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The effect of prior cyclic loading on creep behavior of P92 steel was investigated. Creep tests on prior cyclic loading exposure specimens were performed at 650 °C and 130 MPa. In order to clarify the influence of prior cyclic loading on creep behavior, optical microscope, scanning electron microscope and transmission electron microscope were used. Experimental results indicate that the prior cyclic loading degrades the creep strength significantly. However, the degradation tends to be saturated with further increase in prior cyclic loading. From the view of microstructural evolution, the recovery of martensite laths takes place during prior cyclic loading exposure. This facilitates the dislocation movement during the following creep process. Therefore, premature rupture of creep test occurs. Additionally, saturated behavior of degradation can be attributed to the near completed recovery of martensite laths. Based on the effect of prior cyclic loading, a newly modified Hayhurst creep damage model was proposed to consider the prior cyclic loading damage. The main advantage of the proposed model lies in its ability to directly predict creep behavior with different levels of prior cyclic loading damage. Comparison of the predicted and experimental results shows that the proposed model can give a reasonable prediction for creep behavior of P92 steel with different level of prior cyclic loading damage. [ABSTRACT FROM AUTHOR]

Results of a large set of tensile and compressive creep tests on pure Al were reanalyzed for the influence of low- and high-angle grain boundaries on the deformation resistance at the temperature T  = 473 K = 0.51 T m where T m is the melting point. Thermomechanical treatment by equal channel angular pressing followed by heating to T led to strong increase of areal fraction of high-angle boundaries in a structure of subgrains of ≈10 −6  m in size, accompanied by significant reduction of subgrain strengthening and of the stress sensitivity of the deformation rate. (Sub)grain strengthening by low-angle boundaries is most effective; the strengthening effect virtually disappears during creep as the boundary spacings coarsen toward their stress-dependent, quasi-stationary size w qs . The same type of coarsening is found for (sub)grain structures with large fraction of high-angle boundaries; in the quasi-stationary state they lead to softening at low and strengthening at high stresses, and a significant increase in tensile fracture strain to values up to 0.8. The results are analogous to former results for Cu and are explained in the same way by the influence of boundaries on storage and recovery of crystal defects and the homogenization of glide. [ABSTRACT FROM AUTHOR]