AZ31镁合金的形变孪生行为及孪生机制
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摘要
{1012}孪生在镁合金的塑性变形中起着重要的作用，是导致镁合金低屈服、拉压屈服不对称以及各向异性等的主要原因。要解决这些问题，研究基于孪生的材料强化机制是关键。但是到目前为止，材料学中的强化机制几乎都是建立在阻碍位错运动的基础上的，基于孪生的强化机制还没有完善，主要原因在于对孪生的行为及机制还没有完全弄明白，因此对镁合金的形变孪生行为及其机理展开进一步的研究显得尤为必要。本文从研究AZ31挤压镁合金在孪生变形过程中的组织与织构演变情况入手，揭示了挤压镁合金压缩孪生变形过程组织演变的本质，探讨了通过预变形引入片层状{1012}孪晶改善镁合金拉压不服不对称的可能性。在此基础上，为了深入地研究镁合金中{1012}孪生的机制，本文从孪生过程原子运动的角度出发，建立了镁合金{1012}孪生原子群单元运动模型，揭示孪生过程原子运动的规律。主要结论如下：
①对挤压AZ31镁合金进行沿挤压方向（ED）的单向压缩变形，发现在光学显微组织分析中孪晶数量先是随着变形量的增大而增大，之后随着变形量的继续增大孪晶逐渐减少甚至消失。从组织观察结果分析似乎这个过程产生了退孪生，但是织构演变分析表明这个过程并没有发生退孪生。造成这一组织演变的实质是孪晶形核、长大与合并，导致在显微组织分析中呈现出类似于退孪生的现象。
②沿镁合金ED方向的预先压缩对随后垂直于ED方向压缩的力学性能有重要影响。研究表明，ED方向上的预先压缩能够明显地提高垂直于ED方向压缩的屈服强度，这主要是因为ED方向上的预压缩导致合金晶粒取向发生转变，以致在随后垂直于ED压缩时变形机制由基面滑移容易启动转变到不容易启动，导致屈服强度比不存在预变形的试样高。同时，由于不同施载模式下孪生变体启动的数量不一样，孪晶的形貌也不一样。对于挤压镁合金，当沿ED方向压缩3%时从垂直于ED的横截面上观察，同一个晶粒内的孪晶几乎都是相互平行的。但是对于先沿ED方向预压缩8%再垂直于ED压缩3%的试样，对于同一方向上的观察面，由于后者孪生变体启动的情况不一样，孪晶特征也有较大的差别，有的晶粒内孪晶是相互平行的，有的晶粒内孪晶则是相交的，交角约为60°。
③在挤压态镁合金中通过预变形引入片层状{1012}孪晶对随后拉伸、压缩变形的力学行为有重要的影响。在预压缩变形中，产生了片层状的{1012}孪晶。在随后的变形中，压缩屈服强度随着预变形量的增加而逐步增大，而拉伸屈服强度则随着预变形量从零开始到1.7%左右迅速降低，此后随着预变形量的增加，拉伸屈服强度几乎保持不变。导致拉伸屈服强度降低的原因主要是在反向拉伸过程中产生了退孪生。预变形量小于约1.1%时，拉压不对随着预变形量的增加逐渐降低，但此后随着预变形量的继续增大，拉压屈服不对称又开始升高。由此可推断，如果控制好合适的预变形量，对改善镁合金的拉压不对称是有利的。
④退孪生本质上是在已孪生区发生的孪生过程。通过控制预变形量即可实现在随后的反向受力变形中得到不同的最大退孪生体积，考察不同退孪生体积试样的加工硬化行为能够更加直观地反映孪生对加工硬化的贡献。在挤压态AZ31镁合金中通过控制压缩预变形量可以得到不同的孪生体积，这些含有不同已孪生体积的试样在随后反向拉伸变形的过程中表现出不同的加工硬化行为。没有预变形的试样在拉伸变形中加工硬化率不存在随应变量增大而上升的阶段；3%和9%预变形试样虽然都存在这个阶段，但是3%预变形试样在较低伸长量时加工硬化率就开始下降，而9%预变形试样的加工硬化率则延续到较高伸长量时才开始下降。由于预变形量小的试样退孪生率先完成，这验证了由{1012}孪生引起的织构强化对合金的加工硬化有着重要的作用。同时，由于退孪生对塑性变形的贡献，预变形量越大的试样在随后反向拉伸时具有越高的伸长量。
⑤通过建立直角坐标系计算孪生过程原子的位移矢量，可把孪生过程原子的运动归结为原子群单元的旋转和平移。对AZ31镁合金，原子群单元的旋转角为15.86°，每一层单元体之间的相对平移矢量为bT(bT=0.049nm)。无数个原子群单元的旋转和平移即可得到一定厚度的孪晶。
⑥镁晶胞中靠近孪生面的B位原子运动1/6〈1010〉矢量可导致原子群单元旋转，而原子群单元的旋转能够促进〈1011〉方向孪生位错的生成，使得单元体产生平移，到达孪生位置。1/6〈1010〉不全位错可能由晶界缺陷或在晶界附近受阻的位错分解得到。
{1012} twinning plays an important role in magnesium and its alloys duringplastic deformation and it is always responsible for the low yield stress, high yieldasymmetry between tension and compression and anisotropy in these alloys. In order toresolve these problems, it is very important to understand the strengthening mechanismbased on twinning. However, nearly all the strengthening mechanisms in materialsscience are based on slip and the strengthening mechanism based on twinning is stillvery limited up to these days because of ambiguity in twinning behaviour and