A comparative study of the Al-Al(3)Ti and Cd-Cd(3)Ag (epsilon) peritectic systems

Abstract

The thesis describes an investigation of the solidification behaviour of two peritectic alloy systems, chosen to permit observation of the differences between systems with faceted and non-faceted primary phases. The Al-Al3Ti system has a faceted primary phase (Al3Ti) and was studied in the range 0-5 wt. % Ti; in the Cd-Cd3Ag system the primary e phase (Cd3Ag) has a wide range of solid solution and grows dendritically. It was studied in the range 0-20 wt. % Cd.Controlled unidirectional solidification was used to investigate some theoretical aspects of peritectic solidification but this necessitated studies of particular aspects of the behaviour of each alloy which are of intrinsic interest. The first part of the thesis deals with these aspects in separate sections dealing with metallography and tensile properties of both systems, the crystallography and nucleating ability of Al3Ti and the thermal stability of Al-Al3Ti microstructures. Material from these chapters is used in the second part of the thesis dealing with peritectic transformation theory in relation to unidirectional solidification, thermal analysis and nucleation.The metallographic studies of unidirectionally grown samples showed that the plate-like primary Al3Ti and the dendritic primary Cd3Ag tended to grow parallel to the imposed growth direction but that their orientation became increasingly random at higher growth rates. In low titanium content alloys grown at high rates the primary Al3Ti tended to form equi-axed particles of star and petal-like form. The plate-like Al3Ti grew as two-dimensional dendrites in the (001) plane with (hko) growth directions and faceting on all (100) planes and also exhibited foliated growth of new dendrite layers in the (001) direction. The petal-like Al3Ti particles formed with 4 to 12 petals each of which grew in a similar manner to the plate-like particles. It was found that each petal could act as a separate nucleant for a (Al), thus providing an explanation for the grain-refining ability of Al3Ti.The tensile tests showed that plate-like Al3Ti lowered the ultimate tensile strength and toughness to a value which was constant regardless of Ti content and growth rate. The Cd-Cd3Ag alloys strained in the manner of fibremreinforced composites exhibiting two elastic regions.Distinction is made between the peritectic reaction and. the peritectic transformation. The peritectic reaction refers to the initial envelopment of the primary phase (b) by a film of peritectic product (a) after nucleation of a on the b surface. The reaction is self-stifling and accounts for a negligible proportion of the total a product. The peritectic transformation is the subsequent diffusion-limited thickening of the a envelope, which is usually anisothermal. The a phase can also form by direct crystallisation from the melt as temperature falls and it is difficult to distinguish metallographically between this process and the peritectic transformation, which involves dissolution of the primary (b) phase. A diffusion analysis was carried out to calculate the relative rates of formation of a by the peritectic transformation and by direct crystallisation from the melt. It was found that in the Al-Al3Ti system the a is formed principally by direct crystallisation, without attack on the b phase. In the Cd-Cd3Ag system the peritectic transformation goes almost to completion and direct crystallisation from the melt plays a minor part.The solute distribution at quenched interfaces of Al-Al3Ti and Cd-Cd3Ag alloys which have been unidirectionally solidified was analysed and a revised theory of unidirectional solidification of peritectic alloys is presented. Thermal analysis of Cd-Ag and Zn-Cu alloys showed substantial superheat of the peritectic arrest temperature at high cooling rates, as previously observed for Al-Al3Ti. Nucleation of the peritectic product above the equilibrium peritectic temperature appeared to be a normal occurrence.