铝合金高压扭转变形及其强化模型
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摘要
高压扭转(High Pressure Torsion,HPT)是一种有效获得超细晶金属的剧烈塑性变形(Severe Plastic Deformation,SPD)方法,采用该方法能细化金属材料组织至亚微米甚至纳米尺度。在HPT过程中,应变路径对组织细化与力学性能有重要影响；而且,建立HPT变形过程中材料微结构与力学性能之间的数学模型,具有十分重要的学术价值,但至今在HPT中尚未得到充分研究。此外,开展HPT变形下合金化对强化影响的研究,对改善现有合金性能和设计新合金均具有现实意义。
本文主要围绕上述问题开展工作,采用HPT技术对1050、2XXX以及5XXX铝合金进行室温改性处理,利用维氏硬度表征合金的力学性能,通过光学显微镜、扫描电子显微镜和透射电子显微镜及差热分析研究了HPT对合金组织的影响,并探讨HPT强化机制,建立强化模型。主要研究结果如下：
(1)首次提出单反高压扭转变形(sr-HPT)法,并结合单向高压扭转变形(m-HPT)法、循环高压扭转变形(c-HPT)法,强化1050合金。HPT变形后,圆盘试样中心区域硬度低,边缘硬度高。研究表明：在较低HPT旋转圈数下,试样中心区域晶粒粗大、硬度低；随HPT旋转圈数增多,试样中心组织细化、硬度升高。m-HPT的快速强化效果主要和总旋转圈数有关；c-HPT的较弱强化效果受总旋转圈数和每循环旋转圈数的共同影响；sr-HPT初始合金硬度降低,其最大硬度降低出现在试样中心区域,随着反向圈数增多,硬度再次升高。
(2)从位错角度阐述了1050合金的HPT强化行为和微观组织演化机理。变形试样的强化与几何必需位错(GND)和统计存储位错(SSD)密度有关。试样中心强化主要受GND密度影响；随着远离试样中心,SSD强化逐渐成为主要强化机制；GND和SSD共同作用使m-HPT强化效果显著；GND密度改变被用于解释c/sr-HPT引起的应变软化行为。基于上述研究结果,建立了一个新的强化模型：
σy=σ0+σgb+M(?)
该模型能定量地预测m/c-HPT条件下1050合金的强化,并能对sr-HPT引起的强化给予定性解释。模型指明HPT引起的强化主要是由于GND和SSD,晶界强化有限,通常小于10%。
(3)研究HPT-热处理组合工艺对两种2XXX铝合金强化的影响,探讨其析出强化顺序。HPT变形过程中,Cu-Mg原子集聚强化和位错强化为主要强化机制。依据在HPT变形过程中Cu-Mg原子集聚数量的改变,提出Cu-Mg原子集聚-位错竞争强化机制概念,合理解释其实验现象。通过引入Cu-Mg原子集聚数量改变因子(ηcl),对HPT强化模型进一步扩展。
(4)研究了合金化对高压扭转强化的影响。Mg元素添加能有效地改善HPT引起的强化效果,随着远离试样中心,其硬度先快速上升,然后趋于某一稳定值。经c/sr-HPT变形的5XXX铝合金与1050合金,试样中心区域硬度随半径增大表现出斜率异号的硬度增量曲线。m-HPT变形Al-1Mg-0.4Cu合金微观组织为板带状组织,反向变形后,其组织为等轴晶组织；选区衍射分析表明m-HPT条件下平均晶界取向角大,sr-HPT降低其平均晶界取向角,c-HPT变形时平均晶界取向角较小
High pressure torsion (HPT) is a severe plastic deformation (SPD) procedure with the ability to refine the grain size in polycrystalline materials down to micrometer and even to nanometer level. Although the grain refinement and the mechanical properties are dependent of the strain paths, there are a few descriptions of evaluating the effect on them of strain paths applied during HPT processing. Another objective of the present investigation is to analyse microstructural development and hardening during HPT, and provide a model that captures the main mechanisms for the hardening. The third objection is the effect of alloying elements on the strengthening of Al alloys under HPT.
Present investigation focuses on above three aspects of 1050, two 2XXX and four 5XXX aluminium alloys processed by HPT at room temperature. Microhardnes testing was performed to evaluate the strength and work hardening of the alloys. Experiments by means of optical microscopy (OPM), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and differential scanning calorimetry (DSC) were also carried out to provide the relevant information for the microstructure evlotuion and precipitate behavior of the alloys processed by HPT. The primary innovation points and conclusions are:
(1) Three strain paths including monotonic HPT (m-HPT), cyclic HPT (c-HPT) and single reversal HPT (sr-HPT) were employed to strengthen the 1050 alloys. The results show the microhardness is lower and there is less grain refinement in the central regions of the disks in the initial stages of torsional straining but the microstructures become reasonably homogeneous across the disks at high imposed strains. Hardening is lower for c-HPT as compared to m-HPT. The extent of strengthening produced by m-HPT mainly depends on total strain/turns, whereas the extent by c-HPT does on not noly total strain but also strain per cyclic deformation. The sr-HPT initially reduces hardness drastically, and that decrease is most marked for the centre region. As strains by sr-HPT proceed, the hardness increases again.
