Fracture Toughness, Structural Alloys

R.M. Hemphill, D.E. Wert, High strength, high fracture toughness structural alloy, US Patent 5 087 415, U.S. Patent and Trademark Office, Washington, DC. 

The intermetallic alloy NiAl is discussed as a potential base alloy for high temperature structural materials. Its use is currently limited by low room temperature ductility and fracture toughness. Consequently, substantial research efforts have been directed towards understanding its mechanical behaviour [1, 2] so that detailed experimental [3, 4, 5] and theoretical [6, 7, 8] analyses of the deformation of NiAl are available today. 

Material factors. The main metallurgical properties of importance are alloy composition, distribution of alloying elements and impurities, microstructure and crystal structure, heat treatment, mechanical working, preferred orientation of grains and grain boundaries (texture), mechanical properties (strength, fracture toughness, etc.).31... 

It is clear that structure-sensitive dynamic properties [low-cycle fatigue (LCF) and fracture toughness (Klc)] notably increase for the alloy with the nondendritic structure. One more proof of the advantage of the nondendritic structure are the results on mechanical properties measured in three directions of forgings from a 1933 grade alloy obtained from large-scale ingots of 845 mm in diameter (Table 16). 

Moreover, it is evident that the intrinsic mechanical strength and fracture toughness of such nanoporous structures is low [55, 65]. Insofar, gross dealloying represents a serious form of corrosive deterioration. In addition to this, there is little doubt with respect to an association between dealloying and the stress corrosion cracking of certain alloys. 

For structural alloys, the first-year program includes fatigue tests on alloys 21/6/9, AISI 304, and 316 initiation of multiple product and heat tensile tests on alloys AISI 304 and 316 and tensile and fracture toughness tests of a commercial heat of alloy 21/6/9. All tests will be performed at 300, 76, and 4K. In alloy 21/6/9, the fracture toughness at 4 K is marginal compared with AISI 304 and 316 therefore, more tests than are presently available are required to reliably describe the fracture behavior. 

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