【作者】 王晓溪；

【导师】 薛克敏；

【作者基本信息】 合肥工业大学 ， 材料加工工程， 2012， 博士

【摘要】 随着材料科学的飞速发展以及加工工艺的不断完善,块体超细晶材料因其具有不同寻常的物理和力学性能,近年来逐渐成为材料领域研究的热点之一。金属材料的性能与其组织有着密切的联系,细化材料晶粒一直是提高和改善材料综合性能的一种有效手段。目前,大塑性变形工艺(Severe Plastic Deformation, SPD)已被公认为获取无污染、无孔洞块体超细晶材料最行之有效的方法。等径角挤扭(Equal Channel Angular Pressing and Torsion, EC APT)是在等径角挤压(Equal Channel Angular Pressing, ECAP)和挤扭(Twist Extrusion, TE)基础上发展起来的一种新型复合大塑性变形工艺。它综合了ECAP和TE两种变形的特点,在传统ECAP模具的水平端型腔内加工出了螺旋状型槽。因此,在材料可加工性能允许的条件下,可在不改变试样横截面形状和尺寸的前提下,实现多道次重复变形,最终促进了材料晶粒的显著细化和性能的大幅提高。粉末材料是材料领域内的重要组成部分。然而,由于其塑性变形能力相对较弱,其变形、致密和细化的机理尤为复杂。传统的粉末塑性加工技术通常温度较高且工序繁琐,而且往往伴有材料的高温氧化和晶粒长大现象,很难制备出高致密度以及性能优良的块体超细晶材料,从而在某种程度上限制了它的发展和应用。大塑性变形法的出现,为以粉末材料为原料,制备和合成高性能的块体超细晶材料提供了一种有效而可行的新途径。为此,本文采用有限元数值模拟和实验分析研究相结合的手段,综合运用有限元分析技术、金属塑性成形原理、先进塑性成形技术、材料科学基础等科学知识,对纯铝粉末材料ECAPT变形过程中的变形致密行为、微观结构和力学性能演变以及晶粒细化机制进行了深入、系统地探讨与分析,并取得了一些有意义的成果。这些研究对于深入理解ECAPT工艺变形本质,促进大塑性变形在粉末材料领域内的研究和应用具有极其重要的理论意义和参考价值。本文首先基于可压缩连续介质理论,综合考虑变形场和温度场的影响,建立了用于求解粉末材料塑性变形的可压缩刚粘塑性热力耦合有限元方程,将变形材料视为多孔体,对带有包套的纯铝粉末材料单道次ECAPT变形过程进行了有限元模拟。结果表明,ECAPT工艺对粉末材料具有良好应变累积和组织致密效果,当试样头部退出螺旋通道时,挤压载荷达到了峰值。ECAPT变形过程中,试样在经过模具转角和螺旋通道时所受的剪切应变量最大,材料的内部静水压力值也最高。1道次ECAPT变形后,试样累积等效应变量约为1.4,整体相对密度高达0.999,但变形呈现出了不均匀分布的现象。温度场在试样纵向呈现出了逐渐递减的梯度分布,这表明在本文所设定的模拟条件下,坯料与模具之间的热交换以及坯料对周围环境的辐射传热要大于由塑性变形功转化而成的热能。在前述单道次ECAPT有限元模拟的基础上,对相同模拟条件下的ECAP和TE变形工艺进行了有限元分析。结果表明,ECAPT工艺在增大试样累积应变量、提高材料致密程度以及改善变形均匀程度三个方面均具有显著的优势。单道次ECAPT变形后,试样的有效累积应变量相比于ECAP和TE工艺,分别增大了17.6%和9.3倍。分析认为,单道次ECAPT变形过程中,螺旋通道的二次剪切和反向背压作用是使得粉末材料获得更大应变累积和更强致密效果的关键所在。为了进一步研究分析纯铝粉末材料在多道次ECAPT工艺下的变形和致密行为,设计了用于模拟不同变形工艺路径(A、BA、Bc和C)的连续多通道ECAPT模具。结果表明,随着挤压道次的增加,试样内部所累积的塑性应变量不断增大,材料出现了加工硬化现象,挤压载荷峰值不断上升。与此同时,试样的变形均匀程度随着变形道次的增加而逐渐增大。A和C两种路径可在较低的挤压载荷下实现材料有效的应变累积且试样整体变形较为均匀,是较为理想的挤压变形路径。多道次ECAPT变形有效提高了试样内部的静水压力,大大促进了材料内部残余孔隙的收缩和闭合,对改善材料变形的均匀程度起到了积极作用。在自行设计的ECAPT模具上,顺利完成了200℃下A路径纯铝粉末1-4道次的ECAPT变形实验。单道次ECAPT变形后,试样相对密度和显微硬度增幅明显。螺旋通道起到了反向背压的作用,可以有效提高ECAPT变形过程中材料内部的静水压力和A1原子的自扩散系数。随着变形道次的增加,晶粒的细化程度和材料的力学性能进一步提高。4道次ECAPT变形后,材料达到了完全致密,屈服强度高达123.3MPa,并表现出了良好的塑性-ECAPT变形过程中,{111}面衍射峰的形状和强度发生了变化。这表明随着变形程度的增大,晶粒在不断被细化的同时发生了转动。多道次ECAPT变形时,纯铝粉末材料的致密化过程主要体现在单道次的变形过程中。在剧烈剪切变形和强大静水压力作用下,原始孔隙数目不断减少,形状和体积都发生了明显改变,大量孔隙不断地收缩变小,最终实现了有效焊合。采用电子背散射衍射(EBSD)技术,对各道次ECAPT变形后的组织进行了表征和分析,深入研究了晶粒结构和显微织构的演化规律。结果表明,1道次ECAPT变形后,晶粒尺寸分布不均匀,组织为典型的混晶组织,但多为小角度晶界结构,平均晶粒尺寸约为5.20μm。随着变形道次的增加,晶粒不断被细化,取向差逐渐增大,变形更加趋于均匀。4道次ECAPT变形后,组织取向性消失,以细小、均匀且被大角度晶界所包围的等轴再结晶晶粒为主,平均晶粒尺寸约为1.67μm。ECAPT变形过程中,织构的产生和消失是动态连续变化的过程,存在着“织构起伏”效应,其转变过程为<101>→<111>→弥散状态。这是由于ECAPT变形时,晶粒在外力作用下其偏聚状态发生了改变,使得内应力向相邻晶界传递的过程中原来的聚集状态遭到了破坏。采用透射电镜(TEM),对200℃下各道次ECAPT变形组织的微观结构演变规律进行了观察和分析。结果表明,ECAPT变形初期,材料内部产生了许多近似平行的剪切带,位错密度较高且组态紊乱,形成了大量的位错缠结和位错胞,亚晶界和小角度晶界所占的比例较高。随着变形道次的增加,材料累积应变量进一步增大,晶界处位错密度大大升高,晶粒不断被新产生的位错界面所分割。亚晶在尺寸继续减小的同时发生了转动,亚晶之间的取向差增大,逐渐演变成了清晰、平直的大角度晶界,最终使得晶粒显著细化,组织均匀性大大提高,该变形过程中组织发生了连续动态再结晶。分析认为,机械剪切、应变累积和动态再结晶为纯铝粉末材料ECAPT变形过程中的晶粒细化机制。三种机制相互作用,密切相关,缺一不可。变形初期,晶粒细化机制主要为机械剪切和应变累积；而变形后期,连续动态再结晶机制占据了主导地位。更多还原

【Abstract】 With the rapid development of material science and the further improvement of processing technique, bulk ultrafine grained materials (UFG) have attracted enormous attention because of their potential mechanical and physical properties as compared with conventional materials. Generally, the strength of polycrystalline materials is related to the grain size and grain refinement is usually an effective method for the enhancement of the mechanical properties of materials. At present, severe plastic deformation (SPD) is considered to be the most attractive and promising process in obtaining bulk UFG materials without any contamination and residual porosities. Equal Channel Pressing and Torsion (ECAPT) is a newly developed technique of SPD, which combines the advantages of both ECAP and TE. In this method, two processes of ECAP and TE occur subsequently in a