The importance of the crystal boundary in the high-temperature creep of zinc was-investigated by two distinct methods, namely, (i) by testing polycrystalline specimens under constant strain-rate conditions. (ii) by carrying out constant stress experiments on sections of bicrystals. The polycrystalline tests were performed at strain rates of 1%, 0.1% and 0.01% per hour over a temperature range from 23°C to 200°C. Data relating creep stress and ductility to strain rate and temperature were obtained. In addition, metallographic examination enabled the occurrence and extent of intergranular cavitation to be correlated with the test conditions. From these results, it is shown that secondary creep in zinc obeys a relation of the form:- -(Q-BQ)RT Ae where E is the creep rate 0-is the stress Q is the activation energy for creep. Furthermore, the creep activation energy appears to be considerably less than that for self-diffusion. In these respects, zinc differs from many other metals. Reasons for the anomalous behaviour are discussed and possible creep mechanisms considered. In the investigations on bicrystals, most tests were conducted on specimens containing a high—angle, symmetrical tilt—boundary. The load was, in general, applied in shear parallel to the boundary, but a few specimens were tested in tension. It was demonstrated that a large stress concentration can act at certain parts of the boundary and that the locations of this stress concentration may be related to the slip geometry of the component crystals. It is concluded that many of the characteristics of grain boundary sliding, including the progressive boundary hardening which occurs under constant stress, may be understood in terms of the generation of sub—structure which results from the interaction of crystal slip with the boundary. The relevance of these findings to the process of creep deformation and to the nucleation and growth of intergranular cavities is discussed.

The decomposition of single sintered calcium carbonate pellets of different porosities and the decomposition of packed beds of pellets has been investigated. A. thermobalance, modified to allow the simultaneous measurement of the temperature and weight of a decomposing sample was used for the single pellets investigation. It has been shown that the mechanism governing the decomposition reaction is independent of the pellet's porosity up to porosities of 70 %. According to this mechanism the rate of the reaction is controlled by heat and mass transfer. Thus the thermal conductivity of the porous lime and the effective CO2 diffusion coefficients through it are important parameters in the reaction model, and their values were measured in independent experiments, and compared with the values calculated from the decomposition experiments. The excellent agreement between the two sets of results, and the accuracy with which the model predicts measured reaction rates, confirmed the validity of the proposed reaction mechanism.

The reverse martensite transformation in two stainless steel alloys has been investigated; one is a pure ternary alloy (Fe -16% Cr -12% Ni) and the other also contains molybdenum and carbon (Fe -15% Cr - 8½% Ni 2% Mo 0.1% C). Both alloys are fully austenitic at room temperature after solution-treatment but undergo partial transformation to martensite when quenched to -196°C or deformed at room temperature. The transformation has been studied by room temperature measurements of specific saturation magnetic intensity, by hardness measurements and by transmission electron microscopy after heating the partially martensitic specimens for short times (1 or 2 minutes) in molten salt baths. Reversion during slower rates of heating has also been studied by a differential dilatometry technique and by elevated temperature measurements of dynamic Young's modulus. The results suggest that the reverse martensite transformation in these alloys occurs mainly by a diffusionless (shear) mechanism but that diffusion-controlled reversion and, possibly, stabilisation effects are also present when slower heating rates are employed. The 'reversed austenite' contains a high density of tangled dislocations together with stacking faults and reversal twins and suggestions have been made regarding the formation of these structural imperfections in the austenite. Strengthening and stabilisation effects produced by repeated transformation cycling, γ⟶∝'⟶γ , have also been studied. The process of 'annealing-out' of the reversed austenite structure in the ternary alloy has been studied and it is suggested that this occurs by a continuous recovery mechanism rather than by the movement of high angle boundaries through the deformed austenite.

The previous work on the silver-oxygen system from the thermodynamicists' view point, has been reviewed. The design consideration of high pressure equipment in general, and of that used here in particular, have been discussed. Fusion points of silver and silver oxide subjected to oxygen pressures ranging from 0.2 to 750 atm. have been deter-mined to derive the pressure-temperature relation for the silver -oxygen system. The eutectic point corresponding to Ag(s) ⇌ liq. ⇌ Ag20(s) is found to be at 530°C and 530 atm. Compositions of the silver-oxygen melts have been measured thermogravimetrically along the silver liquidus and on the isotherms at 950° and 1050°C in the all liquid region in the pressure range 1 to 100 atm. The extrapolated oxygen content at the eutectic point is 0.22 atom fraction. A phase diagram for the silver-silver oxide system has been constructed. Thermo-dynamic quantities for the process, ½02 (g) ⇌ 0 (liq. silver) have been derived.

