Elevated temperature intergranular cracking of heat-resistant alloy under tensile stress
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In a 2.25Cr1.5W heat-resistant alloy, it is shown that the time to intergranular failure under tensile stress t can be expressed by t0 rn expðQ=RT Þ, where t0 is the constant of proportionality, n is the stress exponent, and Q is the activation enthalpy. It is shown that the dimples observed at elevatedtemperature intergranular fracture surfaces are not the micro-ductile fracture areas but the interfaces between the grain boundary carbides and the neighboring grains. It is also shown that the segregation concentration of solute atoms is much higher at the grain boundary carbide interfaces than at the carbide-free grain boundaries. Under tensile stress, the elevated-temperature intergranular cracking occurs through the decohesion of grain boundary carbide interfaces, which is followed by the eventual carbide-free grain boundary cracking.
I. INTRODUCTION
Effects of applied stress on grain boundary segregation behavior of solute atoms have been first investigated in some studies.1,2 When a tensile stress is applied to the specimen that the equilibrium grain boundary segregation concentration of the solute atoms at a temperature has established, the grain boundary segregation concentration of the solute atoms continues to increase to a maximum, after which it decreases gradually to restore the original segregation concentration. Based on a research,1 it is observed that the tensile stress enhances the grain boundary segregation kinetics of the solute atoms through its influence on the bulk diffusivity and promotes the capability of the grain boundaries to absorb the solute atoms. Meanwhile, a compressive stress results in the opposite segregation behavior1 of the solute atoms, and the bismuth segregation3 at zinc-oxide grain boundaries is nearly suppressed under a high hydrostatic pressure (;1 GPa). The research on the effects of tensile stress on grain boundary segregation behavior of the solute atoms was originally motivated by the occurrence of stress relief cracking of low alloy steels.4–8 Reheat or stress relief cracking in heat-resistant alloys has been explained by the combination of a precipitation-strengthened matrix and a soft depleted zone formed adjacent to prior austenite grain boundaries9–12 or by the prior austenite grain boundary segregation of the solute atoms including P.4,7,8,13–17 In the former mechanism, the precipitation of M3C and M23C6 forms a C- and Cr-depleted zone along the earlier austenite grain boundaries. Also, interior of the grain is strengthened
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Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2011.30 1006
J. Mater. Res., Vol. 26, No. 8, Apr 28, 2011
http://journals.cambridge.org
Downloaded: 14 Mar 2015
by the precipitation of fine MC (M5W, Mo, V and Nb) carbides. Therefore, most of the strain that results from a residual or thermal stress is concentrated on the soft depleted zone, causing the intergranular cracking, in which the cracked surface includes many dimples. In the latter mechanism, the segregation of the solute atoms to the prior
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