纳米结构Al-1%Si合金的组织、热稳定性及力学行为研究
摘要
纳米结构纯金属有很高的强度，但面临两个最基本的问题：一是因界面密度高，材料中的储存能高而导致的热稳定性下降的问题；另一个是因加工硬化能力下降而导致的塑性严重降低。在本论文工作中，设计制备了一种纳米结构Al-1%Si合金，该合金以超高纯铝（99.9996%）为基体，在基体中弥散分布了质量比1%的Si第二相。目的是研究弥散分布的纳米颗粒第二相对于纳米结构Al-1%Si材料的热稳定性和力学行为的影响，以期探寻一种优化纳米结构材料结构和力学性能的新方法。
由于室温下Si在超高纯铝基体中的固溶度几乎为零，因而1%的Si元素都以第二相的形式弥散分布在基体中。通过将热轧后的材料进行98%形变量的冷轧，获得了用于研究其热稳定性和力学性能的纳米结构材料。通过对冷轧和不同工艺下热处理后材料的力学行为进行系统的测试，以及利用透射电镜(TEM)、扫描电镜（SEM）和高分辨EBSD等方法对材料的微观组织进行了系统表征，得出以下结论：
通过冷轧变形制备出了ND方向(Normal Direction)平均界面间距为230nm、RD方向(Rolling direction)平均界面间距为950nm和基体中弥散分布着尺寸约为20nm的Si颗粒相的层状纳米结构材料。Si第二相颗粒绝大多数都沿晶界分布，对材料的形变组织起到了很好的稳定作用。制得的纳米结构Al-1%Si材料的屈服强度为210MPa，拉伸强度为230MPa，拉伸延伸率为15%。与文献中利用ARB制备的高纯铝（99.99%）与工业纯铝Al1050（99.5%）和Al1100（99.2%）相比较，纳米结构Al-1%Si材料具有最好的强塑匹配。
通过对纳米结构Al-1%Si材料在大温度范围内（100-600℃）的热稳定性进行研究发现，纳米结构Al-Si材料在200℃以下退火1小时只发生回复；当退火温度大于200℃时，材料发生再结晶形核与长大；在200~250℃之间退火1小时，材料发生部分再结晶，当退火温度高于250℃时，材料在1小时内完全再结晶。基体中的纳米尺寸第二相Si颗粒在受热过程中发生粗化，退火温度越高，颗粒的平均尺寸越大。尽管在材料中有少量直径在数微米的粗大的第二相Si颗粒，在其周围形成的高形变区内并没有发现明显的PSN（Particle Stimulated Nucleatin）现象，这是由于周围基体的弥散颗粒对界面移动的钉扎作用所致。认为粗大第二相颗粒对于材料的热稳定性不产生重要影响。98%冷轧形变后的Al-1%Si材料在较低温度下再结晶退火后形成近似随机织构，在高温下退火时再结晶的性质与传统的铝和铝合金形变材料类似，形成旋转立方织构，退火温度越高，织构越强。
在相同的温度范围内还对材料的力学行为进行了系统研究。纳米结构Al-1%Si合金在回复阶段（<200℃退火）强度随退火温度的增加下降较快，而塑性（总延伸率）变化不大，保持在15%的水平。这与纳米结构工业纯Al回复退火造成的拉伸塑性急剧下降形成鲜明对比。并且，在Al-1%Si合金并没有出现纳米结构工业纯铝在中温退火条件下所出现的屈服点现象，即没有发现屈服点和Lüders带的形成。Al-1%Si合金的Hall-Petch系数（斜率）是粗晶(>10m)纯Al的Hall-Petch系数的2倍，表明对于所研究的Al-1%Si合金可通过细化晶粒，更有效地实现强化。以上结果表明形变和退火材料的拉伸稳定性得到了显著提升，这与基体中弥散的Si颗粒阻止了基体中位错组织的回复、促进了拉伸过程中与运动位错的交互作用从而提高加工硬化能力有关。
通过与文献报导的具有类似名义成分（99%）的纳米结构工业纯Al的退火行为比较发现，本研究所设计的Al-1%Si合金在大于100MP的强度范围内具有显著的强塑匹配优势。
本论文的研究结果表明，在纳米结构中引入弥散分布第二相的方式可以显著提高热稳定性和改善力学行为（包括去除回复导致的流变失稳和屈服点现象），是一种实现纯Al材料高强度、高塑性的有效途径。
Nanostructured pure metals show enhanced strength but suffer from twofundamental problems: reduced thermal stability due to the high stored energyassociated with the high density of grain boundaries and reduced ductility due to thelack of capability of work hardening. In this thesis we have designed and produced ananostructured Al-1%Si alloy with the addition of1%Si present in the form of dispersedparticles in the Al matrix. The principal objective of the present study is to explore theeffects of dispersed particles on the thermal stability and mechanical behavior of thenanostructured Al-1%Si alloy, with an aim to establish a new strategy to optimize thestructure and mechanical properties of nanostructured metals.
