Mg具备密度低、储量大以及环境友好的特点,在结构材料的轻量化等领域具有广阔的应用前景[1]。然而,作为hcp结构金属,Mg在室温下可开动的独立滑移系少,难以满足冯∙米塞斯准则(Von Mises criterion),造成其室温成形能力差[2]。此外,相比于钢铁和铝合金,镁合金在变形过程中各向异性明显且合金强度低、塑性差,严重限制了其工程应用[3,4]。通常,合金化[5,6]和晶粒细化[7,8]被认为是提高镁合金力学性能的有效途径。然而,对于时效析出镁合金,其析出相尺寸较大、数量密度低且取向单一[9],因此时效硬化响应缓慢且硬化效率低[5,9]。值得注意的是,镁合金的Hall-Petch斜率较大,细晶强化具有明显优势[10,11]。鉴于此,通过热机械加工细化微观结构对强化镁合金和拓宽其应用具有重要意义。
对镁合金而言,晶粒尺寸[20]和异质结构[21~24]对变形机制和力学行为有显著影响。Zheng等[25,26]采用大塑性变形工艺制备了不同晶粒尺寸的完全再结晶块体Mg-Zn-Zr-Ca合金样品,发现孪生和基面<a>滑移在粗晶样品中作为主要变形机制,但随着晶粒尺寸的减小,锥面<c + a>滑移逐渐占据主导地位。Xu等[21]发现,Mg-Gd-Y-Zn-Zr合金在变形过程中非基面<a>滑移和基面<a>滑移分别在回复组织和再结晶组织中激活,可有效缓解局部应力集中现象,从而表现出良好的强塑性匹配。可见,充分利用镁合金变形机制对微观组织的强烈依赖性,设计调控异质结构特性,是实现镁合金性能优化的一种有效而可行的途径。
本工作以Mg-3Gd (质量分数,%)为研究对象,采用累积叠轧(accumulative roll-bonding,ARB)和退火相结合的工艺,通过调控回复和再结晶的体积分数及组织形貌,分别制备了异构样品和均匀组织样品,异构样品是由回复组织层和再结晶组织层交替组成的层状异构样品,均匀组织样品是晶粒尺寸不同的均匀等轴再结晶结构样品,对比分析这2类样品的力学行为和变形机制,揭示层状异构样品能够填补均匀等轴晶结构样品在强塑性匹配上的不足,以期为异质结构镁合金的微观结构设计和宏观性能调控提供新的指导。
1 实验方法
实验材料为Mg-3Gd合金热轧退火板材,线切割加工制得尺寸为70 mm × 25 mm × 1 mm的初始薄板试样。采用ARB工艺对初始薄板试样进行两道次轧制,试样在每道次变形前进行400℃保温8 min的预热处理,总变形压下量为75%[27]。为保证ARB叠层间洁净,在每道次轧制前进行表面机械抛光处理[28]。经290~550℃系列温度1 h退火后,Mg-3Gd样品的微观结构可分为2类：(1) 290和300℃退火样品的微观组织为部分再结晶的层状异构,由层状分布的回复区和再结晶区组成,回复区包含亚微米到纳米尺度的位错结构和孪晶结构,而再结晶区由微米尺度的再结晶晶粒组成；对回复区组织,测定了包含位错界面和孪晶界面的平均尺寸,对再结晶区组织,测定了平均再结晶晶粒尺寸,经回复区和再结晶区的体积分数加权计算,得到290和300℃退火样品的等效平均晶粒尺寸分别为1.3和2.5 μm；(2) 310~550℃退火样品的微观组织为均匀等轴再结晶组织,平均晶粒尺寸为3.3~114 μm[20]。将厚度为1 mm的ARB薄板沿轧制方向(RD)切取拉伸试样,其中试样的标距段长为13 mm、宽为5 mm,利用AGX.50型拉伸机进行室温拉伸实验,采用高精度光学引伸计实时测量应变,应变速率控制在10-4 s-1。采用机械抛光结合电解抛光的方法制备电子背散射衍射(EBSD)样品；透射电子显微镜(TEM)样品的制备采用Gatan PIPS 691型离子减薄仪。微观结构的分析和表征采用配备EBSD的AURIGA双束扫描电子显微镜(SEM)和JEM 2100型TEM。
2 实验结果
ARB变形Mg-3Gd合金微观组织的TEM像如图1a和c所示。可以看出,经过ARB变形后,Mg-3Gd样品的微观组织已经得到明显细化,但微观结构并不均匀,有的区域为层状结构(lamellar structure),主要由与轧制方向近似平行的层状界面(LBs)和层状内部的位错界面组成；有的区域为复杂孪晶结构(twin block),主要由孪晶界面及其内部的层错组成。图1a和b分别为层状结构的TEM像和结构示意图,层状界面(图1b中白虚线)的平均间距约为200 nm,层状内部的小角度位错界面(图1b中黄虚线)横向连接层状界面。这种层状结构与高应变变形铝合金中的层状位错结构类似[29~31]。图1c和d分别为孪晶结构的TEM像和结构示意图(基于文献[27]报道的实验结果)。可以看出,孪晶结构区域主要由大量亚微米到纳米尺度的孪晶片层(TBs)构成,孪晶片层内有大量层错(SFs),区域内也有小角度位错界面(LABs)存在。图1d中插图为孪晶结构区域内Mg基体和孪晶T1的电子衍射花样。
