A.L. Greer

Modelling of inoculation of metallic melts: application to grain refinement of aluminium by Al–Ti–B

A numerical model is presented for the prediction of grain size in inoculated castings and is tested against measured grain sizes obtained in standard grain-refiner tests on aluminium alloys. It is shown that for potent nucleants, such as commercial grain refiners for aluminium, the nucleation stage itself is not the controlling factor. The number of grains is determined by a free-growth condition in which a grain grows from a refiner particle at an undercooling inversely proportional to the particle diameter. With measured particle size distributions as input, the model makes quantitatively correct predictions for grain size and its variation with refiner addition level, cooling rate and melt composition. The model can assist in optimizing the use of existing refiners and in developing improved refiners.

Unusual room-temperature compressive plasticity in nanocrystal-toughened bulk copper-zirconium glass

Cast Cu50Zr50 alloy rods with a diameter of 1 mm have been found to consist of a glassy phase containing fine crystalline particles with a size of about 5 nm. They have a glass transition temperature T g of 675 K, and a large supercooled-liquid region extending 57 K above T g. The rods exhibit a high yield strength of 1860 MPa and a Youngs modulus of 104 GPa. Because they contain a dispersion of embedded nanocrystals, the as-cast bulk metallic glass rods can sustain a compressive plastic strain at room temperature of more than 50%, an exceptional value which is explicable by compensation of any shear softening by nanocrystal coalescence and pinning of shear bands.

Mechanical properties of Fe-based bulk glassy alloys in Fe-B-Si-Nb and Fe-Ga-P-C-B-Si systems

Mechanical properties of cast Fe-based bulk glassy alloy rods with compositions of (Fe 0 . 7 5 B 0 . 1 5 Si 0 . 1 ) 9 6 Nb 4 and Fe 7 7 Ga 3 P 9 . 5 C 4 B 4 Si 2 . 5 were examined by compression and Vickers hardness tests. The Youngs modulus (E), yield strength (σ y ), fracture strength (σ f ), elastic strain (∈ e ), fracture strain (∈ f ), and Vickers hardness (H v ) were 175 GPa, 3165 MPa, 3250 MPa, 1.8%, 2.2%, and 1060, respectively, for the former alloy and 182 GPa, 2980 MPa, 3160 MPa, 1.9%, 2.2%, and 870, respectively, for the latter alloy. The σ f /E and H v /3E were 0.019-0.017 and 0.020-0.016, respectively, for the alloys, in agreement with the previous data for a number of bulk glassy alloys. The agreement suggests that these Fe-based bulk glassy alloys have an elastic-plastic deformation mode. The syntheses of high-strength Fe-based bulk glassy alloys with distinct compressive plastic strain and elastic-plastic deformation mode are encouraging for future development of Fe-based bulk glassy alloys as structural and soft magnetic materials.

Partially or fully devitrified alloys for mechanical properties

The origins of the mechanical properties of partially or fully devitrified alloys are discussed with particular reference to hardness and wear resistance. Quantitative evidence is presented that nanophase composites, consisting of a dispersion of nanometre-scale crystallites in a glassy matrix, show a novel hardening mechanism based on solute enrichment of the glassy matrix. The importance of microstructural scale is considered, particularly for wear resistance. Focusing on aluminium alloys, the novel nanophase composites and the glassy alloys from which they are derived are compared with conventional high-strength alloys.

Differential scanning calorimetry study of solid‐state amorphization in multilayer thin‐film Ni/Zr

Differential scanning calorimetry (DSC) has been used to study solid‐state amorphization and subsequent crystallization in sputtered multilayer Ni/Zr thin films. Initial results provide quantitative information about the thermodynamics and kinetics of these processes. An analysis of DSC data enables the activation energy and pre‐exponential factor for interdiffusion of Ni and Zr in a‐NiZr to be found.

Micro structure and hardening of Al-based nanophase composites

Abstract Part amorphous-part crystalline Al-Ni-Y alloys can be produced by devitrification of melt-spun fully amorphous alloys. These materials have a novel structure of nanometre-sized crystals of α-A1 in an amorphous matrix and can be regarded as nanophase composites. Devitrified microstructures are studied by X-ray diffraction and transmission electron microscopy. The significant hardening accompanying devitrification is characterised for various heat treatments, and is mainly attributed to the solute enrichment of the remaining amorphous phase. The hardening kinetics are, however, also influenced by the composition uniformity of amorphous matrix.

Shear band melting and serrated flow in metallic glasses

Scanning electron microscopy observations of shear steps on Zr-based bulk metallic glasses show direct evidence of shear band melting due to heat generated by elastic energy release. The estimated range of attained temperatures and the observed morphologies are consistent with shear steps forming at a subsonic speed limited by a required redistribution of local microscopic stresses. The calculations indicate that a 0.2μm layer melts in the vicinity of a shear band forming a 1μm shear step. The plastic part of the stress strain curve is serrated but a majority of shear events are not associated to serrations.

Solid-state intermetallic phase tranformations in 3XXX aluminium alloys

Abstract During homogenisation of direct chill cast ingots of Al alloys for beverage-can bodies, the degree of transformation of Al6(Fe,Mn) phase to α-Al-(Fe,Mn)-Si phase is important for subsequent processing. The morphology of the transformation and the associated local composition changes are studied using a range of electron microscope techniques, including energy-filtered transmission electron microscopy, convergent-beam electron diffraction and field-emission-gun scanning electron microscopy. Nucleation of the α-phase controls the rate of the transformation, which proceeds by eutectoid decomposition of the Al6(Fe,Mn) phase into α-phase and aluminium solid solution. The transformation requires intake of silicon from the surrounding matrix; intake of manganese is not required but does occur on continued annealing. A basic description is given of the transformation kinetics.

Nanocrystallization in Al-based amorphous alloys

Abstract Al-Ni-Y alloys are melt spun into a fully amorphous state and then annealed to obtain a nanometre-scale dispersion of a-Al crystallites in an amorphous matrix. The isothermal kinetics of the crystallization are analysed. The significant hardening effect of nanocrystallization is studied for several alloy compositions. It is attributed mainly to the solute enrichment of the remaining amorphous phase.

Diffusional aspects of the solid state amorphization reaction

Abstract Diffusional aspects of the solid state amorphization reaction (SSAR), specifically in early transition metal-late transition metal (ETM-LTM) systems, are considered. Self- and tracer diffusivities of ETM and LTM components in crystalline and amorphous phases are reviewed. The alloy characteristics leading to fast diffusion of LTM species and to good glass-forming ability are described. The nucleation of the amorphous phase in the SSAR is analysed, drawing parallels with interstitial-substitutional diffusion in semiconductors. Interdiffusion in amorphous ETM-LTM phases is considered in detail, taking into account stress effects, structural relaxation and the mixing thermodynamics of the amorphous phase.