High purity Al (99.9996%) was used as the matrix and the addition of1%Si was toform dispersed Si particles as the solubility of Si in Al is almost zero at roomtemperature. The cast Al-1%Si ingot was hot deformed and then cold-rolled to athickness reduction of98%to obtain a nanostructured state of the alloy, which was usedas the base material for investigating the effects of annealing on thermal stability andmechanical behavior. The specimens of as-cold-rolled and after different annealingtreatments were characterized by tensile tests and by microstructural analysis usingtransmission electron microscopy (TEM), scanning electron microscopy/electronbackscatter diffraction (SEM/EBSD). The main findings are summarized as follows:
After cold rolling to98%thickness reduction a nanostructured Al-1%Si alloy wasproduced with a nanoscale lamellar structure containing nanoscale (~20nm) Si particles.The average boundary spacings are230nm and950nm along the normal direction (ND)and the rolling direction (RD), respectively. Large amount of second phase particlesdisperse mostly along lamellar boundaries which stabilize the microstructure. Thenanostructured Al-1%Si alloy has a yield strength of210MPa, a UTS of230MPa and atensile elongation of15%. Comparing with nanostructured high purity Al (99.99%),commercial purity Al1050(99.5%) and Al1100(99.2%), the as-processednanostructured Al-1%Si exhibits the best combination of strength and tensile ductility.
The thermal stability of nanostructured Al-1%Si alloy was investigated over a widerange of temperature from100–600℃. It is found that the nanostructured Al-1%Sialloy is relatively stable when annealing for1hour at temperatures below200℃where only recovery takes place. Nucleation and growth occur when annealing for1 hour at temperatures higher than200℃. In more detail, partial recrystallization takeplace when the annealing temperature is between200and250℃. After annealing for1hour at temperatures higher than250℃, the material is fully recrystallized. Secondphase particles are coarsening, resulting in an increase in average size with an increasein annealing temperature. Highly strained regions are formed around second phaseparticles with diameters of several micrometers. Nevertheless these large particles donot stimulate preferred nucleation and growth around them during annealing due to thepinning effect of fine particles on the boundary migration. Therefore they have littleeffect on the thermal stability of the nanostructure. Nearly random texture is developedafter low temperature annealing, and rotated cube texture is developed after annealing athigh temperatures. The higher the annealing temperature is, the stronger the textureforms.
The effect of annealing on mechanical behavior was investigated over the sametemperature range as for the thermal stability study. The strength drops rapidly withincreasing annealing temperature in the recovery region but the plasticity (totalelongation) shows nearly no change. This observation shows a distinct contrast to thatobserved in nanostructured commerical purity Al alloys, in which the elongation isreduced greatly after recovery annealing. Furthermore, no yield point phenomenonassociated with a yield drop and Lüders deformation appears after annealing at mediumtemperatures, as observed in nanostructured commerical purity Al after a similarannealing treatment. The Hall-Petch slope of the nanostructured Al-1%Si is twice ofthat obtained in the coarse grain range (>10m). This manifests that grain refinementinduced strengthening is much more effective when the grain size is in thesubmicrometer and nanometer range. The above results show a significant improvementin the tensile stability for both the as-processed structure and after recovery annealing.This improvement is related to the fine dispersion of Si particles in the microstructure,which retraded the recovery of dislocations during recovery and enhanced theinteraction with dislocations during tensile deformation.
Comparing with nanostructured commercial purity Al alloys with similar nominalcompositions, the present nanostructured Al-1%Si shows a greatly improvedcombination of strength and ductility at strength levels of higher than100MPa.
