CYCLIC DEFORMATION OF COARSE GRAINED POLYCRYSTALLINE PURE Al Ⅰ. SLIP CHARACTERISTICS

研究了粗大晶粒(直径10mm以上)纯Al多晶材料在循环形变中的滑移行为,与一般的多晶材料(晶粒大小为微米量级)的滑移行为比较有如下特点:(1)在一个晶粒内存在由于不同滑移系统的开动所造成的不同滑移区,可称之谓形变畴或滑移畴;在同一畴区内也可有几个滑移系统同时或先后强烈地操作,在试样表面形成美丽而整齐的滑移图像。(2)对于较大∑值或随机晶界结构,晶界对滑移形变的影响区约在距离晶界50—120μm的范围内。这类晶界往往在疲劳寿命早期就可萌生裂纹。晶界迁移、晶界滑动现象也能经常观察到;对那些低∑值的重位晶界,一般情况下滑移线都能连续地穿越晶界,在晶界区的形变不协调性小,这样的晶界往往不易成为裂纹的萌生地点。(3)三叉晶界的节点抑制了晶界滑动,是形变不协调较大的区域,造成较大的应力集中,尤其三条晶界中的高∑值的或随机的晶界,往往是沿晶开裂的优先部位。     

粗晶纯Al多晶材料的循环形变——Ⅰ.滑移特征

研究了粗大晶粒(直径10mm以上)纯Al多晶材料在循环形变中的滑移行为,与一般的多晶材料(晶粒大小为微米量级)的滑移行为比较有如下特点:(1)在一个晶粒内存在由于不同滑移系统的开动所造成的不同滑移区,可称之谓形变畴或滑移畴;在同一畴区内也可有几个滑移系统同时或先后强烈地操作,在试样表面形成美丽而整齐的滑移图像。(2)对于较大∑值或随机晶界结构,晶界对滑移形变的影响区约在距离晶界50—120μm的范围内。这类晶界往往在疲劳寿命早期就可萌生裂纹。晶界迁移、晶界滑动现象也能经常观察到;对那些低∑值的重位晶界,一般情况下滑移线都能连续地穿越晶界,在晶界区的形变不协调性小,这样的晶界往往不易成为裂纹的萌生地点。(3)三叉晶界的节点抑制了晶界滑动,是形变不协调较大的区域,造成较大的应力集中,尤其三条晶界中的高∑值的或随机的晶界,往往是沿晶开裂的优先部位。     

多晶Nb的纳米压痕研究

晶界硬化和滑移越过晶界传播的机制是材料变形研究中的两个重要问题，纳米压痕技术是研究滑移与晶界关系的有效手段之一．本文利用这一技术研究了多晶Nb晶界附近的变形行为，结果表明，当压痕打在一些晶界附近时，诱导了一种位移突变现象即晶界pop-in现象．与对应于材料初始塑性的pop—in不同，这种晶界pop-in现象与晶界二侧晶粒取向差密切相关．晶界pop—in与晶界附近硬度的变化无明显关联．     

密排六方钛中基面堆垛层错导致的孪晶界滑移、孪生台阶及孪晶生长:第一性原理研究

基于第一性原理计算研究密排六方结构钛中{101n}共格孪晶界和滑移孪晶界的结构和能量,探讨滑移孪晶界的形成机理及其与孪晶生长的关系.结果表明,共格孪晶界与基面堆垛层错的相互作用可使共格孪晶界产生滑移,从而形成对应的滑移孪晶界.这种滑移最终能在孪晶界处形成一对单层孪生台阶,并恢复共格孪晶界的结构.孪生台阶的塞积可导致高分...     

Mg-Gd-Y-Zn稀土镁合金中的位错滑移转移现象

通过原位拉伸对Mg-Gd-Y-Zn稀土镁合金中位错在相邻晶粒内的滑移转移现象进行了研究。使用Luster-Morris因子m′评估相邻两个晶粒内滑移系之间的几何关系。研究发现,Mg-Gd-Y-Zn稀土镁合金中裂纹更容易在硬取向和软取向晶粒的交界处形成。对实验结果进行统计分析后发现,位错滑移转移发生时,相邻晶粒滑移系的m′大于0.77。当晶界取向差小于34.2°时,发生基面位错-基面位错滑移转移现象;当晶界取向差大于48.8°时,发生基面位错-锥面位错滑移转移现象。此外,研究发现,m′与基面滑移系的Schmid因子相乘,可以定量地判断基面位错-基面位错的滑移转移现象,而对于基面位错-锥面位错的滑移转移现象,m′与Schmid因子相乘的方法并不适用。     

