Robert F. Cochrane

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

A melt encasement (fluxing) technique has been used to systematically study the velocity-undercooling relationship in samples of pure Ge and Ge doped with 0.01 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.

Microstructural evolution and growth velocity-undercooling relationships in the systems Cu, Cu-O and Cu-Sn at high undercooling

A melt encasement (fluxing) technique has been used to systematically study the velocity-undercooling relationship in samples of Cu and Cu-O and Cu-3 wt% Sn at undercoolings up to 250 K. In pure Cu the solidification velocity increased smoothly with undercooling up to a maximum of 97 m s-1. No evidence of grain refinement was found in any of the as-solidified samples. However, in Cu doped with >200 ppm O we found that samples undercooled by more than 190 K had a grain refined microstructure and that this corresponded with a clear discontinuity in the velocity-undercooling curve. Microstructural evidence in these samples is indicative of dendritic fragmentation having occurred. In Cu-Sn grain refinement was observed at the highest undercoolings (greater than 190 K in Cu-3 wt% Sn) but without the spherical substructure seen to accompany grain refinement in Cu-O alloys. Microstructural analysis using light microscopy, texture analysis and microhardness measurements reveals that recrystallisation accompanies the grain refinement at high undercoolings. Furthermore, at undercoolings between 110 K and 190 K, a high density of subgrains are seen within the microstructure which indicate the occurrence of recovery, a phenomenon previously unreported in samples solidified from highly undercooled melts.

Mechanical deformation of dendrites by fluid flow during the solidification of undercooled melts

Abstract Mechanical interactions between growing dendrites and their parent melt are normally considered to be of little significance. During conventional solidification processing this is undoubtedly true. However, during the solidification of undercooled melts the twin conditions required to produce mechanical damage to dendrites, high flow velocities and very fine dendrites, may exist. This is most likely in strongly partitioning alloy systems where the tip radius experiences a local minimum at undercoolings in the range of 50–100 K. In this paper we present a model for the skin stress resulting from fluid flow around a family of realistically shaped dendrites. We find that within a narrow undercooling range about the minimum in the tip radius, mechanical deformation of the growing dendrite is likely. Experimental evidence is presented from the Cu–3wt%Sn and Cu–O alloy systems that appear to show evidence of extensively deformed dendritic structures consistent with mechanical damage. Other mechanisms for causing dendritic bending during growth are considered and shown to be unlikely in this case.

A phase field model for spontaneous grain refinement in deeply undercooled metallic melts

The origin of spontaneous grain refinement in deeply undercooled metallic melts has been of enduring interest within the solidification literature. Here we present the results of phase field simulations of dendritic growth into pure undercooled melts, at growth velocities up to 35 m s−1. We find that, at low growth velocities, dendrite morphologies are broadly self-similar with increasing growth velocity. However, above ≈15 m s−1 the initiation of side-branching moves closer to the dendrite tip with increasing growth velocity. This appears to be related to the level of kinetic undercooling at the tip. Once side-branch initiation begins to occur within 1–2 radii of the tip, profound morphological changes occur, leading to severe thinning of the dendrite trunk and ultimately repeated multiple tip-splitting. This process can be invoked to explain many of the observed features of spontaneous grain refinement in deeply undercooled metallic melts.

Effects of Ni addition on liquid phase separation of Cu–Co alloys

Abstract This article deals with the effects of Ni addition on liquid phase separation of Cu–Co alloys. The results reveal that Ni addition can partly restrain the liquid phase separation of Cu–Co alloys, resulting in a decrease of the volume fraction for the Co-rich particles from the liquid phase separation and in refined microstructures. The composition analysis indicates that Ni is dissolved in both the Co-rich and the Cu-rich phases, but the Ni content in the Co-rich phase is much higher than that in the Cu matrix. The decreasing amount of Co-rich particles from the liquid phase separation and the refined microstructure are responsible for the GMR improvement of Cu–Co alloys with Ni addition.

