Influence of intergranular carbide density and grain size on creep of Fe-15Cr-25Ni alloys
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Influence of Intergranular Carbide Density and Grain Size on Creep of Fe-15Cr-25Ni Alloys P.E. LI, J.S. ZHANG, F.G. W A N G , and J.Z. JIN Recently, we have studied the effect of grain-boundary carbides on creep behavior of Fe-15Cr-25Ni alloys by comparing a two-phase alloy containing only intergranular carbides with a single-phase matrix material, t~'21 A decrease in creep rate of about one order of magnitude was observed due to intergranular carbides which furthermore gave rise to an increase in creep stress exponent and activation energy. These experimental data have been well rationalized by a concept of "boundary obstacle stress," O~bo = m ( b k G / d ) tr~/2.t21 The boundary obstacle stress characterized an opposition to deformation in the vicinity of a grain boundary and can be seen as a back stress from grain-boundary carbides. In the expression of trbo, k is a constant which is usually taken as 20, TM b the magnitude of the Burgers vector, G the shear modulus, d the line intercept grain size, and m a stress concentration factor which is dependent on the intergranular carbide density p (an area fraction of
P.E. LI, formerly Ph.D. Student, Dalian University of Technology, Dalian, China, is Postdoctoral Fellow, Department of Materials Science, Shanghai Jiao Tong University, Shanghai 200030, China. J.S. Z H A N G and J.Z. JIN, Professors, and F.G. W A N G , Associate Professor, are with the Department of Materials Engineering, Dalian University of Technology, Dalian 116023, China. Manuscript submitted July 1, 1991. METALLURGICAL TRANSACTIONS A
the grain boundary covered by intergranular carbides). The value m = 1 corresponds to very dense carbide arrays or a carbide layer at a grain boundary (p = 1), whereas m = 0 corresponds to a carbide-free grain boundary (p = 0). The two extreme cases have been documented in previous works. ~ The aim of this communication is to examine further the general circumstances for 0 < m < 1, corresponding to a dilute carbide distribution at a grain boundary. Also, we have noted trbo oc d ~/2, which predicts a different grain size dependence of creep from that in a solid solution alloy, t4~ Consequently, the influence of grain size on creep behavior in the presence of grain-boundary carbides is also a topic of interest in the present communication. The alloys corresponding to the cases of p = 0 and 1 are identified as C I and C2, respectively, whose experimental details were described in References 1 and 2. A heat of new alloy (labeled as C21) was used in this study, with a chemical composition (in weight percent): C 0.053, Cr 15.74, Ni 25.88, Si 0.24, Mn 0.07, S 0.24, P 0.005, and balance Fe. A grain size range of 60 to 220/xm was obtained in the alloy through a solution plus aging treatment procedure. The C21 alloy has a similar microstructure to the C2 alloy after the treatment, i . e . , with carbides only at grain boundary, and the intergranular carbide thickness of the two alloys is also similar (about 1 /zm). However, the intergranular carbide density (p) of the C21 alloy
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