ECAP技术对不同组织结构的LY12铝合金高温压缩变形与损伤行为影响的研究
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摘要
ECAP技术制备的超细晶材料因其具有各种独特的力学行为近年来引起越来越多研究者的兴趣,但是对此类材料的表面变形特征,尤其是在高温环境下的变形行为的认识还很不全面,而这些研究对ECAP制备材料的开发和应用具有其重要的实际意义。本论文选取了LY12铝合金作为研究对象,制备了初始态、450℃/3h、初始态+1ECAP、450℃/3h+1ECAP、500℃/3h+1ECAP、初始态+2ECAP和450℃/3h+2ECAP七种不同微观组织结构的LY12铝合金样品,考察了它们在不同温度(25～400℃)下的压缩变形行为、表面变形及损伤特征以及相应的微观结构变化。
研究发现,不同处理工艺对LY12铝合金的压缩力学行为具有不同的影响,且七种状态LY12铝合金的压缩变形及损伤特征与实验温度密切相关。具体表现如下:
初始态样品内部均匀分布着的大量细小S沉淀相(AlCuMg)的强化作用十分显著。对初始态样品进行450℃/3h退火处理后,晶粒尺寸长大,内应力降低,均匀分布的细小沉淀相在退火过程中长大形成针状S相,致使450℃/3h样品的屈服强度及流变应力在各温度压缩条件下均低于初始态样品。由于晶粒尺度影响,退火后样品在压缩变形过程中,粗大的晶粒承担了更多的变形,导致其在压缩后表面产生了晶内滑移和沿晶裂纹。
利用ECAP技术对初始态LY12铝合金样品直接进行变形处理后,一方面,可明显细化晶粒,引入高密度位错,使样品得到应变强化;另一方面,在ECAP制备过程中,大量细小的S相在严重的塑性变形过程中被基体所溶解,使沉淀强化作用减弱。两种强化机制的共同作用导致ECAP样品强度低于初始态样品。但是,ECAP变形处理使样品高温塑性变形能力有所增加,在压缩过程中,表面除出现大尺度剪切裂纹和剪切带外,还有许多沿晶微裂纹出现,随压缩温度的升高,剪切带减少,剪切裂纹尺度降低,当压缩温度达到400℃时,剪切裂纹、剪切带和微裂纹基本消失,在晶内和晶界上形成了许多微孔。而此时初始态样品强度依然较高,其表面仍然存在尺度较大的剪切裂纹。
对初始态样品进行450℃/3h退火后再进行ECAP变形处理,晶粒得到细化,位错密度增加,大量强化作用较弱的大尺寸针状S相在严重塑性变形过程中被折断而发生了重溶或尺寸减小,此时,由ECAP变形处理所引入的应变强化作用显得更加突出,不同的应变强化机制和沉淀强化机制导致了退火+ECAP和退火样品的流变应力相当,但是应变强化机制有效地提高了退火合金的屈服强度。与直接ECAP变形处理样品相类似,经退火+ECAP处理后压缩变形,表面同样出现了大量的剪切带、大尺度裂纹和沿晶微裂纹。在300℃以下进行压缩时,450℃/3h+2ECAP样品屈服强度虽然低于初始态样品,但始终保持在较高水平(320～350 MPa),表现出较好的高温变形稳定性。
In recent years, ultrafine-grained (UFG) materials prepared by the equal channel angular pressing (ECAP) technology have been attracting more and more interests of research, owing to their unique mechanical behaviors. However, the surface deformation characteristics of such materials, especially the deformation behavior in high temperature environment, is still far from comprehensive studies, and these studies are of practical significance for the development and application of such ECAPed materials. Therefore, in the present work, LY12 Al alloys are selected as the target materials, and seven kinds of samples with different structures induced by various treatments, such as original, 450℃/3h annealed, 1 ECAPed, 450℃/3h annealed+1ECAPed, 500℃/3h annealed+1ECAPed, 2ECAPed, 450℃/3h annealed+2ECAPed, are prepared. Then, the compressive deformation behavior, the surface deformation and damage characteristics, and the corresponding changes in microstructures of these samples are considerably examined at different temperatures (e.g. 25℃-400℃).
It is clearly found that the LY12 Al alloy exhibits different compressive mechanical behavior depending on different treatments, and the compressive deformation characteristics and damage behavior of LY12 Al alloys with seven treating states are closely related with the experimental temperature. The details are presented as follows.
