Investigations Of Strengthening Precipitate Evolutions In High-performance Automotive Aluminum Alloys For Tailoring Their Microstructures And Properties

Revealing the atomic-scale structural and chemical change of hardening precipitates is essential for understanding and tuning of microstructure in complex multi-phase alloys. Based on advanced electron microscopy investigations and performance tests in conjunction with first-principles calculations, this thesis systematically studied the dynamic evolution of nano-scale precipitates as well as its relationship with property in high-performance automotive age-hardening Al alloys. In addition, novel strategies are proposed to tune the nano-precipitation and improve the property of Al alloys.1. Multiple precipitating nanotwins of {111}/[112] type have been observed forming on the eutectic Si particles in Al-7.0Si alloys upon annealing. High-resolution transmission electron microscopy(HRTEM) reveals that these precipitating nanotwins may possess very rich microstructures, including five-fold nanotwins and various twin junctions. This study presents experimental evidences that a prolonged spheriodization(solution treatment) has a detrimental influence on age hardening due to the porosity in the cast Al-7.0Si-0.3Mg(A356) alloy. The enrichment of Mg on the internal surface of the porosity was observed. This phenomenon is consistent with the degraded aging kinetics and hardening potential of the A356 alloys solution treated for a long time, since the reduction of Mg solutes in the Al matrix shall lead to a decrease of the volume fraction of the Mg–Si-containing strengthening precipitates, mainly the monoclinic β" phase.2. Combining quantitative HRTEM, annular dark field scanning transmission electron microscopy(ADF-STEM) and computational analysis, a comprehensive structural picture for the evolution of distinct precipitates at the atomic scale is established in Al-Mg-Si-(Cu) alloys. There are two distinguished paths for different types of hardening particles to evolve, leading to the possibilities of coexistence of complex multiple phases and the enhanced properties of the alloys. The scenario of the precipitation sequences in Al Mg Si Cu alloys is revealed in detail. These findings are significant to clarify the effects of Mg/Si ratios and Cu content on the microstructure evolution and thus mechanical properties of Al Mg Si Cu alloys. The evolution map revealed in our study also provides the basis for interpreting the reaction mechanism of the addition of other elements such as Ag and transition metals in Al Mg Si alloy to modify the precipitation behaviors and thus performances.3. The effect of Cu addition on the quick age-hardening response of Al-Mg-Si alloys with various Mg/Si ratios and the corresponding mechanism which controls these phenomena are clarified. Significant quick age-hardening responses are acquired for the Al-Mg-Si-Cu alloys even with a Mg/Si weight ratio approaching 2. The microstructural information with regard to the size, type and proportion of the strengthening precipitates formed during the early-stage artificial aging were systematically investigated using HRTEM and ADF-STEM. By optimizing the alloy chemistry(Mg/Si=1~2), a novel automobile body panel Al-Mg-Si-Cu alloy with enhanced formability and thermal stability, while maintaining the quick-bake hardening response, is developed.4. A new method through controlling the aggregation states of alloying elements before cold-rolling is proposed to tune the nano-precipitation in Al alloys processed by combining deformation and aging. Through controlling chemical states, mainly the aggregation and distribution, of alloying elements before cold rolling and a post annealing, we successfully obtained Al-Mg-Si-Cu sheets with a remarkable enhancement in strength without any decrease in ductility. A considerable improvement in strength up to 30%, without any loss of ductility, had been achieved for Al-Mg-Si-Cu alloys compared with the T6 peak-aged one. A bi-modal distribution of the precipitates in the annealed sample pre-treated by natural aging was found. In addition to the discrete precipitates existing in the Al matrix, a large number of continuous precipitates also occurred. The discrete precipitate is lath-like Q "-type phase, while the continuous precipitate, whose composition is identical to Q’-type phase, possesses a much larger size and is curved. A short-term artificial aging which induced a dispersion of nano-sized coherent particles, mainly GP zone of the monoclinic β″-phase, before cold-rolling led to the formation of Q"-type precipitates with a dense mono-modal distribution. It was inferred that the solutes aggregation state before deformation affected significantly the re-precipitation of solutes during the subsequent annealing. The proposed strategy in this paper is applicable to many precipitation-hardened alloy systems. The Al-Mg-Si-Cu alloys produced by combining deformation and ageing also have superior corrosion resistance and electrical conductivity to the T6 aged alloy.5. The evolution of microstructure and mechanical property during the post-annealing of the cold-rolled Al-Mg-Si-Cu alloys were investigated to provide deep insight into the interplay between solutes segregation and annihilation and rearrangement of structural defects. By applying precipitation annealing or ageing(at a temperature from 70 ℃ to 180 ℃) to a cold-rolled Al-Mg-Si-Cu alloy that is work-hardened but poor in ductility, the excellent combinations of strength and ductility can be achieved for the processed material. The underlying mechanism of the enhanced properties was investigated by monitoring the microstructure evolution of the cold-rolled alloy upon the post-annealing, using(scanning) transmission electron microscopy in association with differential scanning calorimetry. It is demonstrated that the interplay between solutes diffusion and defects annihilation upon the post-annealing plays the key role in this mechanism. Three types of solute aggregations or precipitations including cell-boundary segregation, bulk precipitation and grain-boundary segregation and their corresponding roles in tuning the combined properties were identified. The solutes may rapidly precipitate at the dislocations that delineate cell-boundaries before they partially released and rearranged(recovery) upon annealing, leading to the cell-boundary segregation to enhanced the strength while the alloy becomes ductile. Bulk precipitation of lath-like disordered Q"-phase particles occurs at an ageing temperature above 120 ℃. The optimum combination of strength and ductility can be achieved when both the cell-boundary segregation and the bulk precipitation occur. At an ageing temperature above 150 ℃, grain-boundary segregation(Q phase and S phase) may initiates and declines significantly both the strength and the ductility of the alloy. It is inferred that the solutes have a strong tendency to migrate toward the nano-grain boundaries formed by simultaneous partial re-crystallization.6. The effects of initial solutes aggregation states, which were modified by a pre-treatment of natural aging(NA), artificial under-aging(AUA) and artificial peak-aging(APA), on the mechanical property and microstructure in Al-Mg-Si-Cu alloys processed through cold-working followed by post-ageing were investigated using hardness test, tensile test, differential scanning calorimetry and transmission electron microscopy. We show that the pre-treatment allows for a tunable way to change the mechanical property of Al-Mg-Si-Cu alloys and has a significant effect on the precipitation reactions during post-ageing. By means of solutes re-precipitation, strength enhancement of 20-40 % without compromise in ductility was achieved in samples pre-treated by NA and AUA compared with conventional T6 temper. The ductility of the sample pre-treated by APA was quite low though the strength increased 40-50%. The precipitations during post-ageing in the four naturally pre-aged samples took place in a similar manner with the formation of isolated lath-like Q " precipitates and long curved dislocation cell-boundary segregates. The initial-stage β" precipitates in AUA sample before cold-rolling transformed to a fine uniform dispersion of lath-like Q " precipitates after post-ageing. The monoclinic β"-type precipitates in APA sample experienced a considerable loss of ordering though still existed after deformation. The results demonstrate that the solutes from pre-existed precipitate in artificially pre-aged samples were limited in a local region after cold-rolling. Then self-organization and ordering occurred during post-annealing. Our findings provide key information in deconvoluting the cooperative effects of the pre-existed solutes states on both the deformation and subsequent ageing.