Anisotropy of thermal conductivity and mechanical properties in Mg–5Zn–1Mn alloy

The microstructure, texture, thermal conductivity and mechanical properties of the as-extruded Mg–5Zn–1Mn (ZM51) magnesium alloy were investigated on specimens with the extrusion direction (ED), the transverse direction (TD) and the normal to the extrude plane (ND), respectively. The results indicated that the thermal conductivity of ZM51 alloy at room temperature is 125 (W/m K), almost twice as high as other conventional commercial magnesium alloys, such as Mg–Al series and Mg–RE series. The effect of texture on anisotropy of mechanical properties and thermal conductivity has been analyzed. The strong crystallographic texture typical of Mg alloys results in much higher yield strength and tensile strength (UTS) in the extrusion direction, but higher ductility and thermal conductivity in the transverse direction.     

Dynamic compressive behaviour and microstructural evolution of extrusion-shear Mg–6Zn–1Cu–1Y–0.6Zr alloy

In this work, the alloy Mg–6Zn–1Cu–1Y–0.6Zr was prepared using an extrusion-shear method, which combines traditional extrusion with the equal channel angular pressing. Dynamic compressive behaviour and microstructural evolution were studied along the extrusion direction with strain rates in the range of 695–1995?s^?1 using a Split-Hopkinson pressure bar. The dynamic compression properties have a distinct positive strain rate strengthening effect. A texture transition from ?10–10? into ?0001? is found in the Mg–6Zn–1Cu–1Y–0.6Zr alloy. Analysis of the microstructural evolution shows that {10–12} extension twinning and (0002) basal-type slip are major deformation mechanisms. The absorption energy density dramatically increases as the strain rate increases, results indicate that dynamic recrystallisation and high yield strength are mainly responsible for the high energy absorption capacity.     

Microstructure and mechanical properties of LA51 and LA51–0.5Y alloys with different accumulated strains and rolling temperatures

Mg–5Li–1Al (LA51) and Mg–5Li–1Al–0.5Y (LA51–0.5Y) alloys were smelted and rolled with different accumulated strains (36% and 68%) and rolling temperatures (373 K and 573 K). The microstructure, mechanical properties, fracture morphology and texture of the specimens were investigated. Results show that, due to the PSN (particle stimulate nucleation) mechanism, the addition of 0.5 wt.% Y improves the deformation resistance and weakens the basal texture of LA51 alloy. The effect of Y on UTS (ultimate tensile strength) of as-rolled alloys is more obvious than that of the as-cast alloy. Accumulated strain and rolling temperature could influence the twinning number, slip systems and DRX (dynamic recrystallization), thus affecting the microstructure and mechanical properties of the alloy. Under the proper combination of the above factors, the as-rolled LA51–0.5Y alloy with an accumulated strain of 68% at 573 K possesses the best comprehensive mechanical properties.     

Deformation behaviour of ultrafine grained Mg–7Gd–5Y–1.2Nd–0.5Zr alloy under high strain rates

The mechanical behaviour of Mg–7Gd–5Y–1.2Nd–0.5Zr (wt. %) alloy with ultrafine grains was measured by split Hopkinson pressure bar method under the strain rates of 1000, 1500, and 2000 s^?1 at room temperature. Dynamic tests were carried out along extrusion direction (ED), transverse direction (TD), and normal direction (ND). The results demonstrated that the flow stress increased with the increase of strain rate, showing a positive strain rate strengthening effect. There was no obvious anisotropy in dynamic compression along ED, TD, and ND, which was caused by rare earth elements and multi-pass deformation. This led to the adoption of plastic deformation mode dominated by non-basal slip and participated by tension twinning.     

