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I.

INTRODUCTION

S E V E R A L articles in this symposium have dealt with specific aspects of brittleness in intermetallic compounds. Brittleness is manifested as lack o f tensile ductility, low fracture toughness, high notch sensitivity, and in a few cases, extremely rapid fatigue c r a c k growth. Unless these problems can be overcome o r circumvented, the use o f these alloys in structures will be severely hampered. Consequently, a very large effort has been devoted to improving the various measures o f toughness, in many cases utilizing techniques, such as grain refinement, that have been successful with metallic materials. However, the restrictions on slip in intermetallics, the high sensitivity o f intermetallics to small compositional changes, and their extreme susceptibility to hydrogen-containing environments have complicated efforts to improve properties. This article will s u m m a rize the various approaches that have been utilized to address these problems. These include microalloying, macroalloying, microstructural control, and composite toughening (Table I). However, in addition to these major categories, one may cite the use o f hydrostatic pressure and suppression o f environmental embrittlement.

II.

MICROALLOYING

The discovery that small additions o f boron can change the fracture mode and sharply increase the ductility o f Ni3AI marked the beginning o f modern microalloying approaches to intermetallic toughening.tL2] It quickly became apparent that there were significant limitations to the boron effect in Ni3A1. Boron-induced ductile behavior occurs only in a narrow compositional range and only if the aluminum content is substoichiometric, t2~ Further, boron is effective only in finer grained material; cast ingots with coarse grain size could not be reduced by mechanical working. There is continuing controversy as to the role o f boron in improving ductility. [3.4,5l While there N.S. STOLOFF, Professor of Materials Engineering, is with the Rensselaer Polytechnic Institute, T r o y , NY 12180-3590. This article is based on a presentation made in the symposium "QuasiBrittle Fracture" presented during the T M S fall meeting, Cincinnati, OH, October 21-24, 1991, under the auspices of the T M S Mechanical Metallurgy Committee and the A S M / M S D Flow and Fracture Committee.

METALLURGICAL TRANSACTIONS A

is no doubt that boron segregates preferentially to grain boundaries, it is not c l e a r whether there is a direct effect on cohesion o r a facilitation o f slip transmission across boundaries, perhaps through formation o f a very thin disordered layer, t4,51 Moreover, there is not even agreement whether such disordered layers exist and, if so, how t h i c k they are. Boron has now been added to a large n u m b e r o f intermetallics, with the results summarized in Table II. Most recently, 0.1 at. pct B has been shown to slightly improve the ductility o f Ni3Ge.[6] There is no c l e a r pattern distinguishable, except that stoichiometric and hyperstoichiometric L12 alloys receive no benefit fro