Microstructure and Phase Formation in a Rapidly Solidified Laser-Deposited Ni-Cr-B-Si-C Hardfacing Alloy

In this study, microstructural evolutions and phase selection phenomena during laser deposition of a hardfacing Ni-Cr-B-Si-C alloy at different processing conditions are experimentally investigated. The results show that even minor variations in the thermal conditions during solidification can modify the type and morphology of the phases. Higher undercoolings obtained at faster cooling rates suppressed the primary borides and encouraged floret-shape mixtures of Ni and Cr5B3 via a metastable reaction. Variations in the boride phases are discussed in terms of nucleation-and growth-controlled phase selection mechanisms. These selection processes also influenced the nature and proportion of the Ni-B-Si eutectics by changing the amount of the boron available for the final eutectic reactions. The results of this work emphasize the importance of controlling the cooling rate during deposition of these industrially important alloys using laser beam or other rapid solidification techniques. (C) The Minerals, Metals & Materials Society and ASM International 201

The Stability of Al11Sm3 (Al4Sm) Phases in the Al-Sm Binary System

The relative stability of Al11Sm3 (Al4Sm) intermetallic phases was experimentally investigated through a series of heat treatments followed by microstructural, microchemical, and X-ray diffraction (XRD) analyses. The principal findings are that the high-temperature tetragonal phase is stable from 1655 to 1333 K and that the low-temperature orthorhombic phases, α and γ, have no range of full stability but are metastable with respect to the crystalline Al and Sm reference states down to 0 K. Thermodynamic modeling is used to describe the relative energetics of stable and metastable phases along with the associated two-phase mixtures. Issues regarding transition energetics and kinetics are discussed

Modeling of Thermodynamic Properties and Phase Equilibria for the Al-Sm Binary System

The thermodynamic properties and associated phase equilibria for the Al-Sm binary system are examined, and experimental results regarding the stability of the Al3Sm, Al11Sm3, and Al4Sm intermetallics are incorporated. In the analysis presented, the liquid phase is described using a three-species association model, the intermediate phases are treated as stoichiometric compounds, and the terminal phases are treated as solid solutions with a single sublattice model. In addition to the stable phases, thermodynamic descriptions of the metastable Al11Sm3-α and Al4Sm-γ phases are employed, and both stable and metastable phase equilibria are presented over the full composition range, providing a general model, which is consistent with available experimental data. Metastable liquidus curves are examined with respect to the observed crystallization behavior of amorphous Al-Sm alloys

The Influence of the effect of solute on the thermodynamic driving force on grain refinement of Al alloys

Grain refinement is known to be strongly affected by the solute in cast alloys. Addition of some solute can reduce grain size considerably while others have a limited effect. This is usually attributed to the constitutional supercooling which is quantified by the growth restriction factor, Q. However, one factor that has not been considered is whether different solutes have differing effects on the thermodynamic driving force for solidification. This paper reveals that addition of solute reduces the driving force for solidification for a given undercooling, and that for a particular Q value, it is reduced more substantially when adding eutectic-forming solutes than peritectic-forming elements. Therefore, compared with the eutectic-forming solutes, addition of peritectic-forming solutes into Al alloys not only possesses a higher initial nucleation rate resulted from the larger thermodynamic driving force for solidification, but also promotes nucleation within the constitutionally supercooled zone during growth. As subsequent nucleation can occur at smaller constitutional supercoolings for peritectic-forming elements, a smaller grain size is thus produced. The very small constitutional supercooling required to trigger subsequent nucleation in alloys containing Ti is considered as a major contributor to its extraordinary grain refining efficiency in cast Al alloys even without the deliberate addition of inoculants.The Australian Research Council (ARC DP10955737)

GRAIN GROWTH IN UNSTRAINED GLACIAL ICE

We use theories for grain growth driven by surface tension and curvature of grain boundaries to explain published data on grain growth in cold (⩽-10°C) glacial ice that is not deforming rapidly. Among other results, we propose that small grain sizes in Wisconsinan ice are caused by large concentrations of soluble impurities. Major observations that we seek to explain are Gow, (1), (2) ; Gow and Williamson, (3) ; Duval and Lorius, (4) ; and Alley et al., (5) (i) in most post-Wisconsinan ice and firn, the average cross-sectional area of grains increases linearly with time. The rate of increase is nearly the same in firn and ice, although it may be slightly less in ice ; (ii) Grain sizes are smaller in ice rich in volcanic tephra than in adjacent, clean ice ; (iii) Bubbles, which form on grain boundaries at the firn-ice transition, separate from grain boundaries in a discrete depth interval below the transition ; and (iv) Grain size decreases downward across the firn-ice transition. When grain growth is driven by grain-boundary curvature and surface tension, grain-growth theory (reviewed by Higgins, (6)) predicts that the average cross-sectional area of grains in pure, fully consolidated materials will increase linearly with time. Bubbles, inert second-phase particles (microparticles), and dissolved impurities can slow grain growth and cause deviation from this linear area-time relation. For typical glacial ice, including Wisconsinan ice, we calculate that microparticles are too sparse to affect grain growth significantly, a conclusion reached previously by Duval and Lorius (4). For tephra-rich layers such as those in the Byrd core, however, we calculate that microparticles slow grain growth significantly, in accord with observations (Gow and Williamson, (3)) ; impurity drag and strain probably are important in the Byrd tephra layers also. Model calculations show that compression of bubbles deeper than the firn-ice transition reduces their mobility below that of grain boundaries so that bubble-boundary separation should occur ; such separation is observed (Gow and Williamson, (3). Observed bubble-boundary separation requires about 10 % of the driving force for grain growth and should reduce growth rates by about 10 %. Such a change would be largely masked by observational error, but may be observed. The downward decrease in grain size across the Holocene-Wisconsinan boundary correlates with a downward increase in soluble impurities, especially NaCl (Petit et al., (7)). The correlation has the form expected if the grain-size decrease is caused by the impurity increase. Furthermore, the interaction energy between grain boundaries and Nacl required to cause the grain-size decrease is of the same order as our best estimate of this interaction energy. We thus propose that the small grain sizes in Wisconsinan ice are caused by high concentrations of NaCl and possibly other soluble impurities