CoCrFeNiMn系高熵合金高温变形与断裂行为研究

发布时间：2018-01-10 09:20

本文关键词：CoCrFeNiMn系高熵合金高温变形与断裂行为研究　出处：《中国科学技术大学》2017年博士论文　论文类型：学位论文

【摘要】：多主元高熵合金作为一种新型的合金材料,它是由五种或五种以上主要元素构成且每种元素的原子百分比在5-35%之间。由于该合金多主元特性产生的高熵效应、晶格畸变效应、缓慢扩散效应等,使合金在结构上易于形成简单的固溶体结构(面心立方、体心立方或两相混合),而非复杂的金属间化合物。另外在一些热处理条件下可能在基体中伴随生成纳米析出相。这种特殊结构产生的固溶强化、析出强化效果使高熵合金表现出优异的力学性能,例如较高的拉伸强度和延展性,优异的抗高温蠕变性能和热稳定性等。然而,关于高熵合金高温变形和断裂行为发生的机理尚无完善的论述,需要进行深入的研究。本文利用真空电磁感应熔炼方法制备并浇铸了 CoCrFeNiMn高熵合金。为了研究热机处理工艺对合金组织演化和力学性能的影响,对合金进行了均匀化热处理、冷轧和再结晶退火等工序。对热机处理态的合金进行室温和高温的拉伸试验,通过变形微结构观察和理论分析研究了锯齿流变行为,并对合金高温拉伸蠕变的变形机理进行了探讨。为了研究A1元素的添加对合金微结构和力学性能的影响,通过同样的工艺路线制备了AlxCoCrFeNiMn(x=0.4,0.5,0.6)高熵合金(记为Al0.4、Al0.5和Al0.6)。对不同Al含量合金的相组成、组织结构和拉伸性能进行了表征和分析,并选取Al0.5合金对其高温拉伸蠕变行为进行了研究。研究发现,热机处理后CoCrFeNiMn高熵合金由粗大的树枝晶偏析结构转变为成分均匀的等轴晶结构,在此过程中保持单一的面心立方相不变。轧制比例越高,再结晶温度越低,得到的等轴晶粒越细。其中最细晶粒的合金(轧制比40%,退火温度900℃/1h),平均晶粒尺寸为25μm,室温下抗拉强度达到580MPa,延伸率为56%左右。合金在中温区表现出优异的动态应变强化能力,应变强化指数在500℃达到0.42。与此同时在300～600℃温度区间观察到了显著的锯齿流变行为,锯齿类型随着温度的升高或应变率的降低发生A→A+B→B→B+C→C的转变。其中最大锯齿幅度出现在3×10-4s-1/600℃的C型锯齿(～6.7MPa)和500℃/1×10-5 s-1的B+C型锯齿(～8.9MPa)。对室温、400℃和600℃的缺陷组织观察发现,低应变下(～1%)较低密度的短直位错呈平行堆积排列,高应变下(～20%)较高密度的纠缠位错呈胞状结构分布。400℃和600℃时出现大量的位错弯曲和扭折,证明了位错被溶质原子"钉扎"的过程。基于溶质拖拽模型和准静态时效模型,在300～500℃温度区间,锯齿流变的激活能为116 kJmol-1,说明位错的"钉扎"受溶质原子经位错管道扩散过程控制;在500～600℃温度区间,激活能为295 kJmol-1,位错与溶质原子的相互作用受多原子协同晶格扩散控制,其中扩散最慢的Ni元素对变形速率起主要作用。具有25μm晶粒尺寸的合金高温蠕变行为表现出两个不同特征的应力区域。低应力区的应力指数为5-6,平均激活能为268 kJmol-1;而在高应力区,应力指数为8.9-14,平均激活能为380 kJmol-1。微结构分析观察到大量割阶结构,表明蠕变过程中发生了位错攀移。另外,高应力区还观察到显著的动态再结晶现象,并且在晶界处产生了大量的纳米析出相(M23C6和富含Cr的σ相)。因此,低应力区和高应力区的蠕变均为晶格扩散控制的位错攀移机制,但是高应力区明显升高的应力指数是由于蠕变过程中动态再结晶和晶界析出的共同作用导致的。对热机处理后的AlxCoCrFeNiMn(x=0.4,0.5,0.6)高摘合金的微结构研究发现,富含AlNi的体心立方相随着A1含量的升高而增多,合金的平均晶粒尺寸随着Al含量的升高而减小。体心立方相的强化作用和晶粒尺寸的"Hall-Petch"作用使Al0.6合金的室温屈服强度和最大强度分别达到348MPa和801MPa。与CoCrFeNiMn合金类似,Alx合金在中温区(300～600℃)表现出了较高的动态应变强化能力,这与此温度区间发生的锯齿行为有关。Al0.5合金的蠕变行为表现出与温度有关的转变。500℃和550℃应力指数为2.6-3,平均蠕变激活能为201 kJmol-l,说明合金的蠕变由溶质原子管道扩散控制的位错黏滞性滑移过程主导:600℃和650℃的应力指数为4.6-5.4,平均蠕变激活能为411 kJmol-1,说明蠕变过程为元素品格扩散控制的位错攀移机制。
[Abstract]:Multi element high entropy alloy as a new alloy material, which is composed of five or more than five kinds of main elements and the atomic percentage of each element in the range of 5-35%. Due to the high entropy effect of the alloy multi principal element properties, lattice distortion effect, slow diffusion effect, which is easy to form a solid alloy the solution structure is simple in structure (FCC, BCC or mixed), rather than complex intermetallic compounds. In addition some heat treatment conditions may be accompanied by the generation of nano precipitates in the matrix. The solid solution strengthening the special structure, the effect of precipitation strengthening high entropy alloy performance the excellent mechanical properties, such as high tensile strength and ductility, excellent high temperature creep resistance and thermal stability. However, the mechanism of the occurrence of high entropy alloy high temperature deformation and fracture behavior is not perfect in this need For further research. In this paper, using vacuum induction melting and casting method for preparation of CoCrFeNiMn high entropy alloy. In order to study the effect of heat treatment on microstructure evolution and mechanical properties of the alloy, homogenizing heat treatment, cold rolling and recrystallization annealing process. The tensile test at room temperature and high temperature thermomechanical treatment of alloy the state, through the deformation microstructure observation and theoretical analysis of the serrated flow behavior and deformation mechanism of the alloy, high temperature tensile creep are discussed. The effects of adding A1 on the microstructure and mechanical properties of the alloy, through the same process to prepare AlxCoCrFeNiMn (x=0.4,0.5,0.6) high entropy alloy (remember for Al0.4, Al0.5 and Al0.6). The phase composition of Al alloy with different content of structure, microstructure and tensile properties were characterized and analyzed, and selected the Al0.5 alloy on its high temperature tensile The creep behavior was studied. The study found