图1
图1
累积叠轧(ARB)变形Mg-3Gd合金的非均匀微观结构的TEM像和结构示意图(基于文献[27]报道的实验结果)
Fig.1
TEM images (a, c) and sketches (b, d) showing a lamellar structure (a, b) and a twin block structure based on the experimental data reported in Ref.[27] (c, d) of the ARB-deformed Mg-3Gd alloy (RD—rolling direction, ND—normal direction, ARB—accumulative roll-bonding, LBs—lamellar boundaries, LAB—low-angle boundary, TB—twin boundary, SF—stacking fault. Inset in Fig.1d shows the corresponding electron diffraction pattern of a deformation twin (T1) and matrix)
图2所示为经290℃部分再结晶退火形成的层状异构Mg-3Gd样品的微观组织。图2a和b分别SEM像和EBSD像。可以看出,经290℃退火1 h样品的局部区域发生了再结晶,其微观结构主要包含部分再结晶区(recrystallized zone)和回复区(recovered zone)。从图2a可以看出,再结晶区和回复区沿RD呈带状或层状平行分布,其中衬度均匀且细小的再结晶晶粒清晰可见。从图2b可以看出,再结晶区主要由取向较为随机的再结晶晶粒组成,而回复区仍含有大量的小角度晶界(白色界面)。图2c和d为层状异构样品的TEM像。可以看出,大量的再结晶晶粒沿RD镶嵌于回复基体内部,而回复区域的衬度则较为复杂,仍呈现高位错密度的变形组织特征。统计结果显示,再结晶晶粒的平均晶粒尺寸为2.4 μm,再结晶区和回复区的体积分数分别为47%和53%。综上可知,ARB样品经适当的热处理工艺,可形成回复层和再结晶层相间的层状异构。
图2
图2
Mg-3Gd合金经290℃部分再结晶退火形成的由层厚不均匀的回复层和再结晶层组成的层状异构
Fig.2
SEM image (a), EBSD image (b), and TEM images (c, d) taken from the Mg-3Gd sample partially recrystallized at 290oC showing a layered heterostructure composed of alternating recovered and recrystallized layers of varying thicknesses
图3给出了不同微观结构Mg-3Gd合金的应力-应变曲线和加工硬化率曲线,包括ARB变形态样品、层状异构样品(Hetero)和均匀再结晶结构样品(Homo)。从图3a可以看出,ARB变形样品的强度高、塑性差；而均匀粗晶再结晶样品(114 μm)的强度低,延伸率约为15%。随着晶粒的细化,细晶均匀结构样品的强度和延伸率得到同步提升,且应力-应变曲线表现出由粗晶样品(114和12.5 μm)的连续流变(应力-应变曲线连续变化,无应力突然下降的屈服点现象)向细晶样品(5.0和3.3 μm)的不连续流变(有屈服点现象)转变。比如,平均晶粒尺寸为3.3 μm的细晶样品的屈服强度达173 MPa,延伸率达37%,其应力-应变曲线表现出屈服点现象和随后明显的加工硬化现象。值得注意的是,当层状异构形成时(伴随着等效晶粒尺寸的进一步减小),其屈服强度相比于细晶均匀结构样品进一步提升,且保持良好的延伸率(大于20%)。图3b~d分别为Mg-3Gd合金的真应力-真应变曲线、加工硬化率和真应变、以及加工硬化率和真应力的关系曲线。如图3c所示,粗晶均匀再结晶结构样品在变形初期阶段表现出很高的加工硬化能力,但随着应变的增加,加工硬化率快速降低。这是因为,粗晶样品在变形早期容易诱发孪生,从而提高加工硬化率[10]。然而,细晶样品在经历变形初期的加工硬化率降低后迅速回升,并在随后的整个变形过程始终稳定在较高水平。此外,如图3d所示,随真应力增加,ARB变形样品和粗晶均匀结构样品的加工硬化率快速降低,细晶均匀结构样品的加工硬化率介于2者之间；在同等真应力条件下,层状异构样品的加工硬化率最高。
