低温轧制结合退火热处理下锆的微结构与强韧化研究
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摘要
本文采用液氮低温轧制进行强塑性变形，再结合低温退火热处理的工艺对纯Zr进行处理，研究了其微结构的演变和强韧化机理。研究结果显示：液氮低温可以有效抑制动态回复，低温轧制能够使Zr的累积位错密度达到更高的稳定状态。在平均应变速率为2.238s-1的时候，将纯Zr强塑性变形至累积等效应变量为2.869，再经450℃条件下退火一小时后，获得了具有多尺度的微结构的Zr：其中约有20%的体积分数的亚晶和晶粒为纳米尺度（<100nm），约有56%的体积分数的超细晶（100nm-1000nm），同时有大约24%的体积分数的粗晶（>1000nm）。这种Zr材料具有高的强度和良好的韧性：极限抗拉强度为658MPa，均匀延伸率为8.5%，断裂延伸率为20.7%；其强度是原始粗晶锆的退火样品的1.8倍，同时韧性为127.5×106J/m3，要大于原始粗晶锆的退火样品的韧性110.7×106J/m3。其内在的强韧化机制为多尺度微结构与高角度晶界的综合作用：多尺度微结构中的纳米晶和超细晶提供了高的强度，而粗晶提供了大的变形能力和塑性，三种尺度晶粒之间复杂的相互作用而引起的多种变形路径也使得材料的应变硬化能力增强，此外，高角度晶界的形成对强度和韧性的强化也有利。对制备工艺的研究显示，随着低温轧制过程中的应变速率的增大，变形更加剧烈，退火前Zr中存储的位错、缺陷密度增大，并使得位错、缺陷的分布趋于平行带状，高的储存能促进退火过程中的形核和二次再结晶，更加有利于在退火之后形成多尺度的微结构。
The evolution of microstructure and the enhancement of strength and ductility of Zrhave been investigated in this research by cryorolling via the liquid nitrogen cooling withfollowed low-temperature annealing. The results of this research demonstrate these pointsfollowed: The use of the low temperature suppresses dynamic recovery, allowing thedensity of the accumulated dislocations to reach a higher steady-state level than thatachievable at room temperature. A multimodal grained structure composed of nanoscalegrains and subgrains (20%,<100nm), ultrafine grains (56%,100nm-1000nm), coarsegrains (24%,>1000nm) has been introduced in pure Zr by employing cryorolling withaverage strain rate2.238s-1and strain2.869, combined with subsequent low-temperatureannealing at450℃for1hour. This kind of pure Zr exhibits a high ultimate strength(658MPa), a good uniform elongation (8.5%) and a large elongation to failure (20.7%).Its strength is1.8times as the raw coarse-grained Zr that is processed by annealing. At thesame time, It has a larger toughness127.5×106J/m3than the second one’s110.7×106J/m3. The mechanism of the simultaneous enhancement of strength andductility is the introduction of multimodal grained structure and high-angle grainboundaries. The high strength results from the contribution of nanoscale and ultrafinegrains, while the good ductility is provided by the coarse grains and the strain hardeningcapability is improved by the coarse grains and complex deformation strain paths causedby the multimodal grains. While the strain rate increases, the deformation is more severe,much more dislocations are put in the pure Zr to a higher density. And its distributionbecomes regular as lamellas. The high energy stored lead to more recrystallizationnucleation evens and secondary recrystallization. These have benefits for the production ofmultimodal grained structure Zr.
The evolution of microstructure and the enhancement of strength and ductility of Zrhave been investigated in this research by cryorolling via the liquid nitrogen cooling withfollowed low-temperature annealing. The results of this research demonstrate these pointsfollowed: The use of the low temperature suppresses dynamic recovery, allowing thedensity of the accumulated dislocations to reach a higher steady-state level than thatachievable at room temperature. A multimodal grained structure composed of nanoscalegrains and subgrains (20%,<100nm), ultrafine grains (56%,100nm-1000nm), coarsegrains (24%,>1000nm) has been introduced in pure Zr by employing cryorolling withaverage strain rate2.238s-1and strain2.869, combined with subsequent low-temperatureannealing at450℃for1hour. This kind of pure Zr exhibits a high ultimate strength(658MPa), a good uniform elongation (8.5%) and a large elongation to failure (20.7%).Its strength is1.8times as the raw coarse-grained Zr that is processed by annealing. At thesame time, It has a larger toughness127.5×106J/m3than the second one’s110.7×106J/m3. The mechanism of the simultaneous enhancement of strength andductility is the introduction of multimodal grained structure and high-angle grainboundaries. The high strength results from the contribution of nanoscale and ultrafinegrains, while the good ductility is provided by the coarse grains and the strain hardeningcapability is improved by the coarse grains and complex deformation strain paths causedby the multimodal grains. While the strain rate increases, the deformation is more severe,much more dislocations are put in the pure Zr to a higher density. And its distributionbecomes regular as lamellas. The high energy stored lead to more recrystallizationnucleation evens and secondary recrystallization. These have benefits for the production ofmultimodal grained structure Zr.
引文
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[8] Zhu Y T, Langdon T G. The Fundamentals of Nanostructured Materials Processed by Severe PlasticDeformation[J]. JOM,2004,56(10):58–63.
[9] Meyers M A, Mishra A, Benson D J. Mechanical Properties of Nanocrystalline Materials[J]. ProgMater Sci,2006,51(4):427–556.
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[11] Gleiter H. Nanocrystalline Materials[J]. Prog Mater Sci,1989;33:223–315.
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[13] Valiev R Z. Nanomaterial Advantage[J]. Nature,2002,419:887.
[14] Youssef K M, Scattergood R O, Murty K L, et al. Ultrahigh Strength and High Ductility of BulkNanocrystalline Copper[J]. Appl Phys Lett2005,87(9):091904.
[15] Chen J, Lu L, Lu K. Hardness and Strain Rate Sensitivity of Nanocrystalline Cu[J]. ScriptaMaterialia,2006,54(11):1913–1918.
[16] Estrin Y, Kim H S, Nabarro F R N. A Comment on the Role of Frank–Read Sources in Plasticity ofNanomaterials[J]. Acta Materialia,2007,55(19):6401-6407.
[17] Hansen N, Huang X, Winther G. Grain Orientation, Deformation Microstructure and FlowStress[J]. Mater. Sci. Eng. A,2008,494(1-2):61-67.
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