Measurement of the Densities of Nickel-Based Ternary, Quaternary and Commercial Alloys

Abstract

The modified sessile drop method (MSDM) and the modified pycnometric method (MPM) have been employed to measure precisely the densities of liquid nickel-based ternary (Ni-Co-Al) and quaternary (Ni-Co-Al-Cr, Ni-Co-Al-Mo, Ni-Cr-Al-Mo) alloys and four commercial superalloys in liquid and mushy states. There was a good agreement between the values measured by the two methods. The measured densities of liquid model alloys and commercial superalloys in liquid and mushy states decreased linearly with increasing temperature for the experimental temperature range. The temperature coefficient of the density of liquid ternary Ni-Co-Al alloys measured in this work can be represented as a quadratic function of the aluminium concentration in the alloys. The density of liquid Ni-Co-Al alloys can be expressed as a function of both temperature and aluminium concentration in the alloys. The recommended equations for the densities of liquid nickel-based ternary and quaternary alloys and four commercial superalloys in liquid and mushy states were obtained by analysing density values measured by both MSDM and MPM. The calculated values from the recommended density equations show good agreement with those measured by both MSDM and MPM. The density (� ) and its temperature dependency (d�= dT) are important input data for simulation of the solidification of Ni-based superalloys, e.g. the prediction of defects such as gas porosity and microsegregation, as well as the evaluation of the convective contribution to heat transport. Accurate experimental values of the densities of nickel and Ni-based model and commercial alloys (including their dependencies on composition and temperature) are needed to develop models to predict the densities of Ni-based superalloys. In order to predict the densities of Ni-based commercial alloys, it is necessary to have an understanding of the effects of the various alloy elements (such as Cr, Co, Ta, W, Al, Mo, Re, Hf, Nb, Ti, etc.) on the densities of liquid nickel alloys. This can be obtained from accurate measurements of the densities of Ni-based binary alloys in relation to the concentration and temperature. Comparisons of the densities of liquid Ni-based ternary alloys with those for the corresponding binary alloys provide useful information on the effects of the second alloy element on the properties of liquid alloys. For some alloy elements, such as Re, their concentrations in the Ni-based superalloys are relatively low but they are very reactive, which may make the accurate density measurement of their Ni-based binary alloys difficult. The effect of these elements on the densities of Ni-based superalloys can be obtained by comparing the densities of liquid Ni-based commercial alloys with those of liquid Ni- based multi-component model alloys with similar chemical compositions to those of the Ni-based commercial alloys. Furthermore, accurate density values of both Ni-based model and commercial alloys can be used to verify the validity of the models developed to predict the densities of liquid Ni- based superalloys. However, there is a large scatter in the reported density values for liquid nickel and its temperature dependence because of the experimental uncertainties associated with the different techniques. Furthermore, there are very few reports on the measurement of the density of Ni-based alloys in relation to the effects of composition and temperature. 1-7) There are no reports on the measurement of the densities of liquid Ni-based multi-component model alloys such as ternary and quaternary alloys. Therefore, in order to develop a model to predict the densities of commercial Ni-based superalloys, we have used two accurate methods, namely, the modified sessile drop method (MSDM) and the modified pycnometric method (MPM) to measure the densities of liquid nickel and Ni-based model alloys and commercial alloys in the liquid state and in the mushy (ie ''liquid-solid coexistence'') state for some alloys. The principles, advantages and experimental proce- dures of the two methods have been described previously. 5-7) The densities of liquid nickel and liquid Ni-Cr, Ni-Co, Ni-Ta, Ni-W and Ni-Al alloys have been measured precisely with both methods, and density values for Ni-Cr and Ni-Al alloys in mushy state have also been determined. 5-7) In this paper, both MSDM and MPM were employed to measure the densities of liquid ternary model alloys (Ni-Co- Al, molar fraction xNi:xCo � 86:14, Al mass% = 0� 11), liquid quaternary model alloys (Ni-Co-Al-Cr, Ni-Co-Al-Mo and Ni-Cr-Al-Mo), and four commercial Ni-based super- alloys in liquid and mushy states. The chemical compositions of these model alloys were similar to those of commercial Ni- based superalloys. The recommended density values for these alloys have been obtained by combining the measured values determined with the two methods.