(2) The hardening behaviours and microstructure evolution of commercially pure aluminium during HPT processing were interpreted in terms of the density of geometrically necessary dislocations (GND) and statistically stored dislocations (SSD). GND dominate the strengthening in the centre of disk. Instead, with increasing the distance from the centre of disk, the SSD dominate strengthening. The density increase of SSD together with GND is responsible for the rapid rise of hardness in m-HPT. The softening on c/sr-HPT was attributed to the change in the GND density. A model has been presented:
σy＝σ0+σgb＋M(?)
The model describes quantitatively the experimental results in m/c-HPT and explains quanlitativly the hardening and softening in sr-HPT. The model indicated that the strength/ hardness is predominantly due to GND and SSD, with grain refinement providing less than 10% of the strengthening effect.
(3) Two 2XXX aluminium alloys were selected for elucidating the relationship between heat treatments as well as HPT processings and the final hardness of the alloys. The strengthening mechanisms in the ageing of 2XXX alloys were interpreted. During HPT, the strength/hardness depends on the strengthening of Cu-Mg co-clusters and strengthening of dislocation density. The influence of quench in liquid nitrogen on strengthening of ageing at room temperature was interpreted in terms of the relationship between the amounts of retained co-clusters (ηcl)and HPT strain, and the strengthening model was further developed.
(4) The effect of alloying on strengthening by strain paths was studied. Mg addition strongly improves the strengthening effect produced by HPT, where the increment of hardness initially rapidly increases with increasing the distance from the centre of disks and then slowly approaches stable values. It was observed that near the centre regions of hardness incremental curves of 1050 and 5XXX Al alloys processed by c/sr-HPT have the slopes with opposite signs and explanations for them were given. It was found that the characteristic structure of Al-1Mg-0.4Cu alloys developed during m-HPT consists of bands of elongated (sub)grains with high angles of misorientation, in contrast, the TEM micrographs and slected SEAD (Selected area electron diffraction) patterns for the same alloy after various reversal HPT processing steps showed that grains are nearly equiaxed with relatively low grain boundary misorientations.
本文主要围绕上述问题开展工作,采用HPT技术对1050、2XXX以及5XXX铝合金进行室温改性处理,利用维氏硬度表征合金的力学性能,通过光学显微镜、扫描电子显微镜和透射电子显微镜及差热分析研究了HPT对合金组织的影响,并探讨HPT强化机制,建立强化模型。主要研究结果如下：
(1)首次提出单反高压扭转变形(sr-HPT)法,并结合单向高压扭转变形(m-HPT)法、循环高压扭转变形(c-HPT)法,强化1050合金。HPT变形后,圆盘试样中心区域硬度低,边缘硬度高。研究表明：在较低HPT旋转圈数下,试样中心区域晶粒粗大、硬度低；随HPT旋转圈数增多,试样中心组织细化、硬度升高。m-HPT的快速强化效果主要和总旋转圈数有关；c-HPT的较弱强化效果受总旋转圈数和每循环旋转圈数的共同影响；sr-HPT初始合金硬度降低,其最大硬度降低出现在试样中心区域,随着反向圈数增多,硬度再次升高。
(2)从位错角度阐述了1050合金的HPT强化行为和微观组织演化机理。变形试样的强化与几何必需位错(GND)和统计存储位错(SSD)密度有关。试样中心强化主要受GND密度影响；随着远离试样中心,SSD强化逐渐成为主要强化机制；GND和SSD共同作用使m-HPT强化效果显著；GND密度改变被用于解释c/sr-HPT引起的应变软化行为。基于上述研究结果,建立了一个新的强化模型：
σy=σ0+σgb+M(?)