In principle the most direct way of obtaining the experimental heats of transport is by measuring the crystal thermopower. This is done for pure NaC1 single crystals using platinum electrodes in the temperature range of 870°K to 1060°K. However, the results of the alkali-halides measurements have not been successfully analysed so far. This is because in the alkali-halides system irreversible electrodes (no common ion between electrodes and crystal) have to be used, and the existing theories for the irreversible electrode-crystal thermopower are unsatisfactory. The possibility that the heterogeneous thermopower might be caused by the electrons, which are transferred from the electrodes to the crystal, and which do not fall into any traps, is investigated. This model is also found to be unsatisfactory. The dynamics of the diffusion of a vacancy in a linear chain is studied. A simple relationship between the heat of transport,Q*,and the activation energy for thermal diffusion, E, is found, namely, Q* = (2+q/T)E, where q is related to the dynamics of the system. The subsequent calculations show that q is negative in agreement with the experimental data for AgBr and AgC1. In the course of calculating q to attempt to give theoretical heats of transport for the alkali-halides, various force constants operating between the ions have to be evaluated. This is first done by evaluating the Schottky -pair formation energies by lattice calculations.This calculations, which prove. very successful,show that the basic Born-Mayer form of potential chosen here is realistic,

A study has been made of the role of the substrate surface on the adhesion of plasma sprayed metallic coatings. The criteria specifically investigated includes the role of substrate oxide films, preheat level and surface preparation on the mechanism of adhesion. It has been established that, when aluminium is plasma sprayed onto oxidized steel substrates, adhesion is associated with the formation of an interfacial alloy layer, Al5Fe2, due either to the spalling of the substrate oxide layer at a preheat level of greater than 550°C or to the mechanical penetration of a thin substrate oxide film. In the absence of interfacial alloying, coating adhesion was found to be dependent on the adhesion of the substrate oxide layer to its parent phase, the oxide layer remaining as a barrier layer between the metallic substrate and the coating. It was further established that in the absence of a specifically grown substrate oxide film, the adhesion of plasma sprayed aluminium to preheated metallic substrates is due to a reduction reaction between the sprayed aluminium and the inherent substrate oxide film. The level of coating adhesion was further dependent on the substrate preheat level and surface preparation. A preheat level of 200°C was found to be required to produce an adherent aluminium coating on polished mild steel whereas this level was below 100°C if the substrate had been grit blasted. The adhesion of the aluminium coating was markedly reduced by the presence of any visible substrate oxide film prior to spraying. An attempt was made to identify the thermo physical factors relating to the occurrence of a chemical adhesive bonding mechanism.

No description available.

Wide application of plasma sprayed coatings to shafts and rotating components has led to concern at the effect that such sprayed coatings have on the fatigue properties of the coated parts. Previous work has been inconclusive as to the effect of such coatings and the mechanism by which they affect the fatigle properties. In the present work throe high strength steels typical of those used industrially for shafts and cams have been sprayed with molybdenum, aluminium and alumina. Pure molybdenum markedly reduced the fatigue limit of the three steels, whilst the self fusing molybdenum containing low melting point constituent did not have this effect. The reasons advanced to account for this difference lay in the pattern of die residual stresses at the coating/substrate interface. It has been shown that although the pure molybdenum coatings contained cracks, initiation of fatigue, occurred at the substrate interface. This is due to the unfavourable tensile residual stresses present on the surface of the substrate which favour crack initiation and a shorter Stage I growth period thereby reducing the fatigue limit. Self fusing molybdenum which contracts over a narrower temperature range than the pure molybdenum coating produces tensile residual stresses at the substrate surfaces which are less severe and do not appear markedly to affect the fatigue behaviour of the substrate material. It was further established that both flame and arc sprayed molybdenum coatings also reduce the fatigue limit of the substrate material. In the case of arc sprayed molybdenum the fatigue life at the finite life portion of the S-N curve was reduced by 40%. This reduction is attributed to the formation of a highly stressed region beneath the martensitic zone formed at the interface on the surface of the substrate as a result of arc sprayed molybdenum coatings. The reduction in fatigue limit in the case of flame sprayed molybdenum coatings is believed to be due to a similar mechanism to that proposed for plasma spraying. No marked effect on the fatigue behaviour of the steel was observed after spraying with either aluminium or alumina. This is believed to be due to the presence of less harmful interfacial stresses because of the lack of metallurgical bonding.

The corrosion of grey cast iron in hot water has been studied under conditions which are similar to those in central heating systems. The rate of hydrogen evolution from a cast iron boiler in a model system at 75°C was measured for 63 days. After an initial increase up to 9 days the rate decreased parabolically (k = 6.5 x 10-2 mg-2 cm-4 h -1 for 3Fe + 4H20 = Fe304 + 4H2). A duplex corrosion scale was found on boiler surfaces which is similar to those reported for corrosion of mild steel in water at 300°C. An increase- in dissolved oxygen stimulated hydrogen evolution, and a mechanism for this process is proposed. The formation of corrosion scales on cast iron at 50°C was studied in vitro. A linear dependence of corrosion rate upon dissolved oxygen in the range 0 to 4.0 ppm was found, the rate, R, being approximately that predicted by R = k[FeOH+] [OH-]2[O2] Scales formed on cast iron in a wide range of solutions contain lepidocrocite, magnetite and a green rust which is similar to, but not identical with, those previously reported. There is evidence for a crystallographic transformation of green rust (G.R) to magnetite CM), the orientation relationship having been partially determined thus: The growth sequences of scales formed at 3.0 ppm and 0.44 ppm O2 are described. The proportions of magnetite and lepidocrocite change continuously with time and with dissolved oxygen level, while there is evidence for electrochemical dissolution of lepidocrocite and precipitation of magnetite as the oxygen level is reduced. A cyclical mechanism for this process is proposed. Scales produced at 3.0 ppm O2 form shell-like "crusts" over a porous subscale; the "crust" exhibits "chimney"-like outgrowths which are orientated in the water-flow direction. Similar morphologies are observed in scales formed on circulating pumps used in central heating systems. These features are attributed to outward diffusion of soluble corrosion products, and possibly hydrogen, from the subscale.

No description available.