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Particulate reinforced Al alloy composites (AlMCs) are recognized as important structural materials due to their lightweight, high modulus and strength and high wear resistance. In order to understand the effect of matrix, reinforcement and secondary processing techniques on the microstructure development and mechanical properties of AlMCs produced by powder metallurgy routes, Al alloy composites reinforced with three types of reinforcements by different secondary processing techniques have been produced and examined. Fabrication of Al or 6061Al alloy based composites reinforced with nano-sized SiC particles (~500nm), micro-sized (<25µm) quasicrystalline alloy particles (hereinafter referred to as “NQX”) and micro-sized Nb particles (~130µm) has been carried out by powder metallurgy routes followed by extrusion or cold rolling. After extrusion, a homogeneous distribution of secondary particles has been obtained with rare interfacial reaction products. The 6061Al/SiC composites exhibit superior mechanical properties than either monolithic alloys or composites reinforced with micro-sized particles with retained ductility while the 6061Al/NQX and 6061Al/Nb composites show limited improvement in tensile strength mainly due to their reinforcement size and poor interfacial bonding. After cold rolling, the evolution in microstructure, texture and strength has been analysed. A typical near β fibre texture with highest intensities near Copper and Brass orientations has been developed for 6061Al/NQX and 6061Al/Nb composites. For 6061Al/SiC composites, a randomized texture with very small grains has achieved due to the presence of the non-deformable SiC particles. Mechanical property tests including microhardness, three-point bending tests and tensile tests have been carried out on cold rolled samples and the results exhibit some level of improvement when compared with as-extruded samples due to work hardening. Finally, the work moves on to the general discussion based on the previous result chapters. The microstructural development related to reinforcement, matrix and interfacial areas during extrusion and cold rolling has been summarised and the correlation between microstructure and mechanical properties has been discussed. The thesis provides a thorough understanding of AlMCs produced by powder metallurgy routes in terms of matrix, reinforcement and processing techniques. It can provide reference to the future development of AlMCs for high strength applications.

Thesis. 1978. Ph.D.--Massachusetts Institute of Technology. Dept. of Materials Science and Engineering. / MICROFICHE COPY AVAILABLE IN ARCHIVES AND SCIENCE. / Vita. / Includes bibliographical references. / Ph.D.

M. Tech. Metallurgical Engineering / Titanium and its alloys exhibit poor tribological characteristics. The poor resistance to sliding wear of Ti6Al4V alloy makes it susceptible to severe wear at the surface during sliding contact. This could cause galling and seizing during sliding contact. Ti6Al4V alloy also have poor corrosion resistance under critical conditions. Some problems with Ti6Al4V MMCs produced by laser cladding technique in most cases is poor bonding as a result of wetting properties between the ceramic and metal powders for reinforcement. Occurrence of porosity is another factor which can reduce the mechanical properties of MMCs. Occurrence of agglomerates is also a concern due to poor mixing of reinforcement powders. This project is aimed at investigating the effect of laser cladding of titanium alloy substrate with zirconium (Zr), titanium carbide (TiC), titanium (Ti) reinforcement additions. The effect of combination of these powders using various fractions and variable cladding parameters on the substrate will be investigated.

A systematic study of the effect of sulphidizing atmosphere on the High Temperature Low Cycle Fatigue (HTLCF) properties of coated and uncoated unidirectionally solidified MARM002 nickel base superalloy was performed at 870°C. The coating systems investigated were, aluminide coating, three types of platinum modified aluminide coatings, and platinum coating. The creep-plasticity mode of the strain range partitioning method was used for creep-fatigue loading. A constant loading regime (Strain range 6.6 x 10-3 ) was used to test the samples in argon, air and Ar + 5%S02 and a lower strain range of3.8 x 10-3 was used to investigate the creep-fatigue properties in Ar + 5%S02 only. The results were analysed using scanning electron microscopy including spot analyses (SEM-EDS), Auger electron spectroscopy (AES) and X-ray diffraction (XRD) techniques. The synergistic effect of sulphidizing environment and the creep fatigue loading (Strain range - 0.66%) resulted in accelerated failure in all the materials systems tested, except for the TYPE I platinum aluminide coated sample. This coating displayed a "self-healing" mechanism which enhanced its fatigue life under sulphidizing conditions. In general, the coatings had an adverse effect on the fatigue properties of the material systems. This was due to the poor mechanical properties of the coating. The mechanical properties of the coating was influenced by the coating microstructure and the chemical composition. The modification of the NiAI zone with platinum in the platinum aluminide coatings improved the fatigue properties of the coating by altering the crack propagation mechanism in the NiAl zone. The higher the platinum content in this region the more brittle it became. The platinum modified aluminide coating showed an improvement in the corrosion fatigue properties in the S02 containing environment at the higher strain range when compared with the uncoated, aluminide coated and platinum coated samples. However, at the lower strain range all the coating systems performed worse than the uncoated alloy. This was mainly due to the brittle failure of the coating. The platinum modified aluminides performed the worst due to the presence of brittle platinum aluminide phases. The interdiffusion and interaction of platinum with the substrate alloying elements, resulted in this coating being ineffective for corrosion protection. The resultant coating layer produced poor corrosion-fatigue properties. Although the coating systems did show evidence of resistance to sulphidation and oxidation there were relatively ineffective under the combination of sulphidizing environment and fatigue loading due to their poor mechanical properties. The mechanism of sulphidation was consistent for all the material systems tested with oxidation proceeding first and sulphidation proceeding at the corrosion scale/substrate interface. The crack propagation in the coating and substrate was controlled by the sulphidation attack at the crack tip and failure of the oxide scales formed in the cracks. / Thesis(M.Sc.Eng.)- University of Natal, Durban, 1998.

