同位相热机械疲劳形变后γ/γ'相界面上的位错网分析

DD8单晶镍基高温合金经过同位相热机械疲劳(TMF)后,在垂直于应力轴的γ/γ'相界面上存在着大量的六角形位错网.对位错网中的位错进行的分析表明,六角形位错网中的位错都是刃型位错,而且这些位错的Brugers矢量都不与滑移过程开动的滑移系相对应.利用双交滑移模型说明了位错网的形成机制.     

DD8单晶镍基高温合金热机械疲劳后的微观结构

透射电镜(TEM)观察表明、DD8单晶镍基高温合金经过热机械疲劳(TMF)后，在同位相(IP)和反位相(OP)加载的条件下，合金内部的位错组态和γ′沉淀相的形貌有很大的区别．在IP加载条件下，垂直应力轴的γ／γ′相界面上存在着密集的六角形位错网、平行应力轴的相界面上存在的是四边形的位错网，而且在小机械应变幅下，γ′相出现明显的筏化现象、并且随着应变幅的增加，γ′沉淀相的筏化现象也越来越不明显．在OP加载条件下，在γ／γ′相界面上则没有位错网被观察到，γ′被层错剪切，并且没有明显的筏化出现．     

FGH95粉末镍基合金蠕变期间位错网的形成与分析

采用TEM观察和衍衬分析研究了FGH95粉末镍基合金蠕变期间的位错组态.结果表明,蠕变初期,1/2〈110〉位错在γ基体{111}晶面开动,使位错不断增殖.蠕变稳态阶段,2组不同Burgers矢量的位错可能滑移至同一晶面并相遇,反应后形成六角位错网络,或在不同滑移晶面相交.形成具有四边形特征的位错网络.位错网的形成可降低位错可动性并抑制位错交滑移,提高合金蠕变抗力.蠕变后期变形特征是形变位错在γ/γ′界面位错网损坏处切入γ′相,切入γ′相的位错可发生分解,形成不全位错和层错的位错组态.     

Calculation of the internal stresses at the γ/γ’ interface of DD8 single crystal nickel base superalloy after thermo-mechanical fatigue

计算了DD8单晶镍基高温合金在同相(IP)和反相(OP)热机械疲劳(TMF)后γ／γ′相界面上产生的位错网的内应力．结果表明：IPTMF条件下，γ／γ′相界面上产生的位错网可以释放掉大部分错配应力，同时因位错网的存在导致了γ′沉淀相发生了明显的筏化现象．OP条件下产生的层错未造成基体内应力分布的不同，因此未发生γ′沉淀相的筏化。     

DD8单晶镍基高温合金热机械疲劳后γ/γ''''界面位错网产生内应力的计算

计算了DD8单晶镍基高温合金在同相(IP)和反相(OP)热机械疲劳(TMF)后γ／γ'相界面上产生的位错网的内 应力.结果表明:IP TMF条件下, γ／γ'相界面上产生的位错网可以释放掉大部分错配应力,同时因位错网的存在导致了γ' 沉淀相发生了明显的筏化现象. OP条件下产生的层错未造成基体内应力分布的不同,因此未发生γ'沉淀相的筏化.     

α2相滑移系对γ-TiAl基多孪晶晶体屈服强度的影响

基于γ-基多孪晶晶体微结构及滑移系和形变孪晶空间晶体学位向分布的变形机制,提出了分析晶体屈服应力对外载轴和片层界面夹角间的依赖关系及影响因素的细观力学解析模型.分析了α2相中柱面和锥面上滑移系的启动对多孪晶晶体屈服应力的影响.模拟计算和分析结果表明:当θ=0°时,位错滑移方向跨越片层界面,α2相中柱面滑移系开动;当θ=45°时,滑移方向平行片层界面;当θ=90°方向时,γ相中与片层界面斜交的孪晶开动,而α2相中锥面滑移系由于其临界剪切应力(CRSS)很大而并不开动.多孪晶晶体中孪晶与普通位错、柱面与孪晶及锥面与柱面之间的CRSS比值关系可确定为k:e:z=1:3:18.     