关于Al-Zn-Mg合金超塑性变形机理的定量金相学研究

一、前言关于细晶超塑性,到目前为止提出过种种变形机理。但是,由于在实际的超塑性变形中观察到显著的晶界滑移,所以把晶界滑移作为主要变形方式来研究变形机理,特别是晶界滑移和扩散流变、以及晶界滑移和晶内滑移的组合这二类被看作是主要的变形机理。在这些变形机理中,扩散流变和晶内滑移在缓和以晶界滑移为基础的晶界交叉点     

纯W高压扭转显微组织演化过程TEM分析

在相对较低温度下完成了纯W不同扭转圈数高压扭转实验,通过EBSD、TEM及HRTEM观察了纯W高压扭转过程中的显微组织形貌及微观结构。结果表明,随着等效应变增大,纯W材料显著细化,位错密度增加,非平衡晶界增多。高压扭转过程中小角度晶界向大角度晶界转化现象明显,且位错结构逐渐转移至晶界,细小晶粒内部无明显缺陷。当等效应变增大至5.5时,由于部分晶粒尺寸与位错平均自由程相近,晶粒变形方式由晶内滑移向晶界滑移转变。     

FCC金属Pd中晶界滑移的研究

使用位能图方法对金属Pd中晶界滑移的研究表明:有序晶界滑移易于开动,有序晶界的滑移可以引起晶界的迁移;与有序晶界相反,无序晶界的滑移很难开动.该结果可以解释形变过程中应力集中和位错发射通常均出现在无序界面处.     

镁合金及其复合材料超塑性的研究现状

以等通道角挤压和粉末冶金等方法为例，讨论了镁合金晶粒细化的处理工艺。在室温下晶粒细化可使镁合金同时具有较高的强度和韧性，而在较高的应变速率或较低的温度下，晶粒细化的镁合金具有一定的超塑性，这说明晶粒细化对改善镁合金的力学性能十分重要。镁合金超塑性变形的主要机制仍然是晶界滑移，但晶界滑移总会在晶界三叉区或材料增强相与基体的相界处产生应力集中，使晶界滑移受到阻碍，这就需要有另外的协调机制来协调晶界进一步滑移。镁合金在超塑性变形过程中更容易发生动态再结晶使晶粒细化，使晶界滑移能够继续进行。镁基复合材料中增强体颗粒很细小且弥散分布，稳定了镁合金在高温下的组织，颗粒与晶粒界面可以充当原子扩散通道，很好地协调了晶界滑移。     

TC6合金拉伸孔洞的形成机制

本文采用TEM、SEM研究了经不同热处理的TC6钛合金拉伸变形滑移特性和孔洞的形成。滑移从晶界、晶界α和魏氏α中开始。马氏体组织的孔洞发生在晶界;具有魏氏组织加晶界α的孔洞在晶界α与β相无取向关系的一侧和α/β相界面形成。拉伸孔洞的形成是位错滑移受阻机制。     

High-cycle fatigue performance of solution-treated metastable-β titanium alloys

The effect of grain boundary misorientation on the high-cycle fatigue performance of solution-treated, metastable-β titanium alloys was investigated. Initial damage during cyclic deformation was associated with the formation of coarse, planar slip bands, these often propagating through several grains without obstruction or redirection when intersecting with a grain boundary. This “continuous” slip through several grains was associated with the presence of a significant number of “low-angle” grain boundaries. Fatigue crack initiation was associated with crack initiation at intersecting planar slip bands at the free surface. The increase in operative slip length occasioned by the presence of low-angle grain boundaries lead to enhanced crack initiation and reduced lifetime. Fatigue crack propagation was characterized by step-like features formed through the interaction of the propagating crack and the coarse slip bands present in the plastic zone ahead of the crack tip. The direction of local fatigue crack propagation was also minimally affected when crossing low-angle grain boundaries.     