twinningmechanism. Therefore, it is still necessary to do more investigations on twinningbehaviour and twinning mechanism in magnesium alloys. In this paper, the essential ofmicrostructural evolution during compression in extruded magnesium alloy AZ31wasrevealed via texture and microstructure analysis. The possibility to decrese the yieldasymmetry via introducing lamellar {1012} twins by pre-compression was investigatedand discussed. It was found that the yield asymmetry can be decreased by pre-strain ifthe amount of pre-strain was appropriate. In order to further understand the twinningmechanism, a model was established to describe the atomic motion in {1012} twinningand the law of atomic motion in {1012} was revealed in magnesium. The majorconclusions can be summarized as follows.
①Uinaxial compression along extrusion direction (ED) was carried out in anextruded magnesium alloy AZ31. The microstuctual analysis showed that the amount oftwins increased as a function of strain if the strain is relatively low. However, it began todecrease with continual increase in strain, or even disappeared at last. It seemed thatdetwinning occurred in this process. However, the texture analysis showed that nodetwinning occurred. The essential of microstructual evolution in this process is actuallytwin nucleation, twin growth and coalescence, leading to an analogous detwinningphenomenon observed in the optical micrographs.
②Pre-compression along ED has an obvious effect on subsequent compressionperpendicular to ED in an extruded magnesium alloy AZ31. The results showed that theyield stress under compression perpendicular to ED increased obviously if there waspre-compression along ED. The reason for this change is mainly the reorientationproduced by twinning during pre-compression, leading to a low basal activity. Therefore,the yield stress of sample with pre-strain is lower than that without pre-strain. And also, samples under different loading modes exhibited different twin characteristics. Forextruded magnesium alloy AZ31, twins are parallel to each other in a grain whencompress along ED, but the twin morphologies are always multiple if the sample issubjected to8%pre-compression along ED and then3%compression perpendicular toED. For the sample subjected to8%pre-compression along ED and then3%compression perpendicular to ED, twins are parallel to each other in some grains butintersectant in most cases. The intersecting angle is always60°.