Effect of icosahedral phase on the crystallographic texture and mechanical anisotropy of duplex structured Mg–Li alloys

Through comparing the microstructure and mechanical properties of Mg–6%Li and Mg–6%Li–6%Zn–1.2%Y alloys, the difference in their mechanical anisotropy have been deeply investigated. Due to the formation of I-phase (Mg₃Zn₆Y, icosohedral structure), the volume fraction of β-Li phases formed in Mg–6%Li–6%Zn–1.2%Y alloy was two times higher than that in Mg–6%Li alloy even these two alloys having the same Li content. Texture analysis demonstrated that I-phase particle stimulated nucleation of recrystallization (PSN) and the formation of higher volume fraction of β-Li phases can remarkably weaken the basal texture of α-Mg phases, resulting in a very weak mechanical anisotropy. On the contrary, the basal texture of the α-Mg phases in Mg–6%Li alloy was quite strong and obvious mechanical anisotropy between differently oriented samples can be observed.     

Effect of crystallographic texture on precipitation induced anisotropy in an aluminium magnesium silicon alloy

The effect of crystallographic texture on precipitation induced anisotropy in yield strength of an aluminium magnesium silicon alloy was investigated. Solutionized samples were subjected to unidirectional and multi-step cross rolling to yield distinct crystallographic textures in the Al–Mg–Si alloy. The rolled sheets were then subjected to annealing followed by second solutionizing treatment to provide sheets with similar grain size and dislocation content but distinct texture. Ageing experiments were carried out on these sheets at 443 K for different time intervals. It was observed that the evolution of anisotropy in yield strength of the age hardened alloy depends on texture. The difference in age hardening response brought about by varying initial texture controls the evolution of anisotropy in mechanical properties of the alloy. This was manifested in terms of transition from anisotropic to isotropic mechanical properties in the unidirectionally rolled samples after peak ageing. On the contrary, a transition from isotropic to anisotropic yield behaviour was observed for multi-step cross rolled samples. This is attributed to enhanced precipitation hardening in crystallographically softer orientations compared to crystallographically harder orientations.     

Grain size effects on the tensile properties and deformation mechanisms of a magnesium alloy, AZ31B, sheet

The grain size dependence of the tensile properties and the deformation mechanisms responsible for those properties are examined for Mg alloy, AZ31B, sheet. Specifically, the Hall–Petch effect and strain anisotropy (r-value) are characterized experimentally, and interpreted using polycrystal plasticity modeling. {1 0 . 2} extension twins, {1 0 . 1} contraction twins, and so-called “double-twins” are observed via microscopy and diffraction-based techniques, and the amount of twinning is found to increase with increasing grain size. For the sheet texture and tensile loading condition examined, {1 0 . 2} extension twinning is not expected, yet the polycrystal plasticity model predicts the observed behavior, including this ‘anomalous’ tensile twinning. The analysis shows that the Hall–Petch strength dependence, of the polycrystal as a whole, is primarily determined by the grain size dependence of the strength of the prismatic slip systems.     

Effects of minor additions of Ce and Y on as cast microstructure of Mg–3Sn–2Ca magnesium alloy

Abstract

The effects of minor additions of Ce and Y on the as cast microstructure of Mg–3Sn–2Ca (wt-%) magnesium alloy are investigated and compared. Results indicate that adding minor Ce or Y to Mg–3Sn–2Ca alloy does not cause formation of any new phases in the alloy. The as cast Mg–3Sn–2Ca alloy with addition of 0·5 wt-%Ce or Y is still composed of α-Mg, CaMgSn and Mg₂Ca phases. However, after adding 0·5 wt-%Ce or Y to Mg–3Sn–2Ca alloy, not only the formation of CaMgSn phase in the alloy is suppressed but also the CaMgSn phases in the alloy are effectively refined. In addition, adding 0·5 wt-%Ce to Mg–3Sn–2Ca alloy exhibits higher refinement efficiency to the CaMgSn phase in the alloy than adding 0·5 wt-%Y. Further investigations need to be considered in order to understand the difference of minor Ce and Y with regard to the refinement of CaMgSn phase in the Mg–3Sn–2Ca alloy.     

