A Sn -1% Bi alloy has been studied to determine the effects of superplastic deformation on the grain growth kinetics.Using both constant crosshead speed and creep tests, the grain size was measured as a function of deformation time and strain over a wide range of strain rates.It was found that during deformation, considerable increases in the grain growth rates occurred when compared to static annealing. The effect was most pronounced at intermediate strain _2 rates (»=10/minute) in the high strain rate sensitivity region.However, the grain growth rates on annealing after deformation were found to be less than static rates.To aid in understanding the mechanism of the enhanced growth, alternating tension-compression tests were performed. The amount of grain elongation and the changes in preferred orientation with deformation were also measured.Grain type and grain size distributions after deformation and after annealing were established and analyzed in terms of a grain coalescence mechanism. However, the most favourable mechanism appears to involve the production of excess vacancies in the grain boundary region leading to increased boundary mobility.

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Ontario Hydro has developed several essentially non-destructive metallurgical methods for assessing the remaining life of high-temperature components in fossil-fuelled electricity generating units. For superheater and reheater tubing, a life-prediction model has been developed and subsequently refined. For headers, piping and associated welds, surface replication has been adopted for the detection and assessment of creep cavitation damage. The scope of replication has recently been broadened significantly to include metal temperature estimation; and also detection of creep strain concentration, both being vital inputs to component life management.

The first CANDU (CANadian Deuterium Uranium) pressurized heavy water reactor (PHWR) went into operation in July 1971. Today, there are several units in operation at the Pickering, Bruce, and Darlington sites in Ontario, Canada. The steam generator tubing materials were manufactured from Monel 400, Inconel 600, and Incoloy 800 for the Pickering, Bruce, and Darlington respectively and are subjected to different operating conditions. This paper presents a review of some of the various types of degradation mechanisms that have been observed on these tubing materials over the operating period of the respective plants. The results presented are based on the metallurgical examination of removed tubes. The mechanisms that have been observed include pitting, stress corrosion cracking, intergranular attack, fretting, and erosion corrosion. The nature of the flaws and causative factors (if known) are discussed.

Stress-corrosion cracking of low-alloy steel turbine discs has emerged as a generic concern in nuclear generating stations. An investigation that made extensive use of field metallographic techniques to examine suspected cracking in such a component is described. The crack position, and its relationship to surface topographic features, were examined and recorded by magnetic rubber and high-resolution dental rubber replicating materials. Corrosion deposits on keyway surfaces and within the crack were collected with acetate foil replicas applied and then stripped from the keyway surfaces. Microstructural details were revealed by the use of field metallographic preparation techniques and replicated by acetate foil for examination with optical and scanning electron microscopes. It was possible by these techniques to establish the cracking mechanism as stress corrosion possibly related to chloride or sulphate ion steam contaminants. Subsequent sectioning and conventional metallography confirmed both the validity of the conclusions and the replication techniques.