③Lamellar {1012} twins produced by pre-strain have a significant effect indeformation behaviour during subsequent tension or compression.{1012}-typetwinning occurred when samples were subjected to compressive pre-deformation. Thecompressive yield strength of sample with compressive pre-deformation increasedgradually with the increase in compressive prestrain. The tensile yield strength of thesample decreased rapidly with the increase in compressive prestrain from zero to~1.7%,but it was nearly unaltered when compressive prestrain was higher than~1.7%.Detwinning occurred in the twinned regions played a role in the subsequent tensileprocess so that the yield stress was lower than sample without prestrain. Yieldasymmetry between tension and compression decreased gradually when compressiveprestrain was lower than~1.1%, but it began to increase after that. It suggests that yieldasymmetry can be effectively controlled by appropriate pre-deformation.
④Detwinning is actually a twinning process occurs in twinned region whensample is subjected to a reversed stress. The maximum detwinning volume can bedetermined by controlling the pre-strain. It is meaningful to investigate the roles oftwinning in strain hardening via investigating the deformation behaviour of sampleswith different pre-compression. In extruded magnesium alloy AZ31, samples withdifferent twinned fraction were obtained by controlling the pre-strains. These samplesexhibited different strain hardening behaviour during subsequent tension. There was nothe stage that strain hardening rate increased with increase in true strain in samplewithout pre-compression, but for samples with pre-compression, that stage existed.Although samples with different pre-strain exihibited a similar strain hardening trend,the strain hardening behaviours were still different. For example, for samples with3%and9%pre-compression, there was the stage that strain hardening rate increased withincrease in true strain, however, for the sample with3%pre-strain, strain hardening ratebegan to decrease when the true strain was relatively low, but it lasted to a relativelyhigh true strain in the sample with9%pre-compression. Because detwinning completed earlier in sample with low pre-strain than that with high pre-strain, the results indicatedthat twinning played a very important role in strain hardening in magnesium alloys. Andalso, sample with higher pre-strain exhibited higher elongation because detwinningcontributed to plastic deformation.
⑤Atomic displacement vectors during twinning were calculated based on thesymmetry principle. The law of atomic motion can be summaried as a movement ofatom group containing rotation and translation. The translation vector between twoadjacent layers of atom group is a twinning dislocation bT(bT=0.049nm). If many atomgroups complete rotation and translation, a twin with a certain thickness will beobtained.