Texture effects on design of Mg biodegradable stents

ZM21 and AZ31B magnesium alloy tubes were hot extruded by a laboratory equipment in order to produce precursors for biodegradable stents. The microstructure and the texture evolution during the extrusion were investigated and related to the resulting mechanical properties. The tubes featured a homogeneous and refined equiaxed grain structure due to dynamic recrystallization experienced during the extrusion process. Electron backscattering diffraction analysis along the longitudinal section of the tubes revealed a typical ‘basal’ type texture featuring hexagonal basal plane aligned parallel to the longitudinal section of the tubes. Tensile and compression tests on different size of tubes showed a strong asymmetry in mechanical properties, the tensile strength being significantly higher than the strength measured in compression. Data about tension-compression behavior were related to crystallographic texture of samples and to the occurrence preferred {10–12} <10–11> twinning in compression. The significance of the tension-compression behavior is analyzed in view of the application of the extruded tubes for biodegradable stents, especially considering the stages of expansion and scaffolding. The importance of considering the above effects in stent design and modeling is highlighted in order to improve performance and reliability of such devices.     

Effect of rare earth elements on deformation behavior of an extruded Mg–10Gd–3Y–0.5Zr alloy during compression

The aim of this study was to identify the influence of rare-earth (RE) elements on the strain hardening behavior in an extruded Mg–10Gd–3Y–0.5Zr magnesium alloy via compression in the extrusion direction at room temperature. The plastic deformation behavior of this RE-containing alloy was characterized by a rapidly decreasing strain hardening rate up to a strain level of about 4% (stage A), followed by a fairly flat linear strain hardening rate over an extended strain range from ∼4% to ∼18% (stage B). Stage C was represented by a decreasing strain hardening rate just before failure. The extent of twinning in this alloy was observed to be considerably less extensive than that in the RE-free extruded Mg alloys. The weaker crystallographic texture, refined grain size, and second-phase particles arising from the addition of RE elements were responsible for the much higher strain hardening rate in stage A due to the increased difficulty on the formation of twins and the slip of dislocations at lower strains, and for the occurrence of quite flat linear strain hardening in stage B at higher strains which was likely related to the dislocation debris and twin debris (or residual twins) stemming from dislocation–twin interactions as well as the interactions between dislocations/twins and second-phase particles and grain boundaries.     

Effect of local strains on the texture and mechanical properties of AZ31 magnesium alloy produced by continuous variable cross-section direct extrusion (CVCDE)

Three different mold structures were designed by changing the parameters of mold cavity to study the effect of local strains on the texture and mechanical properties of AZ31 magnesium alloy produced by continuous variable cross-section direct extrusion (CVCDE) with 2 interim dies. Microstructure and texture evolution of AZ31 magnesium alloy after CVCDE were studied by electron backscatter diffraction (EBSD). Mechanical properties were determined by uniaxial tensile tests along extrusion direction (ED) at room temperature. Due to the differences of local strains among the three schemes, the microstructure of Scheme 1 was the most uniform and the average grain size of Scheme 1 was the smallest. Meanwhile, tensile strength and elongation of Scheme 1 were the highest. Different textures had been formed in the three schemes. Lots of extension twins {10–12} (86°< 1–210 >) occurred in the products of the three schemes. The main deformation modes of Scheme 1 and Scheme 2 were slip and twinning. However, slip was dominant in Scheme 3. The deformation modes provided an essential basic for the design of CVCDE mold structure with more interim dies.     

Microstructure and mechanical properties of asextruded Mg–xAl–5Sn–0·3Mn alloys (x = 1, 3, 6 and 9)

Properties of Mg–xAl–5Sn–0·3Mn alloys (x?=?1, 3, 6 and 9) prepared by hot extrusion are reported. The orientation relationship between Mg₂Sn precipitate and Mg matrix in Mg–9Al–5Sn–0·3Mn alloy was determined. The yield strength of the as extruded alloys initially decreased with increasing Al content, then increased for Al contents >6 wt-%. These changes are interpreted in terms of the effect of texture, grain size and second phase on the yield strength of the alloys.     

