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On the kinetics of the δ′ (Al3Li) phase precipitation in a rapidly solidified Al–Mn–Li–Zr alloy

Published online by Cambridge University Press:  31 January 2011

J. Baram  and
A. N. Sembira
J. Baram
Affiliation:
Materials Engineering Department, Ben-Gurion University of the Negev, Beer-Sheva, Israel
A. N. Sembira
Affiliation:
Chemical Engineering Department, Ben-Gurion University of the Negev, Beer-Sheva, Israel
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Abstract

The precipitation kinetics of the δ′ (Al3Li) phase in two rapidly solidified samples and one conventionally cast sample of an Al–2.3Li–6.5Mn–0.65Zr (in wt. %) alloy are compared. Following high cooling rates, manganese is retained in solid solution in the aluminum matrix (αAl) up to 6.0 wt.%, far beyond the thermodynamic equilibrium value (0.36 wt.% at 500 °C). Extended solid solution of manganese in aluminum induces strain gradients, similar to those produced by dislocations. The effect of such gradients, the size of which is proportional to the solute atomic fraction, is to enhance lithium precipitation by lowering the activation energy, as observed, and also by affecting the rate parameter. Kinetic thermal analysis has been performed in a series of nonisothermal experiments in the heat flux differential scanning calorimetry (DSC) mode. The precipitation of the δ′ (Al3Li) phase is evidenced by an exothermic peak whose characteristics were analyzed. The rate of transformation (precipitation) is assumed to obey the Johnson–Mehl–Avrami equation. The activation energy for the precipitation process as well as the kinetic rate parameter have been evaluated for the rapidly solidified and the conventionally cast specimens. The activation energy for precipitation is lowered, from 107.0 kJ mol−1 for the conventionally cast material, down to 81.8 kJ mol−1 and 77.0 kJ mol−1 for samples that exhibit manganese solid solubility extensions of 2.10 and 6.00 wt.%, respectively. The rate parameter for the precipitation reaction, which has the generally admitted value of 1.50, for a transformation involving diffusion controlled growth, is affected by the strain gradients, too. Its value is reduced from 1.40 for the slowly cast sample to 1.32 and 1.20, respectively, for the two rapidly solidified samples, as a result of competing mechanisms, namely: growth controlled by diffusion and strain-assisted precipitation.

Type
Articles
Information
Journal of Materials Research , Volume 6 , Issue 1 , January 1991 , pp. 46 - 52
Copyright
Copyright © Materials Research Society 1991

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