single die and a twist channel has been manufactured on the horizontal part of conventional ECAP die. Therefore, it is possible to carry out multiple passes in ECAPT without changing the cross section of the sample. In addition, it leads to a larger strain accumulation of the materials which finally accounts for the grain refinement and properties enhancement.Powder metallurgy (PM) material represents an important part of material family. However, due to its weak plastic forming ability, the mechanisms of densification and grain refinement are usually complicated. In conventional powder metallurgy, powder consolidation is always accomplished through sintering at high temperatures, which tends to coarsen the special microstructures of the particles and limits the development and application of PM materials as well. Recently, SPD methods have been widely used to fabricate bulk UFG materials by the consolidation of powder particles and it has been proved that the efficiency and quality of the consolidated materials are greatly improved.Therefore, in the present work efforts were made to understand the deformation and densification behaviors, the evolution of microstructures and mechanical properties as well as the grain refinement mechanisms of ECAPT consolidation of pure Al powder particles. Many kinds of professional knowledge were employed including finite element method, metal plastic forming technique, advanced technology of plastic forming, basic material science and so on. Additionally, some useful conclusions were also obtained. All this research will play a significant role in promoting the further investigation and industrial application of PM materials fabricated by SPD methods.Based on the compressible continuous medium theories and with the full consideration of deformation and temperature fields, the compressible rigid viscoplastic thermodynamic coupling finite element formulas were established. Then the deformation behavior of pure Al porous materials with powder in tubes during a single pas of ECAPT was obtained by FEM simulation. It was found that ECAPT had a significant effect on the strain accumulation and compact densification of PM materials. Extrusion load reached to the peak when the head of sample exited from the twist channel. During ECAPT process, both accumulated strain and hydrostatic pressure of the sample achieved maximum when it passed through the intersection part and twist channel of ECAPT die. After a single pass of ECAPT, the strain imposed on each sample was about1.4and the relative density reached to0.999. However, the deformation was not homogeneous in general. On the longitudinal plane of the sample, temperature slowly decreased from the tail to the top, which indicated that under present simulation conditions, exchange and radiation of the heat were much more than the thermal energy converted from the deformation energy.For comparison, FEM simulations of ECAP and TE were also conducted on the pure Al porous materials with the same simulation parameters. The results showed that ECAPT possessed many advantages over ECAP and TE in terms of increasing strain accumulation, promoting material densification and improving the deformation homogeneity. In specific, the imposed strain was increased by17.6%and9.3times as compared to ECAP and TE respectively. This was attributed to the repetitive shearing and back pressure provided by the twist channel.In order to investigate the deformation behavior of pure Al porous materials during multiple passes of ECAPT, continuous multi-pass ECAPT dies were designed for different routes. It is found that with the increasing number of ECAPT passes, the imposed strain was increased. Due to the occurrence