Liquid phase separation in melt-spun Cu70Co30 ribbon

Abstract Due to the large heat of mixing in the Cu-Co system, liquid phase separation is inevitable in Cu 70 Co 30 ribbons during the course of the quenching process. The resulting large Co-rich particles exhibit either elongated or roughly spherical morphology with a distinct Cu-rich shell. The large Co-rich particles are about 100–300 nm and the shell is about 15–40 nm wide. On annealing, diffusion of Cu and Co through the shell region results in a broadening and eventual disappearance of the shell region. A maximum magnetoresistance of 22% results after annealing at 723 K for 1 h, and more investigations are necessary to clarify the contribution of these large Co-rich particles to the observed magnetoresistance.

Highly undercooled germanium: Growth velocity measurements and micro structural analysis

Abstract Growth velocities have been measured in samples of Ge (3–5 mm in diameter) which were undercooled using a fluxing technique. Samples of Ge were heated and undercooled in a viscous soda-lime glass flux contained within thin-walled, clean silica crucibles. Using this particular method, undercoolings of up to 250 K below equilibrium melting temperature were obtained. This maximum undercooling may have been restricted by sample size, experimental conditions or impurities present in the sample. It was possible to determine growth velocities up to undercoolings of 250 K The size of the samples used allowed growth velocities measurements to be made, by measuring recalescence times using a linear photo-diode array. These were measured at a variety of undercoolings, where nucleation was initiated using a thin alumina needle, and a maximum velocity of 7.6 ms −1 was recorded at an undercooling of 250 K. Microstructural analysis of samples revealed a range of transitions occurring as the level of undercooling increased. These include a gradual transition from faceted to non-faceted type growth at ΔT > 160 K, characterised by the observance of growth twins a small undercoolings which were not present in samples undercooled greater than 160 K. By transmission electron microscopy (TEM) examination, it was also confirmed that there was no microtwinning present in the highly undercooled samples. A general grain refinement at ΔT> 210 K from a coarse microstructure incorporating an extensive dendritic substructure to a fine structure with an impurity-rich, cross-shape dendrite fragment at the centre of the grains. These transitions could also be associated with trends seen between measured growth velocities and undercoolings.

Spasmodic growth during the rapid solidification of undercooled Ag-Cu eutectic melts

A melt fluxing technique has been used to undercool Ag-Cu eutectic alloy by 10–70 K and the subsequent recalescence has been studied using high speed imaging. Spasmodic growth of the solidification front was observed, in which the growth front would make a series of quasi-periodic jumps separated by extended periods during which time growth appeared to arrest. Evidence of this previously unreported mode of growth is presented. The high speed images and microstructural evidence support the theory that anomalous eutectics form by the growth and subsequent remelting of eutectic dendrites.

GMR and precipitation kinetics in mechanically alloyed Cu90Co10

Abstract We report here the first investigation of GMR in Cu-Co produced by mechanical alloying. In order to optimize the GMR we have monitored the microstructural evolution and kinetics of Co precipitation using in situ resistivity measurements. It has been found that when Cu 90 Co 10 is annealed at 390 °C for 30 minutes the GMR ~20%. This is the largest value so far discovered in mechanically alloyed material.

Effects of rare earth additions on GMR of melt-spun Cu–Co–Ni ribbons

Abstract The effects of Ni and rare earth (La, Ce, Nd, Dy, Tb) co-addition on GMR of the melt-spun Cu–Co alloys are investigated. The results indicate that Ni addition can increase the GMR of the melt-spun Cu–Co ribbons. The reasons are that Ni addition can restrain the liquid phase separation occurring at high temperature and increase the density of the fine magnetic Co-rich precipitates during the annealing process. Light rare earth, such as Ce and La, can further enhance GMR of the melt-spun Cu–Co–Ni ribbons. Among these elements, Ce is the most effective. However, heavy rare earth additions result in a GMR decline of the melt-spun Cu–Co–Ni ribbons. It is discovered that there exists an optimal addition quantity for Ce. Excessive Ce addition could result in a GMR decline as well. The reasons for the GMR improvement of Re addition are: role of purifying melt, promoting precipitates process and enhancing the magnetic susceptibility of the fine Co-rich precipitates.