UBC Theses and Dissertations
Strengthening mechanisms in aluminum alloys Sahoo, Maheswar
The substructure of pure aluminum and over-aged Al-4Cu has been varied by mechanical and thermal treatments. The nature of this substructure and its resistance to annealing has been studied, together with the effect of the substructure on tensile strength at ambient and elevated temperatures. It has been found that in at least some respects the response of the strength of the over-aged Al-4Cu to mechanical and thermal processing is very comparable to that of oxide-dispersion-strengthened alloys such as S.A.P. and Ni-ThO₂. The over-aged Al-4Cu is strengthened appreciably by cold work. Much of this incremental room-temperature strengthening can be removed by annealing at relatively low temperatures; i.e. temperatures at which the strength of cold worked pure aluminum is not lowered. In common with oxide-dispersion hardened alloys, the yield strength of cold-worked Al-4Cu at elevated temperatures (300°C or 0.62 Tm) is actually improved by a static anneal at 300°C before testing. This benefit increases with increasing amounts of prior cold work. Similar studies have been carried out with an S.A.P. extruded alloy (10 wt. % A1₂O₃) and comparable results have been obtained. Pure aluminum, Al-4Cu and S.A.P. materials have been examined by X-ray line profile analysis to determine the distribution of nonuniform lattice strain and the coherently-diffracting crystallite domain size. The X-ray data have been interpreted in terms of dislocation densities and configurations, and compared with direct observations made by transmission electron microscopy. An attempt has been made to account semiquantitatively for the strength of the deformed and annealed materials at ordinary and elevated temperatures in terms of available strengthening mechanisms. The 0.2 pet yield strengths of simple aged Al-4Cu alloys (no substructure) was found to be consistent with the Orowan model of dispersion-strengthening both at R.T. and at 300°C. The room temperature yield strength (ϭ0.2) of cold worked and annealed pure aluminum and A1-4Cu alloys was related to the subgrain diameter [symbol omitted] by the Hall-Petch equation: °0 2 = ϭ₀ + kg⁻½ where and k are constants. In such cases it was not believed that there was a contribution to strength from the Orowan mechanism. Similarly the 20°C yield strength of the S.A.P. alloy was associated with the fine dislocation substructure produced by thermo-mechanical treatments. The high temperature yield strength of Al-4Cu and S.A.P. was related to the polygonized substructure produced by static annealing, which was much finer and more stable in the case of the oxide dispersion-strengthened alloy.
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