A large number of small S-phase precipitates (AlCuMg) uniformly distributed in the original sample shows a remarkable strengthening effect. As the original sample is annealed at 450℃for 3h, the grain size distinctly increases, the internal stress significantly reduces, and the needle-like S phase evolves from uniformly distributed small precipitates in the process of annealing, causing that the yield strength and flow stress of 450℃/3h sample are markedly lower than that of the original sample under compressive deformation at all temperatures. As a result of the effects of the sample grain size, coarsened grains accommodate large deformation in the process of compressive deformation, resulting in the crystallographic slips within grains and intergranular cracks.
As the original sample of the LY12 Al alloy is directly deformed by ECAP, the grains can be obviously refined and the high-density dislocation can be significantly induced. Thus the sample is strongly strain-strengthened. Meanwhile, a large number of small S-phases in the process of severe plastic deformation (SPD) of ECAP are dissolved into the matrix, and the precipitation-strengthening effect is thus weakened. The combined effect of two kinds of strengthening mechanisms leads to the fact that the strength of ECAP samples is lower than that of the original sample. However, the capacity of high-temperature plastic deformation of the sample is slightly enhanced by ECAP deformation treatment. During compression, many micro-cracks along grain boundaries (GBs) in addition to large-scale shear cracks and shear bands appear on the surface. With the temperature increasing, the number of shear bands and the scale of shear cracks reduce. When the temperature is raised up to 400°C, the shear cracks, shear bands and micro-cracks almost disappear, but a number of micro-voids are found to appear in the grains or along GBs. In contrast, the strength of the original sample still keeps relatively high, and large-scale shear cracks are still formed on the surface.
As the original sample is annealed at 450℃for 3h and then deformed by ECAP, grain refinement takes place and the dislocation density increases. Moreover, lots of the large-size needle-like S phases have been broken in the SPD process so that the resolution of S phases takes place or the size correspondingly decreases. At this point, the strain-strengthening effect resulting from ECAP deformation becomes more prominent, and the joint strain-hardening and precipitation-strengthening effects bring about a comparative flow stress of the annealed-ECAPed sample and the just-annealed sample. However, the yield strength of the annealed alloy can be effectively enhanced by the strain-hardening mechanism. Similar to the sample directly deformed by ECAP, a lot of the shear bands, large-scale cracks and GB cracks are also formed on the surface of the pre-annealed and ECAPed samples. As the testing temperature is below 300℃, although the yield strength of the 450℃/3h+2ECAPed sample is obviously lower than that of the original sample, it always maintains at a relatively high level (i.e., 320-350 MPa). To sum up, the 450℃/3h+2ECAPed sample shows a good high-temperature deformation stability.
研究发现,不同处理工艺对LY12铝合金的压缩力学行为具有不同的影响,且七种状态LY12铝合金的压缩变形及损伤特征与实验温度密切相关。具体表现如下:
初始态样品内部均匀分布着的大量细小S沉淀相(AlCuMg)的强化作用十分显著。对初始态样品进行450℃/3h退火处理后,晶粒尺寸长大,内应力降低,均匀分布的细小沉淀相在退火过程中长大形成针状S相,致使450℃/3h样品的屈服强度及流变应力在各温度压缩条件下均低于初始态样品。由于晶粒尺度影响,退火后样品在压缩变形过程中,粗大的晶粒承担了更多的变形,导致其在压缩后表面产生了晶内滑移和沿晶裂纹。
利用ECAP技术对初始态LY12铝合金样品直接进行变形处理后,一方面,可明显细化晶粒,引入高密度位错,使样品得到应变强化;另一方面,在ECAP制备过程中,大量细小的S相在严重的塑性变形过程中被基体所溶解,使沉淀强化作用减弱。两种强化机制的共同作用导致ECAP样品强度低于初始态样品。但是,ECAP变形处理使样品高温塑性变形能力有所增加,在压缩过程中,表面除出现大尺度剪切裂纹和剪切带外,还有许多沿晶微裂纹出现,随压缩温度的升高,剪切带减少,剪切裂纹尺度降低,当压缩温度达到400℃时,剪切裂纹、剪切带和微裂纹基本消失,在晶内和晶界上形成了许多微孔。而此时初始态样品强度依然较高,其表面仍然存在尺度较大的剪切裂纹。
对初始态样品进行450℃/3h退火后再进行ECAP变形处理,晶粒得到细化,位错密度增加,大量强化作用较弱的大尺寸针状S相在严重塑性变形过程中被折断而发生了重溶或尺寸减小,此时,由ECAP变形处理所引入的应变强化作用显得更加突出,不同的应变强化机制和沉淀强化机制导致了退火+ECAP和退火样品的流变应力相当,但是应变强化机制有效地提高了退火合金的屈服强度。与直接ECAP变形处理样品相类似,经退火+ECAP处理后压缩变形,表面同样出现了大量的剪切带、大尺度裂纹和沿晶微裂纹。在300℃以下进行压缩时,450℃/3h+2ECAP样品屈服强度虽然低于初始态样品,但始终保持在较高水平(320～350 MPa),表现出较好的高温变形稳定性。
