Formation Mechanism Of Stacking Faults And Twins Induced By SiC Nanowires In Al Matrix And Its Effect On Mechanical Behavior

The development of modern science and technology caused harsh requirements for structure materials. Nano metal matrix composites have become one of the excellent structural materials, due to their much higher strengthen efficiency than micro composites. However, the microsture characteristic of nano composites has not deeply investigated. On the other hand, due to high stacking fault energy, it was thought that stacking faults or twins could not be formed in Al. Refining grain to nano-scale could obtain stacking faults(SF) and twins in Al alloys. The current research is mainly focused on the effect of grain boundaries in metals. The effect of phase interface on the defect mec hanism of metal matrix has not been understood yet. To investigate the effect of high content interface on the defects and mechanical properties of Al matrix and to obtain high-performance nano metal matrix composites, we designed to introduce high amount interface(10 to 30 vol.% SiC nanowires) into pure Al and 6061 Al. The defects microstructure and aging behavior of composites have been investigated systematically. Meanwhile, the effect of surface morphology, content of SiC nanowires on mechanical properties of Al matrix composite was also studied.Morphology of SiC nanowires was firstly studied in our work, which was cylindrical with smooth surface or bamboo shape. And both of the two kinds of SiC nanowires are composed of a large number of small fragments that are formed by hybrid 3C-SiC and 2H-SiC structures. The bamboo shaped morphology is due to periodic twins in the SiC nanowires.The microstructure of SiC nanowires reinforced pure Al composites was investigated. No interface reaction or orientation relationship was found between SiC nanowires and Al matrix. The results indicated that the defects in pure Al matrix was dislocations in 15~25 vol.% composites. When the volume fraction was increased to 30 vol.%, the defects have transformed to stacking faults and twins. The matrix contained single twins and twofold twins. Detwining of single twins would be observed under electron beam irradiation for a long time, while this could not be observed on twofold twins. The analysis of the twins in the composites indicated that, single twins in the matrix were zero-strain twins, which can be explained by the random activation of partials(RAP) mechanism; Twofold twins were formed by the migration and reaction of two zero-strain twins and Lomer-Cottrell(L-C) dislocations generated on the node made them very effective in pinning and stabilization of the twofold twins. Based on the critical shear stress for leading partial and full dislocation nucleation, interfacial area and critical interface content to form stacking faults were calculated. The partial dislocations would be emitted firstly to form SFs and twins when the interfacial area larger than threshold. After irradiated by electrons, the temperature at this area would be increased, which changed the shear stress of the matrix and increasing the threshold of the interfacial area, leading to the unstable state and ultimate detwinning of the twins.The aging behavior of SiC nanowires reinforced 6061 Al composites were investigated. 10~25vol.% SiCnw/6061 Al composite have the similar aging behavior. The hardness of SiCnw/6061 Al composite was significantly increased before peak-aging treatment, and then decreased. The time for SiCnw/6061 Al composite to reach the aging peak was 5h. However, the hardness of SiCnw/6061 Al composite was decreased much faster than that of 6061 Al alloy in the over-aging period. GP zones were observed in the under-aged SiCnw/6061 Al composites. Afterward, only β’’(Mg5Si6) precipitates were detected in the peak-aged SiCnw/6061 Al composite. Furthermore, B’ Precipitates were found in the over-aged SiCnw/6061 Al composite, which is significant different to that of 6061 alloy and traditional 6061 Al matrix composites. Due to the segregation of Mg at the interface between SiC nanowires a nd Al matrix, the amount of Mg in Al matrix is insufficient Mg amount to form high Mg precipitates, leading to the inhibition of β’ and β phases and the formation of B’ phase. Introducing 0.8wt.% Mg into 15 vol.%-SiCnw/6061 Al composite, the β phases was formed again, which proved that the formation of B’ phase was due to the segregation of Mg at the interface between SiC nanowires and Al matrix, the amount of Mg in Al matrix is insufficient. Stacking faults(SF) and twins were also found in the as-quenched sample of 30 vol.%-SiCnw/6061 Al composite. The density of SF was significant decreased with the aging treatment and finally transformed to rhombic phases which coherent with Al matrix. The rhombic phases were stacking faults tetrahedra(SFT) by characterizing. The SFTs in the present work is different to the conventional SFTs. The formation mechanism of SFT was discussed. Due to the segregation of Mg at the interface, only Si segregation could form in the Al matrix. The interaction would be occurred at the { 111} crystal planes of Si segregaion, and the corresponding probability is increased with the aging time. The overlapping of SF at intersected(111) plane leads to the formation of Lomer-Cottrell dislocations, which stabilized the overlapped SF, and it eventually evolved to be Si segregation based SFT.The effect of surface morphology, content and distribution of SiC nanowires on mechanical properties of Al matrix composite was also studied. Bamboo shaped SiC nanowires have higher strengthening effect than cylindrical SiC nanowires. With the increase of bamboo shaped nanowires content, the tensile strength and elongation both increasing. The improvement in the tensile strength and elongation of the SiCnw/6061 Al composites could be attributed mainly to the improvement of the interfacial bonding and the increased of interfacial area. The yield strength and micro yield strength of SiCnw/Al and SiCnw/6061 Al composites was increased with the content of SiC nanowires. When the volume content of SiC nanowires was 30 vol.%, a giant leap of yield strength and micro yield strength deviated from the linear variation in the strength was found in composites, which was related to the change of defects in the matrix. The results showed that the contribution of SF and SFT to strength was about 70%. The yield strength formula of the composites was amended finally acquired based on the shear-lag model, which considering the contribution of aspect ratio, size effect, arrangement direction of SiC nanowires and stacking faults strengthening.