⑥In magnesium lattice, the rotation of atom group can be activated by movementof B-type atom which is closed to twinning plane. The movement vector is1/6〈1010〉.The rotation of atom group will promote the occurrence of twinning dislocation in〈1011〉direction, leading to the translation of atom group. Dissociation of grainboundary defects or dislocations pile-up may lead to1/6〈1010〉partial dislocation.
①对挤压AZ31镁合金进行沿挤压方向（ED）的单向压缩变形，发现在光学显微组织分析中孪晶数量先是随着变形量的增大而增大，之后随着变形量的继续增大孪晶逐渐减少甚至消失。从组织观察结果分析似乎这个过程产生了退孪生，但是织构演变分析表明这个过程并没有发生退孪生。造成这一组织演变的实质是孪晶形核、长大与合并，导致在显微组织分析中呈现出类似于退孪生的现象。
②沿镁合金ED方向的预先压缩对随后垂直于ED方向压缩的力学性能有重要影响。研究表明，ED方向上的预先压缩能够明显地提高垂直于ED方向压缩的屈服强度，这主要是因为ED方向上的预压缩导致合金晶粒取向发生转变，以致在随后垂直于ED压缩时变形机制由基面滑移容易启动转变到不容易启动，导致屈服强度比不存在预变形的试样高。同时，由于不同施载模式下孪生变体启动的数量不一样，孪晶的形貌也不一样。对于挤压镁合金，当沿ED方向压缩3%时从垂直于ED的横截面上观察，同一个晶粒内的孪晶几乎都是相互平行的。但是对于先沿ED方向预压缩8%再垂直于ED压缩3%的试样，对于同一方向上的观察面，由于后者孪生变体启动的情况不一样，孪晶特征也有较大的差别，有的晶粒内孪晶是相互平行的，有的晶粒内孪晶则是相交的，交角约为60°。
③在挤压态镁合金中通过预变形引入片层状{1012}孪晶对随后拉伸、压缩变形的力学行为有重要的影响。在预压缩变形中，产生了片层状的{1012}孪晶。在随后的变形中，压缩屈服强度随着预变形量的增加而逐步增大，而拉伸屈服强度则随着预变形量从零开始到1.7%左右迅速降低，此后随着预变形量的增加，拉伸屈服强度几乎保持不变。导致拉伸屈服强度降低的原因主要是在反向拉伸过程中产生了退孪生。预变形量小于约1.1%时，拉压不对随着预变形量的增加逐渐降低，但此后随着预变形量的继续增大，拉压屈服不对称又开始升高。由此可推断，如果控制好合适的预变形量，对改善镁合金的拉压不对称是有利的。
④退孪生本质上是在已孪生区发生的孪生过程。通过控制预变形量即可实现在随后的反向受力变形中得到不同的最大退孪生体积，考察不同退孪生体积试样的加工硬化行为能够更加直观地反映孪生对加工硬化的贡献。在挤压态AZ31镁合金中通过控制压缩预变形量可以得到不同的孪生体积，这些含有不同已孪生体积的试样在随后反向拉伸变形的过程中表现出不同的加工硬化行为。没有预变形的试样在拉伸变形中加工硬化率不存在随应变量增大而上升的阶段；3%和9%预变形试样虽然都存在这个阶段，但是3%预变形试样在较低伸长量时加工硬化率就开始下降，而9%预变形试样的加工硬化率则延续到较高伸长量时才开始下降。由于预变形量小的试样退孪生率先完成，这验证了由{1012}孪生引起的织构强化对合金的加工硬化有着重要的作用。同时，由于退孪生对塑性变形的贡献，预变形量越大的试样在随后反向拉伸时具有越高的伸长量。
⑤通过建立直角坐标系计算孪生过程原子的位移矢量，可把孪生过程原子的运动归结为原子群单元的旋转和平移。对AZ31镁合金，原子群单元的旋转角为15.86°，每一层单元体之间的相对平移矢量为bT(bT=0.049nm)。无数个原子群单元的旋转和平移即可得到一定厚度的孪晶。
⑥镁晶胞中靠近孪生面的B位原子运动1/6〈1010〉矢量可导致原子群单元旋转，而原子群单元的旋转能够促进〈1011〉方向孪生位错的生成，使得单元体产生平移，到达孪生位置。1/6〈1010〉不全位错可能由晶界缺陷或在晶界附近受阻的位错分解得到。
{1012} twinning plays an important role in magnesium and its alloys duringplastic deformation and it is always responsible for the low yield stress, high yieldasymmetry between tension and compression and anisotropy in these alloys. In order toresolve these problems, it is very important to understand the strengthening mechanismbased on twinning. However, nearly all the strengthening mechanisms in materialsscience are based on slip and the strengthening mechanism based on twinning is stillvery limited up to these days because of ambiguity in twinning behaviour and twinningmechanism. Therefore, it is still necessary to do more investigations on twinningbehaviour and twinning mechanism in magnesium alloys. In this paper, the essential ofmicrostructural evolution during compression in extruded magnesium alloy AZ31wasrevealed via texture and microstructure analysis. The possibility to decrese the yieldasymmetry via introducing lamellar {1012} twins by pre-compression was investigatedand discussed. It was found that the yield asymmetry can be decreased by pre-strain ifthe amount of pre-strain was appropriate. In order to further understand the twinningmechanism, a model was established to describe the atomic motion in {1012} twinningand the law of atomic motion in {1012} was revealed in magnesium. The majorconclusions can be summarized as follows.
