| The origins of thermal fatigue are first considered, with reference to the turbine blades of jet turbine aero-engines. The relationship between this problem and the investigation described in the thesis is then explained. A detailed description follows of the development and structure of the alloy to be tested - Nimonic 105. To enable a systematic understanding of the creep curves produced experimentally, the theory of creep in two-phase alloys is reviewed, and in view of the high test temperatures involved, an examination is included of the time dependent changes occurring in two-phase systems produced by precipitation from a supersaturated solid solution. The literature review is concluded with a survey of previous relevant creep investigations, in particular those carried out to compare tensile and compressive creep behaviour. The development of tensile and compressive creep apparatus is described, including detailed consideration of the design of compressive loading devices, and of a creep furnace made in two halves specially designed to suit the type of compressive creep machine chosen. Details of the experimental procedure are discussed at some length since they explain the small scatter obtained in the creep data. As well as the sequence followed in the creep tests, a description is included of the metallographic examination of specimens after creep testing. The results of the creep testing programme are given as a series of graphs of creep strain versus time. Since the tests were carried out at constant load, a procedure is developed for correcting the constant load creep curves to constant stress curves, A knowledge of the strain-rate dependence upon stress is necessary for this correction. This dependence has been measured, and its significance is discussed. Since the theoretical discussion showed that the strain rate dependence upon temperature varied from one nickel alloy to another, the activation energy for creep was measured, both in tension and compression. The resulting values are discussed, both in relation to each other, and to the values found by other workers. The discussion of results ends with a critical comparison between the creep curves measured in tension and compression, which involves a detailed consideration of the nucleation and grown of cracks during creep deformation. The relevance of the Bauschinger effect to thormal fatigue prediction, is pointed out, and the present state of understanding of this effect in two-phase alloys is outlined. The development of a device to measure this effect at high temperature is described. The final chapter draws together the conclusions from the rest of the Thesis and discusses their more general significance in the experimental and theoretical investigation of time-dependent deformation. |