图3
图3
ARB变形态、层状异构和均匀再结晶结构Mg-3Gd合金的应力-应变曲线和加工硬化率曲线
Fig.3
Tensile engineering stress-strain curves (a), tensile true stress-strain curves (b), work hardening rate-true strain curves (c), and work hardening rate-true stress curves (d) of the Mg-3Gd alloy with ARB-deformed structure, layered heterostructures (indicated by Hetero), and homogeneous recrystallized grain structures (indicated by Homo) (d—average grain size)
图4总结了不同晶粒尺寸Mg-3Gd合金的屈服强度与均匀延伸率的关系。可以看出,ARB变形态Mg-3Gd样品(平均晶粒尺寸为0.15 μm)的屈服强度最高；平均晶粒尺寸为114 μm的粗晶样品的强度最低,均匀延伸率约为12%。随着晶粒尺寸的降低,细晶均匀结构Mg-3Gd样品(Homo)的强度逐渐趋于稳定且均匀延伸率达到峰值。需要指出的是,在细晶均匀结构样品和变形态样品之间可以看到明显的性能空白区域,即力学性能趋于变形样品高强低塑区和细晶样品低强高塑区之间的空白。这说明,在微米晶或纳米晶尺度范围内,常规晶粒尺寸均匀结构调控策略似乎很难进一步优化综合力学性能。然而,层状异构Mg-3Gd样品(Hetero)可以实现强塑性的良好匹配。如图4所示,进一步降低平均晶粒尺寸(小于3.3 μm),层状异构开始形成,且层状异构样品在强度继续提高的同时,保持较好的均匀延伸率。很明显,Mg-3Gd样品展现出了不同于常规强塑性平衡关系的反C曲线。
图4
图4
不同微观结构和晶粒尺寸Mg-3Gd合金的屈服强度与均匀延伸率的关系
Fig.4
Uniform elongation versus yield strength of Mg-3Gd alloy with different microstructures and average grain sizes
3 分析讨论
3.1 结构形成机制
鉴于层状异构材料的优良力学性能,了解层状异构镁合金的微观结构形成机制,特别是从变形组织到层状异构的演化过程,对指导设计高性能镁合金至关重要。尽管层状异构在结构金属材料中已有报道,例如：Ti[17]、IF钢[18]和Fe-Mn钢[19]等,但其微观结构的形成机制仍然存在争议[12,13]。特别对于hcp结构金属,滑移和孪生在变形过程中往往同步激发且交互作用明显,其表现为微观结构的细化效率提高,但通常导致变形组织的局部不均匀[6,27]。通过TEM观察结果可知,变形态Mg-3Gd样品的微观结构主要包含层状结构和复杂孪晶结构。统计结果显示,2种结构的体积分数和片层厚度各不相同。这意味着,变形样品在不同结构处的储存能存在差异,而后续退火处理可导致再结晶晶粒的局部优先形核和长大。因此,这种变形结构的不均匀被认为是层状异构形成的主要原因之一。
3.2 力学行为特征
图5
3.3 变形机制
镁合金室温变形时最易启动的滑移系是基面<a>滑移,这是因为激活基面<a>滑移所需的临界分切应力相比非基面滑移要低[1]。然而,基面<a>滑移只能提供2个独立滑移系,不能满足Von Mises准则,因此镁合金室温下的均匀塑性变形能力差。为了提高镁合金的塑性变形能力,非基面滑移系的激活,尤其是锥面<c + a>滑移的开动至关重要[3,34]。最新的研究结果[20,25]表明,有效的微观结构设计策略可对锥面<c + a>滑移的激活起到关键作用。Luo等[20]和Zheng等[25]在细晶镁合金的拉伸变形过程中观察到了高密度的<c + a>位错,而在粗晶样品中基面<a>位错和孪生作为主要变形机制。鉴于此,本工作针对层状异构镁合金的变形机制进行了系统研究。层状异构Mg-3Gd样品拉伸变形后的TEM像如图6所示。图6a中回复区和再结晶区的界面由红色虚线标出。可以看出,回复区仍呈现包含纳米片层界面的变形组织特征,这与图2中回复组织的TEM观察结果相符合。而经拉伸变形后再结晶晶粒内部的衬度复杂化,可观察到高密度的位错,其中靠近界面处的位错密度更高。