该模型能定量地预测m/c-HPT条件下1050合金的强化,并能对sr-HPT引起的强化给予定性解释。模型指明HPT引起的强化主要是由于GND和SSD,晶界强化有限,通常小于10%。
(3)研究HPT-热处理组合工艺对两种2XXX铝合金强化的影响,探讨其析出强化顺序。HPT变形过程中,Cu-Mg原子集聚强化和位错强化为主要强化机制。依据在HPT变形过程中Cu-Mg原子集聚数量的改变,提出Cu-Mg原子集聚-位错竞争强化机制概念,合理解释其实验现象。通过引入Cu-Mg原子集聚数量改变因子(ηcl),对HPT强化模型进一步扩展。
(4)研究了合金化对高压扭转强化的影响。Mg元素添加能有效地改善HPT引起的强化效果,随着远离试样中心,其硬度先快速上升,然后趋于某一稳定值。经c/sr-HPT变形的5XXX铝合金与1050合金,试样中心区域硬度随半径增大表现出斜率异号的硬度增量曲线。m-HPT变形Al-1Mg-0.4Cu合金微观组织为板带状组织,反向变形后,其组织为等轴晶组织；选区衍射分析表明m-HPT条件下平均晶界取向角大,sr-HPT降低其平均晶界取向角,c-HPT变形时平均晶界取向角较小
High pressure torsion (HPT) is a severe plastic deformation (SPD) procedure with the ability to refine the grain size in polycrystalline materials down to micrometer and even to nanometer level. Although the grain refinement and the mechanical properties are dependent of the strain paths, there are a few descriptions of evaluating the effect on them of strain paths applied during HPT processing. Another objective of the present investigation is to analyse microstructural development and hardening during HPT, and provide a model that captures the main mechanisms for the hardening. The third objection is the effect of alloying elements on the strengthening of Al alloys under HPT.
Present investigation focuses on above three aspects of 1050, two 2XXX and four 5XXX aluminium alloys processed by HPT at room temperature. Microhardnes testing was performed to evaluate the strength and work hardening of the alloys. Experiments by means of optical microscopy (OPM), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and differential scanning calorimetry (DSC) were also carried out to provide the relevant information for the microstructure evlotuion and precipitate behavior of the alloys processed by HPT. The primary innovation points and conclusions are:
(1) Three strain paths including monotonic HPT (m-HPT), cyclic HPT (c-HPT) and single reversal HPT (sr-HPT) were employed to strengthen the 1050 alloys. The results show the microhardness is lower and there is less grain refinement in the central regions of the disks in the initial stages of torsional straining but the microstructures become reasonably homogeneous across the disks at high imposed strains. Hardening is lower for c-HPT as compared to m-HPT. The extent of strengthening produced by m-HPT mainly depends on total strain/turns, whereas the extent by c-HPT does on not noly total strain but also strain per cyclic deformation. The sr-HPT initially reduces hardness drastically, and that decrease is most marked for the centre region. As strains by sr-HPT proceed, the hardness increases again.
(2) The hardening behaviours and microstructure evolution of commercially pure aluminium during HPT processing were interpreted in terms of the density of geometrically necessary dislocations (GND) and statistically stored dislocations (SSD). GND dominate the strengthening in the centre of disk. Instead, with increasing the distance from the centre of disk, the SSD dominate strengthening. The density increase of SSD together with GND is responsible for the rapid rise of hardness in m-HPT. The softening on c/sr-HPT was attributed to the change in the GND density. A model has been presented:
σy＝σ0+σgb＋M(?)
The model describes quantitatively the experimental results in m/c-HPT and explains quanlitativly the hardening and softening in sr-HPT. The model indicated that the strength/ hardness is predominantly due to GND and SSD, with grain refinement providing less than 10% of the strengthening effect.
(3) Two 2XXX aluminium alloys were selected for elucidating the relationship between heat treatments as well as HPT processings and the final hardness of the alloys. The strengthening mechanisms in the ageing of 2XXX alloys were interpreted. During HPT, the strength/hardness depends on the strengthening of Cu-Mg co-clusters and strengthening of dislocation density. The influence of quench in liquid nitrogen on strengthening of ageing at room temperature was interpreted in terms of the relationship between the amounts of retained co-clusters (ηcl)and HPT strain, and the strengthening model was further developed.
(4) The effect of alloying on strengthening by strain paths was studied. Mg addition strongly improves the strengthening effect produced by HPT, where the increment of hardness initially rapidly increases with increasing the distance from the centre of disks and then slowly approaches stable values. It was observed that near the centre regions of hardness incremental curves of 1050 and 5XXX Al alloys processed by c/sr-HPT have the slopes with opposite signs and explanations for them were given. It was found that the characteristic structure of Al-1Mg-0.4Cu alloys developed during m-HPT consists of bands of elongated (sub)grains with high angles of misorientation, in contrast, the TEM micrographs and slected SEAD (Selected area electron diffraction) patterns for the same alloy after various reversal HPT processing steps showed that grains are nearly equiaxed with relatively low grain boundary misorientations.
引文
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