Effect of Modified Surface Microstructures through Shot Peening and Subsequent Heat Treatment on the Elevated Temperature Fat

Author:  Gray, Wagner, Lutjering,
Source:  Conf Proc: ICSP-3, (p.467-476)
Doc ID:  1987018
Year of Publication:  1987
Abstract:  
Shot peening is well recognized as a mechanical surface treatment which can significantly improve the fatigue resistance of high-strength materials. The potential application of shot peening is often thought to be necessarily restricted to low temperatures since the residual compressive stresses which are known to be of most importance in improving the fatigue strength at low temperatures can readily relax at elevated temperatures due to recovery of the material. However, previous work on Ti-alloys has demonstrated that the 10(7) cycles fatigue strength at elevated temperatures can be significantly increased without the presence of residual compressive stresses due to the thermal stability of the induced high dislocation density. To take advantage of the beneficial effect of high dislocation densities on fatigue crack nucleation the rough as peened surface was smoothened by additional polishing treatments. Lately, it was observed that smaller grains or finer phase dimensions in Ti-alloys besides the beneficial effect on fatigue crack nucleation can also improve the resistance to propagation of small surface cracks. In the recent past, developments of high temperature titanium alloys have reflected the increased drive for optimizing their all-round capabilities. While it seems to be quite difficult to optimize a microstructure with regard to the resistance against creep and fatigue shot peening may offer a method for combining excellent fatigue with superior creep properties in engineering components, such as gas turbine blades. By shot peening of an engineering component with a coarse creep resistant microstructure and by subsequent annealing that component at sufficiently high temperatures, very fine recrystallized (fatigue resistant) phase dimensions can be produced in near surface regions without changing the coarse (creep resistant) microstructure in deeper regions. In the present investigation, shot peening and subsequent recrystallization was applied on fatigue specimens of Ti-alloys to modify the surface layer microstructures for an improved resistance to elevated temperature fatigue behavior.