that after thermomechanical treatment of CoCrFeNiMn high entropy alloy into uniform components such as crystal structure from coarse dendrite segregation structure, in the process of maintaining the FCC single phase constant. The higher the proportion of rolling, recrystallization temperature is low, the equiaxed grains get finer. One of the most fine grain alloy (rolling ratio 40%, annealing temperature of 900 DEG /1h), the average grain size is 25 m, the tensile strength at room temperature reached 580MPa, the elongation rate is about 56%. The alloy in medium temperature region showed excellent ability to strengthen the dynamic strain, strain hardening index at 500 DEG C to 0.42. at the same time 300 to 600 DEG C temperature range was observed for the serrated flow significantly, sawtooth type with the increase of temperature or strain rate decreasing change A - A+B - B - B+C - C. The maximum amplitude of serration appeared in type C 3 * 10-4s-1/600 C saw The tooth (~ 6.7MPa) and B+C /1 * 10-5 500 C zigzag S-1 (~ 8.9MPa). The room temperature, observed defects 400 degrees and 600 degrees, under low strain (~ 1%) short straight low density of dislocations in parallel packed high strain (~ 20%) higher dislocation entanglement a large number of dislocation density and bending torsion cell structure distribution of.400 DEG C and 600 DEG C, proved that the process of dislocation pinning by solute atoms. The solute drag model and quasi static model based on time, at 300 to 500 DEG C temperature range, the serrated flow activation energy of 116 kJmol-1, said Ming dislocation pinning by solute atoms by dislocation pipe diffusion process control; at 500 to 600 DEG C temperature range, the activation energy is 295 kJmol-1, the interaction of dislocations and solute atoms by atomic coordination lattice diffusion control, the diffusion of Ni element on the slow deformation rate plays a major role. With 25 m grain ruler High temperature creep behavior "showed two different regional stress characteristics. Low stress index was 5-6, the average activation energy of 268 kJmol-1; while in high stress area, the stress index was 8.9-14, the average activation energy observed in large cut order structure of 380 kJmol-1. micro structure analysis that happened in the process of creep, dislocation climb. In addition, the high stress region also observed significant dynamic recrystallization phenomenon, and at the grain boundaries resulting in a large number of nano precipitates (M23C6 and Cr rich phase). Therefore, the low stress area and high stress zone are dislocation creep lattice diffusion controlled climb mechanism, but the high stress area significantly increased the stress index is the result of the interaction of dynamic recrystallization during creep and grain boundary precipitation. The heat treated AlxCoCrFeNiMn (x=0.4,0.5,0.6) micro structure study found that high pick alloy, rich in the body of AlNi Cubic phase with increasing content of A1 increased, the average grain size of the alloy decreases with the increase of Al content. The BCC phase strengthening and the grain size of the "Hall-Petch" of the Al0.6 alloy at room temperature yield strength and maximum strength reached 348MPa and 801MPa. and CoCrFeNiMn alloy is similar to that of Alx alloy at medium temperature area (300 ~ 600 DEG C) showed a higher ability to strengthen the dynamic strain, the occurrence and the temperature interval of.Al0.5 alloy creep behavior of sawtooth behavior related to temperature change of.500 degrees and 550 degrees of stress index was 2.6-3, the average creep activation energy of 201 kJmol-l, indicating the dislocation creep controlled alloy by the solute diffusion pipeline viscosity slip leading process: 600 degrees and 650 degrees of the stress index was 4.6-5.4, the average creep activation energy of 411 kJmol-1, the creep process is diffusion controlled character elements The mechanism of dislocation climbing.

【学位授予单位】：中国科学技术大学
【学位级别】：博士
【学位授予年份】：2017
【分类号】：TG139

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