In conclusion, the results obtained in this thesis show that introducing dispersedparticles in nanostructured pure metals can generate significant beneficial effects instabilizing the mechanical behavior (including removal of recovery enhanced flow instability and of the occurrence of yield point phenomena) and the thermal response,and in achieving improved combination of strength and ductility at the higher level ofstrength of pure Al.
由于室温下Si在超高纯铝基体中的固溶度几乎为零，因而1%的Si元素都以第二相的形式弥散分布在基体中。通过将热轧后的材料进行98%形变量的冷轧，获得了用于研究其热稳定性和力学性能的纳米结构材料。通过对冷轧和不同工艺下热处理后材料的力学行为进行系统的测试，以及利用透射电镜(TEM)、扫描电镜（SEM）和高分辨EBSD等方法对材料的微观组织进行了系统表征，得出以下结论：
通过冷轧变形制备出了ND方向(Normal Direction)平均界面间距为230nm、RD方向(Rolling direction)平均界面间距为950nm和基体中弥散分布着尺寸约为20nm的Si颗粒相的层状纳米结构材料。Si第二相颗粒绝大多数都沿晶界分布，对材料的形变组织起到了很好的稳定作用。制得的纳米结构Al-1%Si材料的屈服强度为210MPa，拉伸强度为230MPa，拉伸延伸率为15%。与文献中利用ARB制备的高纯铝（99.99%）与工业纯铝Al1050（99.5%）和Al1100（99.2%）相比较，纳米结构Al-1%Si材料具有最好的强塑匹配。
通过对纳米结构Al-1%Si材料在大温度范围内（100-600℃）的热稳定性进行研究发现，纳米结构Al-Si材料在200℃以下退火1小时只发生回复；当退火温度大于200℃时，材料发生再结晶形核与长大；在200~250℃之间退火1小时，材料发生部分再结晶，当退火温度高于250℃时，材料在1小时内完全再结晶。基体中的纳米尺寸第二相Si颗粒在受热过程中发生粗化，退火温度越高，颗粒的平均尺寸越大。尽管在材料中有少量直径在数微米的粗大的第二相Si颗粒，在其周围形成的高形变区内并没有发现明显的PSN（Particle Stimulated Nucleatin）现象，这是由于周围基体的弥散颗粒对界面移动的钉扎作用所致。认为粗大第二相颗粒对于材料的热稳定性不产生重要影响。98%冷轧形变后的Al-1%Si材料在较低温度下再结晶退火后形成近似随机织构，在高温下退火时再结晶的性质与传统的铝和铝合金形变材料类似，形成旋转立方织构，退火温度越高，织构越强。
在相同的温度范围内还对材料的力学行为进行了系统研究。纳米结构Al-1%Si合金在回复阶段（<200℃退火）强度随退火温度的增加下降较快，而塑性（总延伸率）变化不大，保持在15%的水平。这与纳米结构工业纯Al回复退火造成的拉伸塑性急剧下降形成鲜明对比。并且，在Al-1%Si合金并没有出现纳米结构工业纯铝在中温退火条件下所出现的屈服点现象，即没有发现屈服点和Lüders带的形成。Al-1%Si合金的Hall-Petch系数（斜率）是粗晶(>10m)纯Al的Hall-Petch系数的2倍，表明对于所研究的Al-1%Si合金可通过细化晶粒，更有效地实现强化。以上结果表明形变和退火材料的拉伸稳定性得到了显著提升，这与基体中弥散的Si颗粒阻止了基体中位错组织的回复、促进了拉伸过程中与运动位错的交互作用从而提高加工硬化能力有关。
通过与文献报导的具有类似名义成分（99%）的纳米结构工业纯Al的退火行为比较发现，本研究所设计的Al-1%Si合金在大于100MP的强度范围内具有显著的强塑匹配优势。
本论文的研究结果表明，在纳米结构中引入弥散分布第二相的方式可以显著提高热稳定性和改善力学行为（包括去除回复导致的流变失稳和屈服点现象），是一种实现纯Al材料高强度、高塑性的有效途径。
Nanostructured pure metals show enhanced strength but suffer from twofundamental problems: reduced thermal stability due to the high stored energyassociated with the high density of grain boundaries and reduced ductility due to thelack of capability of work hardening. In this thesis we have designed and produced ananostructured Al-1%Si alloy with the addition of1%Si present in the form of dispersedparticles in the Al matrix. The principal objective of the present study is to explore theeffects of dispersed particles on the thermal stability and mechanical behavior of thenanostructured Al-1%Si alloy, with an aim to establish a new strategy to optimize thestructure and mechanical properties of nanostructured metals.
