Kun-kun Deng

Hot deformation behaviour of as-extruded micrometre SiCp reinforced AZ91 composite

Abstract The 10 μm 10 vol.-% SiCp/AZ91 composite was fabricated by stir casting. After processing by forging and subsequent extrusion, the composite with 1∼2 μm grain size was obtained. Then the hot deformation behaviour of as-extruded composite was investigated at elevated temperatures (543–673 K). Results show that the existence of micrometre SiCp has obvious effect on stimulating dynamic recrystallization (DRX) nucleation and inhibiting the growth of DRXed grains. Both the flow stress and work hardening of as-extruded composite is lower than that of as-cast AZ91 alloy and composite, which means that the grain refinement strengthening mechanism is not fit for elevated temperatures, and the reasons were analysed. The calculated activation energy Q (∼90 KJ mol− 1) of as-extruded composite is very close to the grain boundary diffusion activation energy (∼92 KJ mol− 1) for Mg. Calculation analysis gives n = 5, which illustrates that the climb of dislocation is the main controlling mechanism of the as-extruded composite during hot deformation.

Effect of SiC Nanoparticles on Hot Deformation Behavior and Processing Maps of Magnesium Alloy AZ91

The hot deformation behavior and processing characteristics of AZ91 alloy and nano-SiCp/AZ91 composite were compared at temperature ranges of 523 K–673 K and strain rates of 0.001–1 s−1. Positive impact of SiC nanoparticles on pinning grain boundaries and inhibiting grain growth was not obvious when deformation temperature was below 623 K, but was remarkable when the temperature was above 623 K. By comparing compressive stress-strain curves of AZ91 alloy and nano-SiCp/AZ91 composites, the addition of nanoparticles could improve the deformation ability of a matrix alloy under high-temperature conditions. There was no essential difference of deformation mechanism between AZ91 alloy and the composite, but hot deformation activation energy of the composite was significantly lower than that of the AZ91 alloy. The AZ91 alloy and the composite had the same workability region of 600 K–673 K and 0.001–1 s−1, while instability region for the composite was reduced compared with that of AZ91 alloy at high temperature.

Microstructure and tensile properties of magnesium matrix composite: effected by volume fraction of micron SiCp

Abstract In this paper, three volume fractions (2, 5 and 10%) of 10 μm SiCp/AZ91 magnesium matrix composite were fabricated by stir casting. The as cast ingots were forged at 420°C with 50% reduction first and then extruded at 370°C with a ratio of 16∶1. The results show that the grain is refined significantly after hot deformation process, which results in the enhancement of tensile properties. The volume fraction of micrometre SiCp has significant effect on grain size by influencing dynamic recrystallisation of magnesium. By the increase in particle volume fraction, the grain size decreases, the dislocation density increases and the load transfer effect is strengthened, thus leading to the significant improvement of yield strength. The ultimate tensile strength of the composite increases with the increase in particle volume fraction due to the improved deformation uniformity of matrix and the enhanced hindering effect on crack propagation.

Development of SiC Nanoparticles and Second Phases Synergistically Reinforced Mg-Based Composites Processed by Multi-Pass Forging with Varying Temperatures

In this study, SiC nanoparticles were added into matrix alloy through a combination of semisolid stirring and ultrasonic vibration while dynamic precipitation of second phases was obtained through multi-pass forging with varying temperatures. During single-pass forging of the present composite, as the deformation temperature increased, the extent of recrystallization increased, and grains were refined due to the inhibition effect of the increasing amount of dispersed SiC nanoparticles. A small amount of twins within the SiC nanoparticle dense zone could be found while the precipitated phases of Mg17Al12 in long strips and deformation bands with high density dislocations were formed in the particle sparse zone after single-pass forging at 350 °C. This indicated that the particle sparse zone was mainly deformed by dislocation slip while the nanoparticle dense zone may have been deformed by twinning. The yield strength and ultimate tensile strength of the composites were gradually enhanced through increasing the single-pass forging temperature from 300 °C to 400 °C, which demonstrated that initial high forging temperature contributed to the improvement of the mechanical properties. During multi-pass forging with varying temperatures, the grain size of the composite was gradually decreased while the grain size distribution tended to be uniform with reducing the deformation temperature and extending the forging passes. In addition, the amount of precipitated second phases was significantly increased compared with that after multi-pass forging under a constant temperature. The improvement in the yield strength of the developed composite was related to grain refinement strengthening and Orowan strengthening resulting from synergistical effect of the externally applied SiC nanoparticles and internally precipitated second phases.

Unique strengthening mechanisms of ultrahigh pressure Mg alloys

Ultrahigh pressure technique remarkably extends solid solubility limitation of Al alloying element (∼25 at.%) in Mg alloys, resulting in unique solid-solution strengthening and age hardening response. Microhardness, yield strength and ultimate compressive strength are improved simultaneously without degrading plasticity by forming homogeneous and globular-shaped Mg17Al12 precipitates of 10–30 nm. In addition, thermal resistance is enhanced by eliminating the dominant growth of (101) plane and anchoring dense stacking faults in phase interface.

Hot deformation behaviour of Mg–Al–Zn alloys

Abstract The hot working behaviour of Mg–Al–Zn alloy was studied in the temperature range of 270–370°C and strain rate range of 0·001–0·1 s−1. In order to eliminate the influence of Mg17AL12 phase, the magnesium alloy was solutionised at 420°C for 24 h and then quenched in water before hot compression. Results show that the microstructures are mainly characterised by twinning and dynamic recrystallised grains during the hot compression process. The twinning forms at lower temperature, while the dynamic recrystallised grains are usually found at higher temperature and lower strain rate. Moreover, constitutive analysis was carried out by the hyperbolic sine law function, which illustrated that the deformation mechanism of the present alloy is controlled by dislocation climb. The critical conditions associated with the onset of dynamic recrystallisation determine the changes of the strain hardening rate (θ), and the amount of dynamic recrystallised grains increases with the increasing strain in the form of sigmoid scheme.