Cu双晶的循环形变行为与疲劳裂纹萌生：Ⅱ.疲劳裂纹萌生与早期扩展

用扫描电镜观察了受应变疲劳载荷作用的Ｃｕ双晶物的表面形貌，发现晶界是疲劳形变双晶是有利的裂纺萌生地点，在滑移带撞击晶界的地方，特别是在几条粗滑移带共同撞击晶界的地方观察到许多疲劳微裂纹；并且发现与平行晶界双晶相比，垂直晶界双晶有有利于疲劳裂纹沿晶界作早期扩展。     

铜晶体的疲劳损伤微观机制

对Cu单晶体、双晶体和多晶体疲劳损伤微观机制的总结结果表明：在中、低应变范围Cu单晶体的疲劳裂纹主要沿驻留滑移带萌生,而在高应变范围则沿粗大形变带萌生;Cu双晶体中疲劳裂纹总是优先沿大角度晶界萌生和扩展,而小角度晶界则不萌生疲劳裂纹;对于Cu多晶体,疲劳裂纹主要沿大角度晶界萌生,有时也沿驻留滑移带萌生,而孪晶界面两侧由于滑移系具有相容的变形特征而未观察到疲劳裂纹萌生．     

GH536高温合金焊接接头疲劳裂纹萌生与扩展原位试验研究

采用原位疲劳试验方法,实时观察了GH536焊接接头疲劳裂纹的萌生和扩展行为,从而揭示了GH536焊接接头疲劳裂纹的萌生和扩展机制:疲劳加载过程中,位错沿滑移带在晶界前沿塞积,晶界阻碍位错运动,裂纹沿滑移带开裂,萌生疲劳裂纹;疲劳裂纹扩展初期,受单滑移的交替作用,裂纹呈“Z”字型向前扩展,随后裂纹的扩展逐渐以主应力控制为主,垂直于加载方向、平直向前扩展;GH536合金焊接接头组织中的晶界和碳化物会阻碍疲劳裂纹的扩展。     

金属材料疲劳损伤的界面效应

总结了不同金属材料在低周疲劳过程中典型的晶界、孪晶界、相界和微电子互连界面的损伤开裂行为. 纯Cu中疲劳裂纹萌生的难易顺序为: 小角度晶界、驻留滑移带和大角度晶界. 对于纯Cu与铜合金中退火孪晶界, 是否萌生疲劳裂纹与合金成分有关, 随合金元素的加入降低了层错能, 退火孪晶界相对容易萌生疲劳裂纹. 对于Cu--Ag二元合金, 由于存在不同的晶界和相界面, 是否萌生疲劳裂纹取决于界面两侧晶体的取向差, 通常两侧取向差大的界面容易萌生疲劳裂纹. 在微电子互连界面中, 疲劳裂纹萌生位置与焊料成分和时效时间有关,对于Sn--Ag/Cu互连界面, 疲劳裂纹通常沿焊料与界面化合物结合处萌生; 对于Sn--Bi/Cu互连界面, 随时效时间增加会出现明显的由于Bi元素偏聚造成的界面脆性.     

Thermal fatigue mechanism of recrystallized tungsten under cyclic heat loads via electron beam facility

Thermal fatigue resistance of plasma facing materials (PFMs) is an inevitable concern for component lifetime and plasma operations, since the temperature fluctuations will always exist in future nuclear fusion facilities and reactors. Accordingly, experiments were performed in the electron beam facility to investigate the thermal fatigue behavior under operational loading conditions. The tungsten is investigated in its stress relieved and fully recrystallized state for a better understanding of the thermal fatigue process when exposed to cyclic heat loads. The heat loads range from 24 to 48 MW/m² and the number of cycles increases from 100 to 1000 times. The results indicate that the thermal fatigue damage (surface roughening) due to plastic deformation strongly depends on the loading conditions and the cycle index. As the power density and the number of cycles increase, the density of the intragranular shear bands in each grain becomes higher and the swelling of grain boundaries becomes more pronounced. The shear bands are generally parallel to different directions for varying grains, showing strong grain orientation dependence. Additionally, extruded flake structures on shear bands were observed in these damaged areas. It found that the shear bands are generally parallel to the traces of {112} slip planes with the surface. The results suggest that slip plastic deformation represent the predominant mechanism for thermal fatigue and a set of schematic diagram is presented to explain the formation of thermal fatigue damage morphology (extrusion and intrusion structures).     

Grain Boundary Responses to Heterogeneous Deformation in Tantalum Polycrystals

The evolution of heterogeneous deformation in a tantalum polycrystal was examined during a three-point bending experiment using electron backscatter pattern mapping. Slip bands formed at strains as low as 1%, and they became more intense with strain. Heterogeneous deformation was evident as intragranular orientation gradients as large as 30° were observed after a strain of about 8%. Nonmonotonic changes in the local average misorientation distribution were observed, implying that dislocation substructure developed in a complex manner. Slip bands were analyzed using plane traces computed from local orientation information. With the assumption of uniaxial stress, Schmid factors for favorable slip systems were identified for each grain and compared with observations, showing evidence for macroscopic activity on both {110} and {112} slip systems. Reconstructed boundary data were used to estimate the geometric potential for slip transfer at grain boundaries. The correlations indicated that when active slip systems were favorably oriented for slip transfer across the boundary, it was often observed in the form of continuous slip bands aligned across the boundary. In boundaries where geometrical alignment and Schmid factors were not favorable for slip transfer, there was a higher likelihood to form ledges (topographic discontinuities) along the grain boundaries. Dislocation pileups at grain boundaries were also correlated with a low potential for slip transfer.     

