Mechanical Properties of Magnesium Alloy AZ31 after Severe Plastic Deformation

Abstract

Grain refinement of magnesium alloy AZ31 was studied in multidirectional forging (MDF) under decreasing temperature conditions. MDF was carried out up to large cumulative strains of 5.6 with changing the loading direction during decrease in temperature from pass to pass. MDF can accelerate the uniform development of very fine-grained structures and an increase of the plastic workability at low temperatures. New grain structures with the minimal grain size of 0.23 mm can be developed by continuous dynamic recrystallization at a final processing temperature of 403 K. As a result, the multidirectional-forged alloy showed excellent higher strength as well as moderate ductility at room temperature, and also a superplastic elongation of over 300% at 423 K. The mechanisms of strain-induced and fine-grained structure development and of the excellent plastic deformation are discussed in detail.

Mechanical Properties of Fine-Grained Magnesium Alloys Processed by Severe Plastic Forging

“…The average grain size of 0.23 μm is almost fully developed after 7th-pass compression to ΣΔε = 5.6 at 403 K. Changes in the average grain size evolved in the Mg alloy AZ31 during isothermal MDF at 623 K and during MDF under decreasing temperature condition are represented by a broken line and a solid line in Fig. 13, respectively (Xing et al, 2008). The average grain size developed during isothermal MDF is almost constant within the experimental scatters and about 6.7 μm at strains up to ΣΔε =4.8.…”

“…Fig. 16 shows the relationships between yield flow stress at a strain of 0.2% (σ y ) or room-temperature hardness (H v ) and the average grain size (d) developed during MDF (Xing et al, 2005(Xing et al, , 2008 (1) and (2) with almost similar slopes of 0.23 and 0.21.…”

“…Fig. 11 shows typical true stress-cumulative strain (σ − ΣΔε) curves during repeated MDF under decreasing temperature condition and also during isothermal MDF at 623 K (Xing et al, 2008). The first flow curve at 623 K shows a sharp stress peak followed by work softening and subsequently steady-state like flow at high strains.…”

“…The relationship between the average grain size and the flow stress developed during MDF with dropping temperature as well as that for single-pass compression at high temperature is depicted in Fig. 14 (Xing et al, 2005(Xing et al, , 2008. …”

“…Tensile tests were carried out at 298 K and at 8.3×10 -3 s -1 . Fig.15 shows changes in the true stress-nominal strain (σ -ε n ) curves with strain-induced grain size at 293 K (Xing et al, 2008). The σ -ε n curve of the 22.3 μm sample shows a monotonous work hardening following yielding and then a stress peak just before fracture at a total elongation of about 35%.…”