Effect of Al Content on the Microstructural and Grain Growth Kinetics of Magnesium Alloys

Abstract

In order to control the grain size in thermomechanical processing, the grain growth behavior of hot extruded Mg–xAl–1Zn (x = 3, 6, 9) alloys and their relationship with second phase particles and solutes were investigated. The growth rate of AZ61 is greater than that of AZ31 and AZ91 at 300 °C, 350 °C, 400 °C, and 450 °C under isothermal annealing. The average grain growth exponents n of Mg–xAl–1Zn (x = 3, 6, 9) alloys were 2.26, 2.33, and 2.53 at 300–400 °C, respectively. The deviation from the theoretical value of 2 was attributed to the hindrance of grain boundary migration of Al-rich second phase particles and solute Al. Microscopic observations show that the grain size of the annealed samples is closely related to the shape, volume fraction, size, and distribution position of the second phase particles. Significantly, the pinning effect is stronger for lamellar and network-like second phase particles. In addition, the pinning effect of Al-rich second phase particles plays a more important role in grain refinement than the dragging of solute Al. The growth of abnormal grains in the microstructure is attributed to the high energy difference between the preferentially oriented <112¯0> grains and the surrounding grains, which drives the grain boundaries to overcome the same pinning force of the second phase particles.

Abnormal grain growth induced by <112̄0> orientation of AZ31 magnesium alloy

“…The formation of such abnormal sub-grain is supposed to be a lack of coherence in the growing precipitates. The difference is that precipitation is not a precursor to abnormal sub-grain, due to the matrix at annealing temperature is a solid solution in AZ31 Mg alloy and AZ61 Mg alloy [28,52]. Significantly, the substructure does not seem to provide a clear driving force for AGG.…”

“…In the body-centred cubic (bcc) metal, abnormal grains with 110 [001] orientation in Fe-Si alloys are obtained by thermomechanical treatment, and the grains with 111 [112] orientation grow faster than others in low-carbon steel [26]. Regarding hcp metals, it has been reported that <11 0>-oriented grains show growth advantages in AZ31 Mg alloy, which will lead to the enhancement of <11 0> type texture [27,28]. Pérez-prado et al [29] reported the predominance of <11 0> recrystallized texture in hexagonal materials and presented <11 0> as a stable orientation of abnormal growth grains in AZ31 Mg alloy.…”

Austenite-Grain-Growth Kinetics and Mechanism in Type 347H Alloy Steel for Boiler Tubes

“…log D = log k + n log t (4) Equation ( 4) enables us to graphically compute the grain-growth exponent, n. The activation energy for grain growth can be calculated as follows [22][23][24][25]:…”