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In the present work, the authors investigated the microstructural evolution process of equal channel angular pressed (ECAPed) AZ91D magnesium alloy during partial remelting, and the effects of pressing pass, pressing route and heating temperature on the semisolid microstructures. The results indicated that the microstructure evolution could be divided into four steps: the initial coarsening due to the dissolution of interdendritic eutectics, structure separation resulting from the melting of the residual eutectic and the penetration of the firstly formed liquid into the recrystallised boundaries, spheroidisation due to the partial melting of primary particles and final coarsening attributed to Ostwald ripening. Correspondingly, a series of phase transformations occurred in turn: β→a, a→β→L and α→L. The variation of the primary particles with heating time obeyed the Lifshitz, Slyozov and Wagner law, D3(t)-D3(0)=Kt, after the semisolid system was up to liquid-solid equilibrium state. Increasing the heating temperature was beneficial for obtaining an ideal semisolid microstructure because of the decreased tendency of the primary particles to merge. With the increase in pressing pass, the size of the primary particles decreased and their morphology tended to be more spheroidal. Simultaneously, the amount of liquid phase slightly increased because an increased amount of structure melted due to the increased energy stored in the alloy. At a given pressing pass, the semisolid microstructure of the alloy processed by route BC was quite ideal for thixoforming while that of the alloy processed by route A was not completely suitable. In addition, because of the difference in the stored energy, the liquid amount of the former alloy was obviously larger than that of the latter alloy. [ABSTRACT FROM AUTHOR]

The effect of solid solution treatment on semisolid microstructure of a Zn alloy ZA27 with developed dendrites was investigated and a new approach to produce non-dendritic semisolid microstructure available for thixoforming was developed. The results indicated that a semisolid microstructure with small and spherical primary particles could be obtained when the alloy with developed dendrites was solution treated for 3 h at 370°C and then partially remelted at a semisolid temperature of 460°C. This method could not only overcome some inevitable shortcomings resulting from using some present popular approaches, but also simplify the procedure of thixoforming. Too short or too long solution treatment duration could increase the shape factor, size and inhomogeneity of the primary particles. All of these microstructural changes were attributed to the dendrite coalescence and phase transformation which occurred during solid solution treatment. [ABSTRACT FROM AUTHOR]

In situ silicon particle reinforced ZA27 composites were fabricated by liquid metallurgy route. The solidification process was specially discussed. The effects of pouring temperature, silicon content and phosphorous modification on microstructures and tensile properties of the composites were investigated. The results indicated that the solidification process included three stages, primary silicon particle crystallisation (the primary phase was α-Al dendrites when silicon content was lower than 1·5 mass%), binary Al–Si eutectic reaction and quaternary Zn–Al–Si–Cu eutectic reaction. The addition of silicon into the ZA27 alloy changed the α dendrites from the original equiaxed shape to a two-dimension (2D) column dendrites and this change was further enhanced with the increase in silicon content. Phosphorous did not only fine the primary silicon particles, but also fined the α dendrites. The sizes of both the silicon particles and α dendrites decreased and the particles distributed more uniformly as the pouring temperature increased. However, the particle size increased and their distribution became more agglomerated as the silicon content increased. The tensile properties increased with increasing pouring temperature while those decreased as the silicon content increased. There were four factors that led the composites to fracture during tensile testing, porosities, fracture of the silicon particles, debonding of the interfaces of silicon particles/matrix and debonding of the interfaces between the 2D column dendrites. The contribution of each mechanism to the fracture varied with the pouring temperature or silicon content. [ABSTRACT FROM AUTHOR]

The friction and wear properties of permanent mould cast AZ91D magnesium alloy were investigated under dry reciprocating sliding conditions by using a ball-on-disk machine. The results indicated that the wear resistance of the alloy was relatively poorer than those of the other commercial alloys. The wear rate per sliding distance continuously decreased while the wear rate per testing time linearly increased with the increase in reciprocating frequency. The friction coefficient slowly decreased with the frequency. As the load increased, both the wear rates increased while the friction coefficient first decreased, then increased and finally decreased again. Under low frequency or load, oxidation wear or micromachining wear dominated. As the frequency or load increased, the dominative mechanism changed in turn to plastic deformation induced wear, severe deformation accompanied by severe adhesion wear and smear wear. The work hardening and thermal oxidation of the contact surfaces played an important role for the wear behaviour. [ABSTRACT FROM AUTHOR]

The effects of quench aging heat treatment on microstructure and tensile properties of thixoformed ZA27 alloy were investigated. The results indicated that the microstructure of the alloy became into polygonal β phase particles after solid solution treatment at 350°C for 48 h. The β particles then decomposed into α and η phases which would coarsen during the subsequent aging at 150°C. It was observed that the Zn concentration in the β phase near the polygonal boundaries was higher than that within the β particles. As a consequence, both the decomposition speed of the former β phase and the subsequent coarsening speed were faster than those of the latter β phase. Thus, α and η phases near the boundaries were always coarser than those within the particles during aging. Owing to the coarsening, the ultimate tensile strength continuously decreased with increasing aging time. The percentage elongation increased up to 10 h, but decreased with aging time owing to bad deformation accommodation and low bonding strength between particles. Cracks initiated from some defects (e.g. inclusions and porosities) during tensile test, and the path for the cracks to propagation changed with the aging time. [ABSTRACT FROM AUTHOR]

The size effect on strength of alumina agglomerates was examined, and the applicability of the often used size effect laws was discussed. It is concluded that there is a significant energetic size effect, which shows that the Weibull statistical size effect does not apply. However, the size effect on the tensile strength of the agglomerates examined can be addressed well by the energetic–statistical size effect law. [ABSTRACT FROM AUTHOR]