Robert F. Cochrane

Bio: Robert F. Cochrane is an academic researcher from University of Leeds. The author has contributed to research in topics: Eutectic system & Dendrite (crystal). The author has an hindex of 24, co-authored 76 publications receiving 2235 citations.

Grain refinement and the stability of dendrites growing into undercooled pure metals and alloys

Abstract: We present an analysis of the stability of a dendrite against a small perturbation to the tip velocity. We find that dendritic growth in pure metals and alloys will become unstable above some upper critical undercooling ΔT2*. In alloys above a critical concentration, dendritic growth may also become unstable below a lower critical undercooling, ΔT1*. In the example systems studied, Ni–Cu and Ag–O, the location of these unstable regions shows remarkably close agreement with the onset of spontaneous grain refinement. We obtain values of ΔT2* for Ni and Ag of 195 K and 160 K, respectively, in good agreement with the observed values of 170 K and 133–153 K.

Estimation of Cooling Rates During Close-Coupled Gas Atomization Using Secondary Dendrite Arm Spacing Measurement

Abstract: Al-4 wt pct Cu alloy has been gas atomized using a commercial close-coupled gas-atomization system. The resulting metal powders have been sieved into six size fractions, and the SDAS has been determined using electron microscopy. Cooling rates for the powders have been estimated using a range of published conversion factors for Al-Cu alloy, with reasonable agreement being found between sources. We find that cooling rates are very low relative to those often quoted for gas-atomized powders, of the order of 104 K s−1 for sub-38 µm powders. We believe that a number of numerical studies of gas atomization have overestimated the cooling rate during solidification, probably as a consequence of overestimating the differential velocity between the gas and the particles. From the cooling rates measured in the current study, we estimate that such velocities are unlikely to exceed 20 m s−1.

Growth velocity-undercooling relationships and microstructural evolution in undercooled Ge and dilute Ge-Fe alloys

Abstract: A melt encasement (fluxing) technique has been used to systematically study the velocity-undercooling relationship in samples of pure Ge and Ge doped with 001 at % Fe at undercoolings up to 300 K The apparatus was designed such that it was possible to view the sample throughout the experiment, allowing solidification velocity measurements to be made These velocity measurements were subsequently correlated with the as-solidified microstructure From a combination of growth velocity measurements and microstructural characterisation it was possible to identify a change in growth morphology from faceted to non-faceted growth in both the pure metal and the dilute alloy This transition occurred at a lower undercooling in the dilute alloy (ΔT > 150 K) than in the pure metal (ΔT > 170 K) Spontaneous grain refinement was also observed at ΔT > 210 K in Ge-Fe and at ΔT > 270 K in pure Ge These transitions are discussed and a mechanisms for the change in growth morphology with small amounts of impurity is suggested

Metal Additive Manufacturing: A Review of Mechanical Properties

Abstract: This article reviews published data on the mechanical properties of additively manufactured metallic materials. The additive manufacturing techniques utilized to generate samples covered in this review include powder bed fusion (e.g., EBM, SLM, DMLS) and directed energy deposition (e.g., LENS, EBF3). Although only a limited number of metallic alloy systems are currently available for additive manufacturing (e.g., Ti-6Al-4V, TiAl, stainless steel, Inconel 625/718, and Al-Si-10Mg), the bulk of the published mechanical properties information has been generated on Ti-6Al-4V. However, summary tables for published mechanical properties and/or key figures are included for each of the alloys listed above, grouped by the additive technique used to generate the data. Published values for mechanical properties obtained from hardness, tension/compression, fracture toughness, fatigue crack growth, and high cycle fatigue are included for as-built, heat-treated, and/or HIP conditions, when available. The effects of test...