基于TEM双束衍射和位错的不可见判据分析,拉伸变形后层状异构样品的再结晶晶粒内部大量的<a>和<c + a>位错被激活[8]。为了更进一步分析<c + a>位错的滑移类型和特征,采用双束衍射条件下的 g = [0002]操作矢量对其进行表征和分析(图6b)。可以看出,<c + a>位错的台阶状双交滑移特征异常明显,如图6b中白色箭头所示,这说明<c + a>位错具备良好运动能力和增殖能力。结果还显示,部分<c + a>位错(红色箭头所示)的两端被钉扎在基面上,其中基面迹线由(0001)标出。这是因为,在交滑移的过程中<c + a>位错中的刃型部分作为交滑移的产物残留在锥面和基面的交线处,在入射电子束方向显示为与基面平行,而这类位错的螺型部分则平行于Burgers矢量继续向前滑动[35,36]。
图6
图6
层状异构Mg-3Gd样品在拉伸变形后位错结构的TEM像
Fig.6
Dislocation structure analyses of the layered heterostructured Mg-3Gd sample after tensile deformation
(a) bright field TEM image (Red line in Fig.6a shows the interface of recovered area and recrystallized area)
(b) two-beam dark field TEM image with g = [0002] near the [
TEM观察结果表明,临近层状异构界面处有大量<c + a>位错被激活,这种位错的激发被认为与再结晶区和回复区的协调变形有关。研究[12,13,17]认为,在层状异构界面处存在明显的应变梯度场,并在循环加载曲线中证实了背应力强化现象。Wang等[19]采用普通轧制和退火工艺制备了层状异构的Fe-34.5Mn-0.04C样品,发现层状异构样品存在混合法则之外的约束应力强化现象,并认为这主要归因于软硬层之间协调变形而引起的附加几何必需位错的大量激活。总体来说,层状异构Mg-3Gd合金在变形过程中激发锥面<c + a>滑移在界面附近开动,而锥面<c + a>滑移的开动可大幅提高塑性变形能力。而且,由于<a>和<c + a>位错Burgers矢量的大小和方向不同,意味着2者在位错增殖过程中不会因回复作用而相互湮灭。相反,由于Burgers矢量的不同,2种位错在变形过程中的交互作用将会加强,从而进一步促进位错增殖能力和位错累积能力,这也意味着加工硬化能力的进一步提升。正因为如此,层状异构镁合金才能表现出优异的力学性能,同时这种新的微观结构设计策略也为高性能镁合金的制备提供了新的思路。
此外,本工作证实了<c + a>位错具备良好的交滑移能力,交滑移能力的提高有助于协调塑性变形,从而进一步缓解应力集中现象。近年来,研究人员在纯Mg和镁合金中对<c + a>位错的位错核结构及其对塑性变形的贡献进行了大量的模拟分析和讨论,然而关于<c + a>位错的交滑移机制尚存在不同学术观点[35,36],还需要深入研究。Liu等[37]在纯Mg的原位压缩实验中观察到<c + a>位错的交滑移现象。Wu等[38]和Ahmad等[39]则认为<c + a>位错不稳定,室温下即可沿基面分解或附着在基面和锥面的交线处。Wu等[40]认为通过有效的调控策略,比如合金化和结构参数控制,可抑制<c + a>位错的分解,提高其运动能力,从而改善镁合金室温下的塑性变形能力。然而到目前为止,为了更加清楚了解<c + a>位错的源机制、交滑移机制及其影响因素,需要结合位错三维重构技术等[37,41],这是下一步要开展的研究工作。
4 结论
(1) Mg-3Gd合金经ARB变形后在样品厚度方向上形成层状分布的位错组织区域和孪晶组织区域,这种不均匀的变形组织是后续再结晶退火形成层状异构的基础。
(2) ARB变形结合系列退火工艺可制备层状异构和均匀等轴晶结构的Mg-3Gd样品,其中层状异构样品的综合力学性能优于均匀等轴晶结构样品,并表现为连续流变行为。
(3) 层状异构Mg-3Gd合金在变形过程中激发锥面<c + a>位错开动,且<c + a>位错具备良好的交滑移能力。<a>和<c + a>位错由于Burgers矢量不同,在交互作用过程中不会因回复作用而相互湮灭,使得位错增殖能力和位错累积能力得到提高,这是导致层状异构镁合金高加工硬化能力的一个重要原因。
参考文献
Research progress on plastic deformation mechanism of Mg alloys
[J].