In recent years, ultrafine-grained (UFG) materials prepared by the equal channel angular pressing (ECAP) technology have been attracting more and more interests of research, owing to their unique mechanical behaviors. However, the surface deformation characteristics of such materials, especially the deformation behavior in high temperature environment, is still far from comprehensive studies, and these studies are of practical significance for the development and application of such ECAPed materials. Therefore, in the present work, LY12 Al alloys are selected as the target materials, and seven kinds of samples with different structures induced by various treatments, such as original, 450℃/3h annealed, 1 ECAPed, 450℃/3h annealed+1ECAPed, 500℃/3h annealed+1ECAPed, 2ECAPed, 450℃/3h annealed+2ECAPed, are prepared. Then, the compressive deformation behavior, the surface deformation and damage characteristics, and the corresponding changes in microstructures of these samples are considerably examined at different temperatures (e.g. 25℃-400℃).
It is clearly found that the LY12 Al alloy exhibits different compressive mechanical behavior depending on different treatments, and the compressive deformation characteristics and damage behavior of LY12 Al alloys with seven treating states are closely related with the experimental temperature. The details are presented as follows.
A large number of small S-phase precipitates (AlCuMg) uniformly distributed in the original sample shows a remarkable strengthening effect. As the original sample is annealed at 450℃for 3h, the grain size distinctly increases, the internal stress significantly reduces, and the needle-like S phase evolves from uniformly distributed small precipitates in the process of annealing, causing that the yield strength and flow stress of 450℃/3h sample are markedly lower than that of the original sample under compressive deformation at all temperatures. As a result of the effects of the sample grain size, coarsened grains accommodate large deformation in the process of compressive deformation, resulting in the crystallographic slips within grains and intergranular cracks.
As the original sample of the LY12 Al alloy is directly deformed by ECAP, the grains can be obviously refined and the high-density dislocation can be significantly induced. Thus the sample is strongly strain-strengthened. Meanwhile, a large number of small S-phases in the process of severe plastic deformation (SPD) of ECAP are dissolved into the matrix, and the precipitation-strengthening effect is thus weakened. The combined effect of two kinds of strengthening mechanisms leads to the fact that the strength of ECAP samples is lower than that of the original sample. However, the capacity of high-temperature plastic deformation of the sample is slightly enhanced by ECAP deformation treatment. During compression, many micro-cracks along grain boundaries (GBs) in addition to large-scale shear cracks and shear bands appear on the surface. With the temperature increasing, the number of shear bands and the scale of shear cracks reduce. When the temperature is raised up to 400°C, the shear cracks, shear bands and micro-cracks almost disappear, but a number of micro-voids are found to appear in the grains or along GBs. In contrast, the strength of the original sample still keeps relatively high, and large-scale shear cracks are still formed on the surface.
As the original sample is annealed at 450℃for 3h and then deformed by ECAP, grain refinement takes place and the dislocation density increases. Moreover, lots of the large-size needle-like S phases have been broken in the SPD process so that the resolution of S phases takes place or the size correspondingly decreases. At this point, the strain-strengthening effect resulting from ECAP deformation becomes more prominent, and the joint strain-hardening and precipitation-strengthening effects bring about a comparative flow stress of the annealed-ECAPed sample and the just-annealed sample. However, the yield strength of the annealed alloy can be effectively enhanced by the strain-hardening mechanism. Similar to the sample directly deformed by ECAP, a lot of the shear bands, large-scale cracks and GB cracks are also formed on the surface of the pre-annealed and ECAPed samples. As the testing temperature is below 300℃, although the yield strength of the 450℃/3h+2ECAPed sample is obviously lower than that of the original sample, it always maintains at a relatively high level (i.e., 320-350 MPa). To sum up, the 450℃/3h+2ECAPed sample shows a good high-temperature deformation stability.
引文
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