High purity Al (99.9996%) was used as the matrix and the addition of1%Si was toform dispersed Si particles as the solubility of Si in Al is almost zero at roomtemperature. The cast Al-1%Si ingot was hot deformed and then cold-rolled to athickness reduction of98%to obtain a nanostructured state of the alloy, which was usedas the base material for investigating the effects of annealing on thermal stability andmechanical behavior. The specimens of as-cold-rolled and after different annealingtreatments were characterized by tensile tests and by microstructural analysis usingtransmission electron microscopy (TEM), scanning electron microscopy/electronbackscatter diffraction (SEM/EBSD). The main findings are summarized as follows:
After cold rolling to98%thickness reduction a nanostructured Al-1%Si alloy wasproduced with a nanoscale lamellar structure containing nanoscale (~20nm) Si particles.The average boundary spacings are230nm and950nm along the normal direction (ND)and the rolling direction (RD), respectively. Large amount of second phase particlesdisperse mostly along lamellar boundaries which stabilize the microstructure. Thenanostructured Al-1%Si alloy has a yield strength of210MPa, a UTS of230MPa and atensile elongation of15%. Comparing with nanostructured high purity Al (99.99%),commercial purity Al1050(99.5%) and Al1100(99.2%), the as-processednanostructured Al-1%Si exhibits the best combination of strength and tensile ductility.
The thermal stability of nanostructured Al-1%Si alloy was investigated over a widerange of temperature from100–600℃. It is found that the nanostructured Al-1%Sialloy is relatively stable when annealing for1hour at temperatures below200℃where only recovery takes place. Nucleation and growth occur when annealing for1 hour at temperatures higher than200℃. In more detail, partial recrystallization takeplace when the annealing temperature is between200and250℃. After annealing for1hour at temperatures higher than250℃, the material is fully recrystallized. Secondphase particles are coarsening, resulting in an increase in average size with an increasein annealing temperature. Highly strained regions are formed around second phaseparticles with diameters of several micrometers. Nevertheless these large particles donot stimulate preferred nucleation and growth around them during annealing due to thepinning effect of fine particles on the boundary migration. Therefore they have littleeffect on the thermal stability of the nanostructure. Nearly random texture is developedafter low temperature annealing, and rotated cube texture is developed after annealing athigh temperatures. The higher the annealing temperature is, the stronger the textureforms.
The effect of annealing on mechanical behavior was investigated over the sametemperature range as for the thermal stability study. The strength drops rapidly withincreasing annealing temperature in the recovery region but the plasticity (totalelongation) shows nearly no change. This observation shows a distinct contrast to thatobserved in nanostructured commerical purity Al alloys, in which the elongation isreduced greatly after recovery annealing. Furthermore, no yield point phenomenonassociated with a yield drop and Lüders deformation appears after annealing at mediumtemperatures, as observed in nanostructured commerical purity Al after a similarannealing treatment. The Hall-Petch slope of the nanostructured Al-1%Si is twice ofthat obtained in the coarse grain range (>10m). This manifests that grain refinementinduced strengthening is much more effective when the grain size is in thesubmicrometer and nanometer range. The above results show a significant improvementin the tensile stability for both the as-processed structure and after recovery annealing.This improvement is related to the fine dispersion of Si particles in the microstructure,which retraded the recovery of dislocations during recovery and enhanced theinteraction with dislocations during tensile deformation.
Comparing with nanostructured commercial purity Al alloys with similar nominalcompositions, the present nanostructured Al-1%Si shows a greatly improvedcombination of strength and ductility at strength levels of higher than100MPa.
In conclusion, the results obtained in this thesis show that introducing dispersedparticles in nanostructured pure metals can generate significant beneficial effects instabilizing the mechanical behavior (including removal of recovery enhanced flow instability and of the occurrence of yield point phenomena) and the thermal response,and in achieving improved combination of strength and ductility at the higher level ofstrength of pure Al.
引文
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