Possible slip and twinning systems and their critical resolved shear stresses of Mg alloys with hcp structure were described. Research works on plastic deformation behavior and micro-mechanism of different kinds of Mg alloys were reviewed. Both microstructure and texture evolutions during different thermomechanical processes, both dynamic and static recrystallization mechanisms of Mg alloys were described and discussed. Deformation and strengthening mechanisms of precipitates hardening Mg alloys were also addressed with emphasis on the interaction between precipitates and twinning/slip.
镁合金塑性变形机理研究进展
[J].
Towards the development of heat-treatable high-strength wrought Mg alloys
[J].
Plastic anisotropy and the role of non-basal slip in magnesium alloy AZ31B
[J].
Anisotropy of wrought magnesium alloys: A focused overview
[J].
Precipitation and hardening in magnesium alloys
[J].
Current research and future prospect on low-alloyed high-performance wrought magnesium alloys
[J].Wrought magnesium alloys have a wide range of applications by controlling the microstructure and optimizing the deformation process to improve the mechanical properties. Low-alloyed magnesium alloys have great advantages in formability, corrosion resistance, and light weight. Moreover, low alloying and high performance have become important trends in the development of wrought magnesium alloys. This study reviews the research progress of low-alloyed, high-strength, high-plasticity, and superplastic wrought magnesium alloys regarding alloy composition design, strengthening and toughening mechanisms, and processing technology. From the perspective of improving production efficiency and expanding application scope, the development trend of low-alloyed wrought magnesium alloys is proposed.
低合金化高性能变形镁合金研究现状及展望
[J].变形镁合金通过调控微观组织、优化变形工艺,提高材料的力学性能,从而具有广泛的应用前景。低合金化镁合金在成形性、耐腐蚀性及轻量化等方面具有较大优势。低合金、高性能成为变形镁合金发展的重要趋势之一。本文重点概述了低合金化高强、高塑、超塑性变形镁合金在合金成分设计、强韧化机制及加工技术等方面的研究进展,并从提高生产效率、扩大应用范围的角度,展望了低合金化变形镁合金的发展趋势。
Grain size effects on dislocation and twinning mediated plasticity in magnesium
[J].
Effect of grain size on the mechanical behavior and deformation mechanisms of Mg-3Gd
[D].
Mg-3Gd合金的力学行为、变形机制和晶粒尺寸效应
[D].
Effects of precipitate shape and orientation on dispersion strengthening in magnesium alloys
[J].
The mechanism for the high dependence of the Hall-Petch slope for twinning/slip on texture in Mg alloys
[J].
Size effects on the strength of metals
[J].The grain size effect and the specimen size effect on the strength of metals are briefly reviewed with respect to their history and current status of research. It is revealed that the fundamental strengthening mechanisms responsible for these two types of size effect are to increase the resistance to dislocation motion and to dislocation generation, respectively. It is shown that both strengthening mechanisms take place in some nanostructured metals, which leads to a suggestion to use these two mechanisms for optimizing the strength and ductility of nanostructured metals. This suggestion is verified by some results obtained in nanostructured pure aluminum.
金属强度的尺寸效应
[J].简要综述了金属强度的晶粒尺寸效应和样品尺寸效应的研究历史和现状, 揭示了它们的基本强化机制分别是增加位错运动的阻力和增加位错产生的难度. 在一些纳米金属中发现这2种机制同时起作用, 从而指出利用这2种机制调控纳米金属强塑性的可能性. 这种可能性在纳米纯Al中得到了验证.
