Compositional effects on the creep ductility of a low alloy steel
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THE
creep fracture of metals, subjected to tensile stresses at the temperatures often encountered in commercial usage (roughly 1/3 to 2/3 of the melting temperature of the material), occurs by two broad mechanisms, both of which involve the nucleation, growth and coalescence of cavities. At high stresses, failure is transgranular. Voids nucleate and grow by plasticity within grains and produce a ductile failure. At low stresses, failure is intergranular. Cavities are nucleated on grain boundaries and grow primarily by diffusion. Although plasticity may play a role here, especially during cavity coalescence, grain boundary failure is generally accompanied by low ductility. This onset of brittle, intergranular failure at low stresses (and consequently long failure times) is common to all metals and their alloys, as illustrated in a recent review of fracture mechanisms by Ashby et al. 1 The conditions of stress and temperature at which this low ductility failure occurs depends on the internal structure of the alloy as determined by composition and heat treatment. Increases in creep strength, which can be achieved by alloying with solution or precipitation hardening elements, increase the maximum stress at which intergranular failure is found. Alternatively, elements which segregate to internal interfaces may affect creep ductility by changing the amount of intergranular fracture, without altering the strength of the alloy. Such elements are unlikely to affect transgranular creep failure since this failure mode is the direct result of plastic flow. Grain boundary failure, however, results from the nucleation and growth of cavities on the boundaries, both of which may be sensitive to small changes in grain boundary composition. The nucleation of cavities during creep depends on the cohesion between the nucleating particles and the D. S. WILKINSON, formerly a Postdoctoral Research Fellow at the University of Pennsylvania, is now Assistant Professor, Department of Metallurgy and Materials Science, McMaster University, Hamilton, Ontario L8S 4L7. K. ABIKO, formerly a Visiting Scientist at the University of Pennsylvania, is now Research Associate, Research Institute for Iron, Steel and Other Metals, Tohoku University, Sendai, Japan. N. THYAGARAJAN, formerly a Graduate Student at the University of Pennsylvania, is now Process Engineer, National Beryllia Corporation, Haskell, NJ 07420. D. P. POPE is Associate Professor, Department of Materials Science and Engineering, University of Pennsylvania, Philadelphia, PA 19104. Manuscript submitted February 11, 1980.
matrix, which can be reduced by impurity segregation. The cavity growth rate is determined by the boundary self diffusion coefficient which may be either increased or decreased by segregated impurities, depending upon their nature. Thus, the minimum creep life at which intergranular failure first appears might be increased or decreased by segregating impurities. A reduction in creep ductility due to increased intergranular failure is often referred to as "creep embrittle
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