A review of the changes of internal state related to high temperature creep of polycrystalline metals and alloys

Abstract

When polycrystalline metals and their alloys are used at high temperature, creep deformation leads to changes in their internal state. The change in internal state manifests itself in many ways, but the two ways that concern us in this review are (i) the creation of internal stress arising from the strain incompatibility between grains and/or the formation of cell/sub-grain structures and (ii) a change in the material resistance. This review aims to provide a clear separation of these two concepts by exploring the origin of each term and how it is associated with the creep deformation mechanism. Experimental techniques used to measure the internal stress and internal resistance over different length-scales are critically reviewed. It is demonstrated that the interpretation of the measured values requires knowledge of the dominant creep deformation mechanism. Finally, the concluding comments provide a summary of the key messages delivered in this review and highlight the challenges that remain to be addressed.

Keywords:

• Internal stress
• Internal resistance
• Back stress
• Deformation inhomogeneity
• Creep
• Dislocation structure
• Polycrystal

Acknowledgement

Authors are grateful for the financial support from EDF Energy. Bo Chen acknowledges with great gratitude the sustained support from his family members in China. Some of the ideas in this review were enlightened by the long-term research collaboration with Dr Shu Yan Zhang at ISIS, UK. Peter Flewitt acknowledges Wolfson College, Oxford University, for facilitating this collaboration. David Smith was additionally supported by the Royal Academy of Engineering and Rolls Royce plc.

Notes

The online version of this review was amended on 17 November 2014 to correct minor errors in the caption to Fig. 5. The source of part c is Ref. 73 and the correct source of parts e and f is Ref. 81.