Heterogeneous materials: A new class of materials with unprecedented mechanical properties
[J].
Heterostructured materials: Superior properties from hetero-zone interaction
[J].
Review on superior strength and enhanced ductility of metallic nanomaterials
[J].
Alloying design and microstructural control strategies towards developing Mg alloys with enhanced ductility
[J].
Multimodal microstructure of Mg-Gd-Y alloy through an integrated simulation of “process-structure-property
” [J].
“工艺-组织-性能”模拟研究Mg-Gd-Y合金混晶组织
[J].以Mg-8Gd-3Y-0.5Zr (GW83K)镁合金为研究对象,探索通过调控混晶显微组织来提高其力学性能的途径。将具有混晶组织的合金抽象为一种不同尺寸的晶粒嵌入到晶界中的颗粒复合体模型,在该模型中基体相为晶界,不同的颗粒相是不同尺寸的晶粒；然后基于Taylor关系的非局部塑性理论修正不同粒子嵌入到基体后对基体性能的影响,建立具有混晶组织特征的有限元平面应变细观力学模型。采用该模型模拟了含有混晶组织的GW83K镁合金在拉伸条件下的应力-应变行为,模拟结果对比实验数据验证了模型的有效性。在此基础上,通过真实时空相场模拟出的不同退火工艺下的混晶组织作为力学模型的几何输入参数,建立“工艺参数-混晶组织-力学性能”之间的关系。模拟结果对比发现,具有混晶组织的GW83K镁合金强度随平均晶粒尺寸的变化符合Hall-Petch关系,粗晶含量和分布显著影响着合金的塑性,当合金在623 K下退火90 min后对应的混晶组织可在保持高强度的同时有效提升塑性,为混晶组织的设计提供了参考。
Heterogeneous lamella structure unites ultrafine-grain strength with coarse-grain ductility
[J].
Fabricating interstitial-free steel with simultaneous high strength and good ductility with homogeneous layer and lamella structure
[J].
Hall-Petch strengthening in Fe-34.5Mn-0.04C steel cold-rolled, partially recrystallized and fully recrystallized
[J].
Transitions in mechanical behavior and in deformation mechanisms enhance the strength and ductility of Mg-3Gd
[J].
Deformation behavior of ultra-strong and ductile Mg-Gd-Y-Zn-Zr alloy with bimodal microstructure
[J].
Microstructure and mechanical properties of non-flammable Mg-8Al-0.3Zn-0.1Mn-0.3Ca-0.2Y alloy subjected to low-temperature, low-speed extrusion
[J].
The synergy effect of fine and coarse grains on enhanced ductility of bimodal-structured Mg alloys
[J].
Strong and ductile AZ31 Mg alloy with a layered bimodal structure
[J].AZ31 Mg alloy was processed by accumulative roll-bonding (ARB) and hot rolling (HR), respectively, followed by annealing. Layered bimodal structures characterized by an alternative distribution of fine-grained layers and coarse-grained layers were obtained in the ARB samples, while mixed bimodal structures were achieved in the HR samples. The ARB samples have superior combinations of high strength and good elongation compared to the HR samples, indicating a clear effect of layered bimodal structures on mechanical properties of the alloy. The strength of the ARB samples is related to the grain size; while the ductility is attributed to the activity of non-basal slip and the strong backstress.
Change of deformation mechanisms leading to high strength and large ductility in Mg-Zn-Zr-Ca Alloy with fully recrystallized ultrafine grained microstructures
[J].Recently, we have found that fully recrystallized ultrafine-grained (UFG) microstructures could be realized in a commercial precipitation-hardened Magnesium (Mg) alloy. The UFG specimens exhibited high strength and large ductility under tensile test, but underlying mechanisms for good mechanical properties remained unclear. In this study, we have carried out systematic observations of deformation microstructures for revealing the influence of grain size on the change of dominant deformation modes. We found that plastic deformation of conventionally coarse-grained specimen was predominated by {0001} <11-20> slip and {10-12} <10-11> twinning, and the quick decrease of work-hardening rate was mainly due to the early saturation of deformation twins. For the UFG specimens, {10-12} <10-11> twinning was dramatically suppressed, while non-basal slip systems containing <c> component of Burgers vector were activated, which contributed significantly to the enhanced work-hardening rate leading to high strength and large ductility. It was clarified by this study that limited ductility of hexagonal Mg alloys could be overcome by activating unusual slip systems (<c + a> dislocations) in fully recrystallized UFG microstructures.
