Microstructural Evolution and Strength in Hot Working of ZK60 and other Mg Alloys

Abstract

The microstructures close to the gauge surface were observed by optical and transmission electron (TEM) microscopy in ZK60 (Mg-5.7Zn-0.65Zr) and in AZ31S (Mg-2.8Al-0.9Zn) after torsion testing over 180 -450 °C and 0.01 to 1.0 s -1 . The flow curves rose rapidly to a peak stress which was lower as temperature T rose and strain rate declined. At or below 240 °C, the stress fell rapidly to fracture but at or above 300 °C the curves descended less rapidly and extended into plateaus until failure. Clearly, grains that are not oriented for slip undergo twinning in the initial stages (e £ 0.15) at all temperatures and some grains undergo the process as they rotate with strain. Basal slip proceeds in suitable grains and in the early formed twins leading to planar arrays. The stress concentrations at grain boundaries and in twins especially near intersections lead to slip on non-basal systems and to the formation of subgrains with greater dynamic recovery at higher T and lower e ◊ . With increasing strain, misorientation builds up resulting in dynamic recrystallization (DRX), first at twin intersections with very fine cells and later at grain boundaries which have become quite serrated. Even after extensive straining at high temperatures, the new grains are found mainly as thick necklaces along the boundaries of the original grains leaving their centers with only basal slip; however, grains that twinned are likely to become completely replaced because of nucleation at twins.Résumé -On a utilisé la microscopie optique et la microscopie électronique à transmission (TEM) pour observer la microstructure près de la surface de l'échantillon de ZK60 (Mg-5.7Zn-0.65Zr) et de AZ31S (Mg-2.8Al-0.9Zn), après épreuve de torsion entre 180-450 °C et à 0.01-1.0 s -1 . Les courbes d'écoulement s'éle-vaient rapidement en une contrainte maximale, plus basse à mesure que la température T s'élevait et que la vitesse de déformation diminuait; à, ou au-dessous de, 240 °C, la contrainte diminuait rapidement vers la rupture mais à, ou au-dessus de, 300 °C, les courbes descendaient moins rapidement et s'étendaient en plateaux jusqu'au point de défaillance. Les grains non orientés pour le glissement subissent le maclage aux étapes initiales (e £ 0.15) à toutes les températures alors que certains grains subissent ce procédé tout en pivotant avec déformation. Le glissement basal se poursuit dans les grains appropriés et dans les macles formés tôt, aboutissant en des réseaux plans. La concentration de contrainte aux joints de grain et dans les macles, plus spé-cialement près des intersections, conduit au glissement dans des systèmes non basaux et à la formation de sous-grains, avec une plus grande restauration dynamique à température plus élevée et à plus faible e ◊ . Avec l'augmentation de déformation, une erreur d'orientation se développe, résultant en une recristallisation dynamique (DRX), d'abord aux intersections de macles, avec des cellules très fines et, plus tard, aux joints de grains devenus plutôt dentelés. Même après une déformation extensiv...

Dynamic recrystallisation of Mg–6Zn–2Nd–0·5Zr alloy

“…These phenomena could be explained by the Zener-Hollomon parameter according to McQueen theory. 14,15 That is, the mean size of the recrystallised grain decreases with the decrease in temperature or the increase in Z parameter. found that the morphology and density of dislocation depends on both deformation temperatures and second phases related to basal and non-basal slip.…”

“…These phenomena could be explained by the Zener-Hollomon parameter according to McQueen theory. 14,15 That is, the mean size of the recrystallised grain decreases with the decrease in temperature or the increase in Z parameter. Figure 5 shows TEM observation for the alloy of Mg-6Zn-2Nd-0?5Zr at different temperature.…”

Non-Steady-State Creep Behavior in Tube Gas Forming

“…With increasing temperature the contribution of the twinning to total deformation gradually decreases. Detailed deformation twinning was reported by several researchers (Ref [31][32][33][34][35]. The twinning process not only accommodates a certain amount of shape change itself, but it also partially changes the crystallographic orientation and thus regenerates favorable basal slip system.…”

A Physically Based Constitutive Model and Continuous Dynamic Recrystallization Behavior Analysis of 2219 Aluminum Alloy during Hot Deformation Process

“…Moreover, different kinds of thermomechanical processing (TMP) may lead to different DRX phenomena, such as discontinuous DRX (DDRX), continuous DRX (CDRX), and geometric DRX (GDRX) [ 20 ]. In order to recognize the differences of the three DRX types, many efforts [ 21 , 22 , 23 , 24 , 25 , 26 , 27 , 28 , 29 , 30 , 31 , 32 , 33 , 34 , 35 , 36 , 37 ] have been made on the observation and revelation of microstructural evolution during the hot forming process. DDRX usually occurs with new strain-free grains nucleation and the grains growth in low-stacking-fault energy (SFE) metals and alloys [ 21 ].…”

“…Based on the mechanisms above, two numerical models of CDRX, the Gourdet-Montheillet model [ 33 ] and Toth et al model [ 34 ], are proposed to characterize this process. Apart from DDRX and CDRX, GDRX is a field with less study, but is still observed in some metals and alloys during a large amount of hot deformation, such as commercial-purity Al [ 35 ] and Mg–Zn–Zr alloys [ 36 , 37 ]. However, the microstructural evolution with constitutive analysis for 2219 Al alloy under the hot deformation process is rarely reported.…”