Effects of icosahedral phase on mechanical anisotropy of as-extruded Mg-14Li (in wt%) based alloys

Through investigating and comparing microstructure and crystallographic texture of as-extruded Mg-14Li and Mg-14Li-6Zn-1Y(in wt%) alloys,the differences in their mechanical anisotropy were investigated.It revealed that the formation of I-phase(Mg_3Zn_6Y,icosahedral structure) can effectively refine grain size.Moreover,compared with Mg-14Li alloy,the texture type of Mg-14Li-6Zn-1Y alloy changed slightly,but its texture intensity decreased remarkably.As a result,the stronger texture contributed to the "normal" mechanical anisotropy of Mg-14Li alloy with higher tensile strength and a lower elongation ratio along transverse direction(TD) than those along extrusion direction(ED).However,for Mg-14Li-6Zn-1Y alloy,the zonal distribution of I-phase particles along ED caused "abnormal" mechanical anisotropy,i.e.higher tensile strength and better plasticity along ED.     

Grain size effect on deformation twinning in Mg–Al–Zn alloy

Abstract

Specimens of Mg–3Al–1Zn alloy with a wide range of grain size distribution were compressed along different directions. The compressed microstructure was examined to clarify the grain size effect on deformation twinning in magnesium alloys. Small strains were used to reveal the twinning behaviour. The results show that the grain size affects the formation of deformation twins in an Mg–Al–Zn alloy. The reason for a different result being previously reported is given. This study also reports the different deformation microstructures in specimens compressed along different directions.     

Fatigue of rare‐earth containing magnesium alloys: a review

Lightweighting in ground vehicles is today considered as one of the most effective strategies to improve fuel economy and reduce anthropogenic environment‐damaging and climate‐changing emissions. Magnesium (Mg) alloy, as a strategic ultra‐lightweight metallic material, has recently drawn a considerable interest in the transportation industry to reduce the weight of vehicles due to their high strength‐to‐weight ratio, dimensional stability, good machinability and recyclability. However, the hexagonal close‐packed crystal structure of Mg alloys gives only limited slip systems and develops sharp deformation textures associated with strong mechanical anisotropy and tension–compression yield asymmetry. For the vehicle components subjected to dynamic loading, such asymmetry could exert an unfavourable influence on the material performance. This problem could be conquered through weakening the texture via addition of rare‐earth (RE) elements. Thus, a number of RE‐containing Mg alloys have recently been developed. To guarantee the structural integrity, durability and safety of highly loaded structural components, understanding the characteristics and mechanisms of cyclic deformation and fatigue fracture of such RE‐Mg alloys is of vital importance. In this review, the available fatigue properties including stress‐controlled fatigue strength, strain‐controlled cyclic deformation characteristics and fatigue crack propagation behaviour are summarized, along with the microstructural change and crystallographic texture weakening in the RE‐containing Mg alloys in different forms (cast, extruded and heat‐treated states), in comparison with those of RE‐free Mg alloys.     

Grain-scale deformation in a Mg−0.8 wt% Y alloy using crystal plasticity finite element method

Magnesium (Mg) alloys with hexagonal close-packed (HCP) structure usually have a poor ductility at room temperature. The addition of yttrium (Y) can improve the ductility of Mg alloys. To understand the underlying mechanism, crystal plasticity finite element method (CPFEM) was employed to simulate the tensile deformation of a Mg−0.8 wt% Y alloy. The simulated stress‒strain curve and the grain-scale slip activities were compared with an in-situ tensile test conducted in a scanning electron microscope. According to the CPFEM result, basal slip is the dominant deformation mode in the plastic deformation stage, accounting for about 50% of total strain. Prismatic slip and pyramidal 〈a〉 slip are responsible for about 25% and 20% of the total strain, respectively. Pyramidal ⟨c + a⟩ slip and twinning, on the other hand, accommodate much less strain.     