of work hardening behaviors, the peak load was also increased. Moreover, sample deformation became more and more homogeneous. Route A and C were two optimal ECAPT routes because the sample could accumulate large strain without the loss of deformation homogeneity. As the hydrostatic pressure was increased under multiple passes, residual porosities in the PM materials were effectively shrunk and closed, which finally contributed to the improvement of homogeneity and density of the compacts.Pure Al particles were consolidated successfully into full dense bulk UFG materials at200℃using ECAPT and further deformed up to4passes. After1pass of ECAPT, relative density and microhardness of the compacts were greatly improved. During ECAPT process, twist channel played a role of back pressure and offered a significant advantage to PM materials such as increasing the hydrostatic pressure of the compacts and self-diffusion coefficient of Al atoms. As the deformation developed, grains were further refined and mechanical properties were largely enhanced. Combined compressive yield strength of123.3MPa and good ductility were observed after4passes of ECAPT. XRD results showed that the shape and intensity of diffraction peak on{111} plane changed during different ECAPT passes, which demonstrated grains were refined as well as rotated under ECAPT. The densification process of pure Al particles mainly occurred at the first pass of ECAPT based on the severe shearing and high hydrostatic pressure provided by ECAPT.After that, Electron backscattered diffraction (EBSD) was employed to characterized the microstructure and microtexture during ECAPT process. It is found that after the1pass of ECAPT, the microstructure consisted of many elongated grains and few equiaxed grains, but most of them were low angle boundaries with the average grain size of5.20μm. With the increasing number of ECAPT passes, deformation became more homogeneous, grains were further refined and the misorientation angle was increased as well. After4passes of ECAPT, PM materials contained fine grains of1.67μm in size and equiaxed in shape with boundaries of higher misorientation angles. The formation of texture was a dynamic process, which showed a fluctuation of<101>→<111>→scattering during ECAPT. This is because the external force acting on crystal class makes the segregation orientation change, which leads to the broken of aggregation state in the process of internal stress transmitted on the neighboring grain boundaries.In the last part of this investigation, microstructures of the processed materials were characterized using TEM for revealing grains of the order of1μm or smaller in size and subgrain structure. At the beginning of ECAPT process, the microstructure consisted of bands of subgrains and high dislocation density with the formation of dislocation tangles and dislocation cells, but most of the boundaries had low angles of misorientation. With the mumber of ECAPT passes, the samples accumulated larger strain and the dislocation density increased. After4passes of ECAPT, many of the subgrain boundaries evolved into high angle boundaries and there was a concomitant evolution of the arrays of well-defined cell or subgrain bands array into reasonably equiaxed ultrafine grains. This evolution was accompanied by the process of dynamic recovery and dynamic recrystallization. Thus, it can be inferred that the mechanism of grain refinement during ECAPT at200℃was the multiple effects of intensive shearing, large accumulated strains and dynamic recrystallization. Simple shearing and large strain were predominant factors initially while dynamic recrystallization became the leading point in the subsequent ECAPT passes.更多还原