Fatigue cracking at twin boundaries: Effects of crystallographic orientation and stacking fault energy

The combined effects of crystallographic orientation and stacking fault energy (SFE) on the cracking behaviors of twin boundaries (TB) under low-cycle fatigue (LCF) tests were studied in pure Cu, Cu–Al and Cu–Zn alloys. A new approach, called the slipping morphology method, based on the crystallographic characteristics of Σ3 TB in face-centered cubic materials, was developed to determine the grain orientations by studying the twin-slip morphology characteristics on the sample surfaces after LCF tests. Through analyzing the dislocation–TB interaction and the damage this causes to TBs, a new parameter, defined as the difference of Schmid factors (DSF), was proposed to describe the effects of crystallographic orientation on the LCF cracking behaviors of TBs. A semi-quantitative relationship was established among DSF, SFE, dislocation slip mode and the critical conditions of TB cracking by systematically studying more than a hundred post-fatigue surface morphologies of pure Cu, Cu–Al and Cu–Zn alloys. It is interesting to find that the TB cracking relies strongly on the cooperation of both DSF and SFE. Furthermore, taking into account the interactions between slip dislocations and different boundaries, the fatigue cracking possibilities of several typical interfaces were compared and discussed. The results demonstrate that low-angle grain boundaries (GBs) are the strongest in resisting fatigue cracking, high-angle GBs are the weakest, and TBs are in between, which contributes the most to the final fatigue performance of materials. This new finding will help understanding of the interfacial properties under cyclic loading and may be beneficial to the design of high-performance materials with optimal fatigue properties in the future.     

Effect of crystallographic orientation and grain boundary character on fatigue cracking behaviors of coaxial copper bicrystals

Four coaxial copper bicrystals were employed to study the slip morphologies and fatigue cracking behaviors during cyclic deformation. Three of them had high-angle grain boundaries (GBs) with nearly the same misorientation and one bicrystal had a twin boundary (TB). Different slip bands (SBs) operated near the GBs and TB, generating different dislocation arrangements, which are mainly determined by the crystallographic orientations of the component grains. The GBs suffered impingement or shear damage caused by slip difference from both sides. It is suggested that there is an energy increase in the interfaces between matrix and persistent slip bands (PSBs), GBs and TBs per cycle during cyclic deformation due to the accumulation of lattice defects, which would make the interface unstable. After a certain number of cycles, fatigue cracks initiated firstly at GBs for some bicrystals while fatigue cracking occurred preferentially at PSBs for the others. It is confirmed that the energy growth rate is an increasing function of the shear stress, strain amplitude and strain incompatibility, which results from slip differences on both sides of the interfaces. Interfaces with different energies and strain incompatibilities have different fatigue cracking resistance. It is found that GBs with defective and complex structure, and hence high interfacial energy accompanied by high modulus of the residual GB dislocation (GBD), are preferential sites for fatigue cracking, while the fatigue cracking appeared predominantly at PSBs when the modulus of the residual GBD is lower than that of a perfect dislocation with simple GB structure and low interfacial energy. The present model for the energy can predict well which kind of interface would form cracks preferentially during cyclic deformation in one coaxial bicrystal and which GB would need more cycles to initiate fatigue cracking between coaxial bicrystals with different GB characters.     

Deformation mechanism at impact test of Al-11% Si alloy processed by equal-channel angular pressing with rotary die

Al-11%Si(mass fraction)alloy was transformed into a ductile material by equal-channel angular pressing(ECAP)with a rotary die.Two mechanisms at impact test,slip deformation by dislocation motion and grain boundary sliding,were discussed.The ultrafine grains with modified grain boundaries and the high content of fine particles(<1μm)were necessary for attaining high absorbed energy.The results contradict the condition of slip deformation by dislocation motion and coincide with that of grain boundary sliding.Many fine zigzag lines like a mosaic were observed on the side surface of the tested specimens.These observed lines may show grain boundaries appeared by the sliding of grains.