①Uinaxial compression along extrusion direction (ED) was carried out in anextruded magnesium alloy AZ31. The microstuctual analysis showed that the amount oftwins increased as a function of strain if the strain is relatively low. However, it began todecrease with continual increase in strain, or even disappeared at last. It seemed thatdetwinning occurred in this process. However, the texture analysis showed that nodetwinning occurred. The essential of microstructual evolution in this process is actuallytwin nucleation, twin growth and coalescence, leading to an analogous detwinningphenomenon observed in the optical micrographs.
②Pre-compression along ED has an obvious effect on subsequent compressionperpendicular to ED in an extruded magnesium alloy AZ31. The results showed that theyield stress under compression perpendicular to ED increased obviously if there waspre-compression along ED. The reason for this change is mainly the reorientationproduced by twinning during pre-compression, leading to a low basal activity. Therefore,the yield stress of sample with pre-strain is lower than that without pre-strain. And also, samples under different loading modes exhibited different twin characteristics. Forextruded magnesium alloy AZ31, twins are parallel to each other in a grain whencompress along ED, but the twin morphologies are always multiple if the sample issubjected to8%pre-compression along ED and then3%compression perpendicular toED. For the sample subjected to8%pre-compression along ED and then3%compression perpendicular to ED, twins are parallel to each other in some grains butintersectant in most cases. The intersecting angle is always60°.
③Lamellar {1012} twins produced by pre-strain have a significant effect indeformation behaviour during subsequent tension or compression.{1012}-typetwinning occurred when samples were subjected to compressive pre-deformation. Thecompressive yield strength of sample with compressive pre-deformation increasedgradually with the increase in compressive prestrain. The tensile yield strength of thesample decreased rapidly with the increase in compressive prestrain from zero to~1.7%,but it was nearly unaltered when compressive prestrain was higher than~1.7%.Detwinning occurred in the twinned regions played a role in the subsequent tensileprocess so that the yield stress was lower than sample without prestrain. Yieldasymmetry between tension and compression decreased gradually when compressiveprestrain was lower than~1.1%, but it began to increase after that. It suggests that yieldasymmetry can be effectively controlled by appropriate pre-deformation.
④Detwinning is actually a twinning process occurs in twinned region whensample is subjected to a reversed stress. The maximum detwinning volume can bedetermined by controlling the pre-strain. It is meaningful to investigate the roles oftwinning in strain hardening via investigating the deformation behaviour of sampleswith different pre-compression. In extruded magnesium alloy AZ31, samples withdifferent twinned fraction were obtained by controlling the pre-strains. These samplesexhibited different strain hardening behaviour during subsequent tension. There was nothe stage that strain hardening rate increased with increase in true strain in samplewithout pre-compression, but for samples with pre-compression, that stage existed.Although samples with different pre-strain exihibited a similar strain hardening trend,the strain hardening behaviours were still different. For example, for samples with3%and9%pre-compression, there was the stage that strain hardening rate increased withincrease in true strain, however, for the sample with3%pre-strain, strain hardening ratebegan to decrease when the true strain was relatively low, but it lasted to a relativelyhigh true strain in the sample with9%pre-compression. Because detwinning completed earlier in sample with low pre-strain than that with high pre-strain, the results indicatedthat twinning played a very important role in strain hardening in magnesium alloys. Andalso, sample with higher pre-strain exhibited higher elongation because detwinningcontributed to plastic deformation.
⑤Atomic displacement vectors during twinning were calculated based on thesymmetry principle. The law of atomic motion can be summaried as a movement ofatom group containing rotation and translation. The translation vector between twoadjacent layers of atom group is a twinning dislocation bT(bT=0.049nm). If many atomgroups complete rotation and translation, a twin with a certain thickness will beobtained.
⑥In magnesium lattice, the rotation of atom group can be activated by movementof B-type atom which is closed to twinning plane. The movement vector is1/6〈1010〉.The rotation of atom group will promote the occurrence of twinning dislocation in〈1011〉direction, leading to the translation of atom group. Dissociation of grainboundary defects or dislocations pile-up may lead to1/6〈1010〉partial dislocation.
引文
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