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I.

INTRODUCTION

TIlE A1-Li alloys represent a new class of light weight, high modulus, high strength, monolithic structural materials. In spite of such attractive properties, these alloys could not be exploited for commercial applications because of their poor ductility, toughness, and high-temperature stability.V] Only during the last decade have an increasing number of research programs been devoted to overcoming these shortcomings. As a result, now a few A1-Li alloys have been successfully employed in aerospace applications.t21 Earlier studies on the monolithic and cyclic deformation characteristics of binary AI-Li alloys showed that the initial problems of low ductility and toughness could be traced to the inhomogeneous slip and strain localization. This resuited from coherent particle hardening by metastable, ordered 6' (A13Li) precipitates. In addition, nucleation and growth of equilibrium (A1-Li) precipitates at the grain boundaries caused the formation of precipitate-free zones (PFZs). This further localized strain could result in premature intergranular failure.13] Such premature failure could also result from cavitation which occurs during deformation. Therefore, potential cavitation sites, namely, coarse rigid inclusions (stringers) and porosities, must be kept to a minimum in the material.t41 Deformation and failure behavior of a material depend on test temperature. The flow stress (or) at a given temperature is related to strain (e) and strain rate (k) by an equation of the type 151 or oc ea k,~

[1]

where the exponent a (conventionally denoted by n) is the

A. THAKUR, formerly B. Tech. Final Year Student, Department of Metallurgical Engineering and Materials Science, Indian Institute of Technology, Bombay, is Graduate Student, Department of Mechanical and Industrial Engineering, University of Manitoba, Winnipeg, MB, Canada, R3T 5V6. B.P. KASHYAP, Professor, is with the Department of Metallurgical Engineering and Materials Science, Indian Institute of Technology, Bombay 400 076, India. M.K. MALIK, Head of Metallic Section, is with the Atomic Fuels Division, Bhabha Atomic Research Centre, Bombay 400 085, India. Manuscript submitted September 29, 1994. 2274~VOLUME 27A. AUGUST 1996

strain-hardening coefficient and m is the strain rate sensitivity index. At low temperatures, m is very small and its value reduces to zero in the intermediate temperature range.t6] During dynamic strain aging in solid solution alloys, m in the latter temperature range even becomes negative, tTJ In the high-temperature region, m increases, approaching the maximum value of one under the conditions of diffusion creep, whereas the value of a becomes zero. The value of a increases with the decrease in temperature. The exponents a and m reflect deformability of a material as they relate to ductility. During high-temperature deformation, the flow stress depends upon ~, test temperature (T in kelvin), and grain size (d) according to the following constitutive relationship:iS] ADoEb

=

kT

or"

(df ( E )

exp(

)

[21

Here, D Oex