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Mode I and Mode III fatigue tests were performed on copper nickel alloys in helium, salt water environments. The hydrogen, oxygen, two alloys used air and in this investigation were 90-10 and 70-30 copper nickel. Both alloys contained iron which was added to improve the erosion corrosion resistance. The extent of cracking varied with the test environment. Tests showed that oxygen and humid air promoted cracking while salt water helium was used as the baseline retarded cracking when environment. Hydrogen promoted cracking when compared to helium but retarded cracking if comparisons were made with oxygen or humid air. The environmental effects (helium as the base case} in the Mode I tests in gaseous environments were manifested in the form of shorter fatigue lives, easier crack initiation, marginally higher crack growth rates and the development of intergranular fracture at the surface. These effects were accompanied by a change in the near surface deformation characteristics. The increases in fatigue life induced by testing in aqueous environments were greatly extended if the copper nickel was galvanically coupled to steel. Mode III tests showed the same ranking of environmental effects as Mode I tests and also showed multiple initiation, brittle fracture and secondary cracking. Two models were proposed to explain the observed results. One was based on the dilation-aided diffusion of oxygen ahead of the crack tip and subsequent oxidation of internal iron particles. The oxidation caused a volume expansion which produced internal tensile strains and facilitated fracture. The other mechanism was based on dilation-aided transport of hydrogen with subsequent accumulation of hydrogen at interfaces, resulting in a lowering of the interfacial strength and promoting intergranular fracture. The observed increases in life in the aqueous environments were rationalized by the reduced oxygen content available in the stagnant solutions. These observations suggest that the presence of iron accelarates fatigue in copper nickel alloys exposed to aggressive environments. Thus, any application involving fatigue loading with simultaneous exposure to aggressive environments should attempt to ensure that the iron content of the copper nickel alloys is minimized. / Doctor of Philosophy

The goal of this work is to establish a life prediction methodology for thermomechanical loading of the Ni-base superalloy Mar-M247 over a larger temperature range than previous work. The work presented in this thesis extends the predictive capability of the Sehitoglu-Boismier unified thermo-viscoplasticity constitutive model and thermomechanical life model from a maximum temperature of 871C to a maximum temperature of 1038C. The constitutive model, which is suitable for predicting stress-strain history under thermomechanical loading, is adapted and calibrated using the response from isothermal cyclic experiments conducted at temperatures from 500C to 1038C at different strain rates with and without dwells. In the constitutive model, the flow rule function and parameters as well as the temperature dependence of the evolution equation for kinematic hardening are established. In the elevated temperature regime, creep and stress relaxation are critical behaviors captured by the constitutive model. The life model accounts for fatigue, creep, and environmental-fatigue damage under both isothermal and thermomechanical fatigue. At elevated temperatures, the damage terms must be calibrated to account for thermally activated damage mechanisms which change with increasing temperature. At lower temperatures and higher strain rates, fatigue damage dominates life prediction, while at higher temperatures and slower strain rates, environmental-fatigue and creep damage dominate life prediction. Under thermomechanical loading, both environmental-fatigue and creep damage depend strongly on the relative phasing of the thermal and mechanical strain rates, with environmental-fatigue damage dominating during out-of-phase thermomechanical loading and creep damage dominating in-phase thermomechanical loading. The coarse-grained polycrystalline microstructure of the alloy studied causes a significant variation in the elastic response, which can be linked to the crystallographic orientation of the large grains. This variation in the elastic response presents difficulties for both the constitutive and life models, which depend upon the assumption of an isotropic material. The extreme effects of a large grained microstructure on the life predictions is demonstrated, and a suitable modeling framework is proposed to account for these effects in future work.

Superalloys have performed well as blade and disc materials in turbine engines due to their exceptional elevated temperature strength, high resistance to creep, oxidation and corrosion as well as good fracture toughness. This study explores the use of a relatively new technique of strain measurement, high resolution electron backscatter diffraction (HR-EBSD) to measure local deformation fields. The heart of the HR-EBSD technique lies in comparing regions in EBSD patterns from a strained region of a sample to those in a pattern from an unstrained region. This method was applied to study the elastic strain fields and geometrically necessary dislocation density (GND density) distribution near hard carbide particles in a nickel-based superalloy MAR-M-002. Significant thermal strains were initially induced by thermal treatment, which included a final cooling from the ageing temperature of 870°C. Elastic strains were consistent with a compressive radial strain and tensile hoop strain that was expected as the matrix contracts around the carbide. The mismatch in thermal expansion coefficient of the carbide particles compared to that of the matrix was sufficient to have induced localized plastic deformation in the matrix leading to a GND density of 3 x 10¹³ m^–2 in regions around the carbide. These measured elastic strain and GND densities have been used to help develop a crystal plasticity finite element model in another research group and some comparisons under thermal loading have also been examined. Three-point bending was then used to impose strain levels within the range ±12% across the height of a bend bar sample. GND measurements were then made at both carbide-containing and carbide-free regions at different heights across the bar. The average GND density increases with the magnitude of the imposed strain (both in tension and compression), and is markedly higher near the carbide particles. The higher GND densities near the carbides (order of 10¹⁴ per m²) are generated by the large strain gradients produced around the plastically rigid inclusion during monotonic mechanical deformation with some minor contribution from the pre-existing residual deformation from thermal loading. A method was developed of combining the local EBSD measurements with FE modelling to set the average residual strains within the mapped region even when a good strain free reference point was unavailable. Cyclic loading was then performed under four point loading to impose strain levels of about ±8% across the height of bend bar samples. Similar measurements as in the case of monotonic deformation were made at several interruptions to fatigue loading. Observations from the cyclic loading such as slip features, carbide cracking, GND density accumulation have been explored around carbide particles, at regions away from them and near a grain boundary.