some detail. Crack surfaces in contact are assumed to develop Coulumb friction from the very beginning. Hence, a condition of slip occurs throughout the crack faces. The plane in which the crack extends is calculated using a finite element method. Slip-line solutions are derived for comparison with the numerically computed asymptotic field. An excellent agreement between numerical and analytical solutions is found. The agreement is good in the region from the crack tip to around halfway to the elastic–plastic boundary. The relation between friction stress and yield stress is varied. The crack is found to extend in a direction straight ahead in shear mode for sufficiently high compressive pressure. At a limit pressure a kink is formed at a finite angle to the crack plane. For lower pressures the crack extends via a kink forming an angle to the parent crack plane that increases with decreasing pressure.     

Microstructure features of polycrystalline diamond synthesized directly from graphite under static high pressure

Recently, ultra-hard polycrystalline diamond was synthesized from graphite by direct conversion under static high pressure. This paper describes the microstructure features of thus formed polycrystalline diamond. Transmission electron microscopy and electron diffraction have revealed that the polycrystalline diamond has a mixed texture of a homogeneous fine structure and a lamellar structure. The former structure consists of fine-grained diamond particles of several tens of nanometers across, which are randomly oriented. The latter structure has bending diamond layers, which may reflect deformed shapes of locally layered graphite of starting material. The experimental results suggest that diamond particles in the homogeneous fine structure are transformed from graphite in the diffusion process, while diamond layers in the lamellar structure are formed in the martensitic process from graphite via the hexagonal diamond phase. It is also noted that significant grain growth occurred at a high temperature of 2700°C, and the lamellar structure was segmentalized to form new grain boundaries.     

Deformation and fracture of an amorphous metallic alloy at high pressure

The influence of hydrostatic pressure ( 6.5 kbar) on the stress for plastic flow in a Pd_77.5Cu₆Si_16.5 amorphous metallic alloy in compression and tension has been examined. The observed effect (ln/P - 5×10^–6 bar^–1) is very close to that exhibited by crystalline metals. The highly inhomogeneous nature of the deformation appears to be unaltered by pressure. As at one atmosphere, failure in tension with high superposed pressure occurs by rupture through a zone of intense plastic shear. The fracture surface topography is strikingly different, however, because cracking inside the shear zone is suppressed in favour of crack initiation at its periphery.     

Electron-beam rapid quenching of an ultra high-strength alloy steel

Rapid quenching using electron beams has been used to treat the surface layers of an ultra high-strength alloy steel. The microstructure of the treated surface layers has been investigated by optical, scanning and electron microscopy and by X-ray diffractometry. The microstructure produced by conventional solid state quenching of the same steel has also been examined for comparison. This microstructural study shows that the rapid quenching process leads to a high degree of grain refinement and an increase in solid solubility which, in turn, increases the amount of retained austenite. The lowering ofM _s temperature due to the high cooling rate and the increased solid solubility favour the formation of twinned martensite. These interlinked phenomena have increased the microhardness of the rapidly quenched layer considerably, with respect to that of the solid state quenched steel.     

Electrical resistance behaviour of Fe-24 wt.%Mn alloy under high pressure

Electrical resistance behaviour of Fe-24 wt.%Mn SMA was studied up to a pressure of ∼6 GPa by using an opposed anvil high pressure device. The system shows a steep rise in resistance up to ∼1 GPa and thereafter a monotonic decrease up to ∼6 GPa during the forward cycle, whereas it shows a monotonic increase during the return cycle. XRD studies of the as-prepared and pressure quenched samples show a mixed α, γ and ε phase in the former and a predominantly ε phase in the latter, indicative of a possible structural transition at ∼1 GPa, as evidenced from the resistance maximum. The decrease in the transition pressure, when compared with alloys of lower Mn concentration, provides a clue that it should be possible to further reduce the transition pressure to the predominantly ε phase by alloying with suitable elements, which may have positive effect on the shape memory of the alloy.     

Homogenization of hardness distribution and distortion in high pressure gas quenching

Quenching with gases rather than oil or other liquid media has the advantages of reducing the risks concerning health and environment, while simultaneously homogenizing the quenching results and minimizing distortion due to a wide range of possible process parameter variations and the pure convective heat transfer. In this contribution, a coupled solution for increasing homogenization of quenching results within high pressure gas quenching will be presented. In the first stage, an experimental test facility was set up for flow investigations and in the second stage a numerical simulation model was generated. The numerical and experimental results of the flow through the chamber were compared for several boundary conditions. Finally, after complete verification of the simulation, the model may be used to assist in parameter variation for optimization of homogeneous high pressure gas quenching.