Sn对时效态ZM61镁合金高温力学性能的影响

利用金相显微镜(OM)、X射线衍射(XRD)、扫描电镜(SEM)和高温拉伸对时效态ZM61-xSn(x=0,6,8,10,质量分数/%,下同)合金的高温拉伸性能及断裂机制进行了研究。结果表明:ZM61-xSn(x=6,8,10)合金的物相由α-Mg,α-Mn,MgZn2,Mg2Sn相组成。添加Sn元素可有效细化ZM61合金组织,提高合金高温强度,但降低合金塑性。ZM61-xSn(x=6,8,10)合金在300℃下拉伸的抗拉强度分别为149,140,145MPa,较相同温度下拉伸的ZM61合金的抗拉强度分别提高了26%,17%,23%。ZM61-xSn(x=0,6,8,10)合金在300℃下拉伸的伸长率分别为39.95%,5.65%,7.01%和6.33%。拉伸温度对ZM61-xSn(x=6,8,10)合金的断裂机制产生显著影响。当拉伸温度低于220℃,合金为穿晶断裂;高于220℃时,合金变为沿晶断裂。     

Deformation behaviour of Al–Cu–Li alloy containing T1 precipitates

In the present work, a systematic investigation of crystallographic texture evolution and strain hardening behaviour was undertaken to comprehend the deformation behaviour in the presence of T₁ (Al₂CuLi) precipitates. Characteristic texture components symbolising multiple slip condition such as Copper and S were observed upon rolling which is in contrast with other Al alloys containing shearable precipitates. Strain hardening ability was also observed to be remarkably high in the presence of T₁ precipitates. The texture and strain hardening results are compared with another age hardenable Al alloy (Al–Mg–Si alloy) containing shearable precipitates to clearly bring out the difference in the nature of T₁ precipitates.     

Effect of beryllium and calcium on aging behaviour of Al–0·75Mg–0·5Si alloy

Abstract

The effect of microadditions of Be and Ca on the aging behaviour of Al–0·75Mg–0·5Si alloy is investigated. It is shown that the addition of 0·1%Be significantly increases the hardening rate and the maximum hardness level attainable when the alloy is aged at various temperatures from room temperature to 300°C, while the addition of 0·2%Ca decreases both the hardening rate and the maximum hardness level attainable. Optical and scanning electron microscopical observations show a significantly higher precipitate density for the Be containing alloy and a slightly lower precipitate density for the Ca containing alloy when compared with the base Al–Mg–Si alloy. The results are consistent with an earlier kinetic study that indicated a Be enhanced nucleation rate for precipitation in the same alloy.

MST/936     

Influence of solution and aging on the microstructures and mechanical properties of complex deformed WE93 alloy

Optical microscopy, scanning electron microscopy, transmission electron microscopy, hardness testing, and mechanical property testing were performed to study the influence of solution (T4) and aging (T6) on the microstructures and mechanical properties of the WE93 magnesium alloy. A reasonable solution treatment and an aging regime were developed, and the fracture features of the alloy in different states were analyzed. Results show that complex deformation produces microstructures that are largely characterized by deformation-precipitated Mg–Y phase (Mg₂₄Y₅), in addition to those with cubic Mg–Y and Mg–MM phases, which are both undissolved after homogenization. The optimum solution treatment condition for the alloy is a holding temperature of 490 °C for 2 h. After solution treatment, the precipitated Mg–Y phase re-dissolves and grain size grows to a limited extent, which may be attributed primarily to the pinning effect of the Mg–MM phase on the grain boundary. The reasonable aging regime was maintained at 225 °C for 40 h. After the solution and aging treatments, the ultimate tensile strength of the alloy at room temperature reaches 375 MPa but the elongation is only 3%. As indicted by the fracture behavior of the alloy, the secondary cracks of the extruded alloy and the solid-solution alloy occur mainly in the Mg–MM phase with few transcrystalline fractures. After peak aging, however, transcrystalline cracks appear on the grains at room and high temperatures. Under a multi-strengthening mechanism, the mutual coordinating effect may depend primarily on service temperature.