Simultaneously enhanced strength and ductility of Mg-Zn-Zr-Ca alloy with fully recrystallized ultrafine grained structures
[J].
Microstructural evolution in Mg-3Gd during accumulative roll-bonding
[J].
Strength and ductility of ultrafine grained aluminum and iron produced by ARB and annealing
[J].
Hardening by annealing and softening by deformation in nanostructured metals
[J].We observe that a nanostructured metal can be hardened by annealing and softened when subsequently deformed, which is in contrast to the typical behavior of a metal. Microstructural investigation points to an effect of the structural scale on fundamental mechanisms of dislocation-dislocation and dislocation-interface reactions, such that heat treatment reduces the generation and interaction of dislocations, leading to an increase in strength and a reduction in ductility. A subsequent deformation step may restore the dislocation structure and facilitate the yielding process when the metal is stressed. As a consequence, the strength decreases and the ductility increases. These observations suggest that for materials such as the nanostructured aluminum studied here, deformation should be used as an optimizing procedure instead of annealing.
Strengthening mechanisms in nanostructured high-purity aluminium deformed to high strain and annealed
[J].
Strengthening mechanisms and Hall-Petch stress of ultrafine grained Al-0.3%Cu
[J].
Yielding behavior and its effect on uniform elongation in IF steel with various grain sizes
[J].
Strategy for managing both high strength and large ductility in structural materials—Sequential nucleation of different deformation modes based on a concept of plaston
[J].
The activity of non-basal slip systems and dynamic recovery at room temperature in fine-grained AZ31B magnesium alloys
[J].
Transmission electron microscopy investigation on dislocation bands in pure Mg
[J].
An electron microscopy study of dislocation structures in Mg single crystals compressed along [0001] at room temperature
[J].
Large plasticity in magnesium mediated by pyramidal dislocations
[J].
The origins of high hardening and low ductility in magnesium
[J].
Designing high ductility in magnesium alloys
[J].The thermally activated pyramidal-to-basal (PB) transition of (c + a) dislocations, transforming glissile pyramidal dissociated core structures into sessile basal dissociated ones, lies at the origin of low ductility in pure magnesium (Mg). Solute-accelerated cross-slip and double cross-slip of pyramidal (c + a) dislocations have recently been proposed as a mechanism that can circumvent the deleterious effects of the PB transition by enabling rapid dislocation multiplication and isolating PB-transformed sessile segments. Here, the theory for solute-accelerated cross-slip is revisited with an explicit atomistic derivation, is extended to include multiple very dilute solute concentrations, and various aspects of the theory are demonstrated computationally. DFT inputs to the theory for a wide range of new alloying elements are presented. The theory is validated by comparing predicted ductility to literature experiments for a range of alloys. The theory is then applied to predict composition ranges for ductility in rare-earth free ternary and quaternary dilute alloys. The wide range of new alloys predicted to be ductile can serve as a guide to experimental development of new ductile Mg alloys. (C) 2019 Acta Materialia Inc. Published by Elsevier Ltd.
Mechanistic origin and prediction of enhanced ductility in magnesium alloys
[J].Pure magnesium exhibits poor ductility owing to pyramidal [Formula: see text] dislocation transformations to immobile structures, making this lowest-density structural metal unusable for many applications where it could enhance energy efficiency. We show why magnesium can be made ductile by specific dilute solute additions, which increase the [Formula: see text] cross-slip and multiplication rates to levels much faster than the deleterious [Formula: see text] transformation, enabling both favorable texture during processing and continued plastic straining during deformation. A quantitative theory establishes the conditions for ductility as a function of alloy composition in very good agreement with experiments on many existing magnesium alloys, and the solute-enhanced cross-slip mechanism is confirmed by transmission electron microscopy observations in magnesium-yttrium. The mechanistic theory can quickly screen for alloy compositions favoring conditions for high ductility and may help in the development of high-formability magnesium alloys.Copyright © 2018, The Authors, some rights reserved; exclusive licensee American Association for the Advancement of Science. No claim to original U.S. Government Works.
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