Bielefeld, Universiẗat, Diss., 2002.

Th. 3e cycle--Génie chim. et chim. phys. appl.--Grenoble--I.N.P., 1985.

Th.--Sci.--Grenoble--I.N.P.G., 1984 : DE 161.

Th.--Sci. phys.--Grenoble--I.N.P., 1982. N°: DE 130.

Th. 3e cycle--Énerg. phys.--Grenoble--I.N.P., 1982. N°: D3 174. / Extr. en partie du Journal de Physique. Lettres, 43, 1982, L801-L807.

Th. 3e cycle--Cristallogr.--Grenoble 1, 1979. N°: 17.

A number of theories and techniques have been used in the past to determine the stress-strain relationship of metals and alloys at high strain rates of up to 2.2 x 10 per second; some of these were approximate whilst others required expensive equipment. In the present study three aluminium alloys, HE15, HE30TF and DTD5044 have been investigated due to their importance in the aircraft, motor and construction industries. En-8 steel was also investigated for the purpose of comparison. A simple new approach using high velocity compression testing in conjunction with a finite-difference numerical technique was adopted, developed, used and modified to suit different situations. Incremental compression tests were conducted on the materials to obtain the quasi-static stress-strain properties and used in the high strain rate (dynamic) deformation theoretical analysis and calculations. The final dimensions of the dynamically deformed specimens (diameter and height) obtained experimentally were plotted against impact velocity. These parameters were also predicted theoretically for given values of material constants and compared with the experimental ones. The material constants were then varied in a systematic manner to obtain the optimum agreement between theoretical and experimental results. The temperature rise during deformation, specimen size and deformation history were investigated and found to alter the material constants and therefore affect the flow stress. However the radial inertia contribution to the flow stress was investigated and found negligible. Friction was investigated and its effect was minimised by using tallow-graphite lubricant, which proved to be adequate and effective. The new approach caters for material inertia, strain hardening, strain rate sensitivity, friction and temperature rise during deformation. This technique provides a simple, inexpensive method of obtaining the stress-strain characteristics of materials at high strain rates without sacrificing accuracy.

No description available.

The development of aluminium alloys that can be processed by Rapid Solidification (RS) techniques for use in high temperature applications has recently been an area of intense study. One of the alloy systems of interest is Al-Cr-Zr-Mn. This work comprises a study of the microstructure and tensile properties of alloys of this system processed by melt spinning, high pressure gas atomisation (HPGA) and chill casting. The RS microstructures of Al-Cr and Al-Zr binary alloys were also compared with those of the quaternary alloys. The variety of microstructures observed in the powders of the quaternary alloys was consistent with the different cooling rates and nucleation temperatures experienced by droplets of different sizes, A cubic phase not previously reported was observed in the finer powder. The transition from a partitionless to a cellular microstructure occurred at estimated solidification front velocities similar to those predicted by morphological stability theory. The distribution of discrete Al[13]Cr[2] intermetallic particles within Al-Cr gas atomised powders of different sizes was found to be consistent with a probabilistic model of nucleants distributed in the volume of the alloy melt. Based on these results the original Al-5.2Cr-1.4Zr-1.3Mn (wt%) alloy was diluted to give an Al-3.3Cr-0.7Zr-0.7Mn (wt%) alloy so that the bulk of the powder (the sub-45mum size fraction) did not contain coarse intermetallic particles but exhibited a mainly cellular microstructure. A relationship has been determined between the thickness of wedge shaped chill castings and powder diameters for. similar microstructures. Prediction of alloy compositions designed to give a particular microstructure in a specified powder size can therefore be tested by a simple casting technique. The mechanical properties of the original and optimised quaternary alloy powders consolidated by Conform and extrusion have been determined and related to the as-consolidated and aged microstructures. The extruded powders of both alloys exhibited better properties than the Conformed powder. A large contribution to the strength of the extruded materials is made by their stabilised fine grain size. The optimised alloy had a consistently better ductility. Neither of the alloys retained its strength after prolonged treatment at 400°C, but the results suggest that a service temperature of 300°C may be possible.

In recent years, considerable concern has been shown about the effects SPF cavitation has on the mechanical properties of superplastic alloys. This investigation was undertaken to ascertain whether Hot Isostatic Pressure (HIP) eliminated this cavitation in Supral 150 and correspondingly brought about an improvement in the mechanical properties. It was found that the density increased with various isothermal anneals; the activation energy for this process was close to that for grain boundary diffusion in aluminium (61.93 KJ mol-1). The rate of cavity sintering was seen experimentally to be enhanced by the application of pressures greater than 7 HPa, and had an activation energy of 62.42KJ mol-1. Complete cavity closure occurred when the external pressure was greater than the flow stress of the material at thaý temperature and strain-rate; the ratio of external pressure (Pe) to flow stress (of) increased with falling HIP temperature. The activation energy at constant strain-rate (Q-) associated with plastic flow under conditions of hole closure was found to be 53.54KJ mol-1; giving a corresponding activation energy at constant stress (Qa), which is close to that for lattice diffusion in aluminium. The alloy used contained a high level of hydrogen which caused blistering on heat treatment, and was also responsible for the reappearance of porosity in subsequently heat-treated material previously returned to theoretical density; the extent of which was decreased by the use of higher temperatures and pressures or by vacuum degassing the material prior to HIP. Post SPF room temperature ductility was enhanced by HIP. The scatter in the 0.2% PS and UTS values found in as-received SPF specimens was not altered by the use of low pressure HIP (up to 35 MPa), although higher pressures (100 NPa) did slightly enhance these values and drastically reduced the scatter. Room temperature fracture of as-received Supral was by a 450 ductile shear mechanism. In the SPF cavitated material, the external characteristics Of fracture had a more jagged appearance, as the cavitation alters the route of the propagating crack. SPF material which has been HIPped to remove cavitation, fails in a manner similar to the as-received material.