6%Re/5%Ru单晶镍基合金的超高温蠕变行为和影响因素EI北大核心CSCD

通过对6%Re/5%Ru单晶镍基合金(质量分数)进行蠕变性能测试和组织观察,研究了合金的超高温蠕变行为和影响因素。结果表明:测定出合金在(1160℃,120 MPa)的蠕变寿命为206 h。中期稳态阶段,位错在基体中滑移和攀移越过γ′相是合金的变形特征,γ基体中溶解的难熔元素可增加位错在基体中运动的阻力。超高温蠕变期间,随温度提高γ′相发生溶解,可减小筏状γ′相的尺寸,提高位错攀移越过γ′相的速率,特别是当温度大于1170℃时,合金的施加温度敏感性使筏状γ′相的尺寸减小,应变速率提高,这是合金蠕变寿命大幅度降低的主要原因。蠕变后期,基体位错可在位错网破损处切入γ′相,其中,切入γ′相的位错可由{111}面交滑移至{100}面形成K−W锁,抑制位错的滑移和交滑移,可改善合金的蠕变抗力。而在颈缩区域较大的有效应力可开动位错的双取向滑移,致使筏状γ′相扭折,并在扭折区域发生裂纹的萌生和扩展,直至断裂,这是合金在超高温蠕变期间的变形和损伤机制。     

α2相滑移系对全片层组织TiAl合金屈服应力影响的初步分析

基于PST晶体的微结构和γ相及α2相普通位错和孪晶滑移启动，结合细观力学方法，通过数值模拟两相中的各滑移系上的分切应力，得出PST晶体屈服应力和外加载荷与片层之间夹角θ的关系。重点分析了α2相中柱面滑移系的启动对PST晶体屈服应力大小变化的影响。α2相中柱面滑移系的启动对PST晶体小加载角方向的屈服应力有比较明显的影响。所采用的细观方法和得到的结论对进一步研究全片层TiAl合金的屈服强度有一定的参考价值。     

基于γ′相尺寸的新型Ni-Cr-Co基合金高温变形行为研究

本文在600 ℃、5×10_^-4 s_^-1条件下对不同γ′相尺寸的毫米级粗晶新型Ni-Cr-Co基合金进行高温拉伸实验,并结合OM,SEM和TEM研究γ′相尺寸对合金高温变形行为和锯齿流变效应的影响。结果表明,γ′相尺寸对合金力学性能影响显著,随着γ′相尺寸的增加,材料强度呈先提高后降低的趋势,合金的主要变形机制由位错切过γ′相转变为位错绕过γ′相;当γ′相尺寸持续增加时,位错运动受阻难以绕过γ′相,溶质原子钉扎可动位错,当应力增大至一定程度时位错脱钉,反复的钉扎与脱钉即动态应变时效导致合金在变形过程中出现锯齿流变效应。可通过调控γ′相尺寸弱化,当γ′相平均尺寸为57.18 nm时,锯齿流变效应微弱,临界应变最大,力学性能较高,因此γ′相最佳尺寸为57.18 nm。     

Ta和Mo点位偏好对Co-Al-W合金γ′相的影响

为了研究Ta元素和Mo元素合金化对Co-Al-W基高温合金γ′相强度和形貌的影响，构建γ′-L12超胞结构，对其六个非等效点位进行掺杂计算，分析能量结构和力学性能；并制备Co-8.8Al-9.8W-2X(X=Ta, Mo)合金，对合金进行5%和10%的压缩变形，使用透射电镜（TEM）技术对合金γ′相形貌和位错形态进行分析。结果表明:Ta原子掺杂时优先占据Al2位置，Mo原子掺杂时优先占据W6位置，Ta原子占据Al2位置会使得γ′-L12掺杂结构的强度以及组织稳定性升高，Mo原子在W6位置的掺杂则效果相反；由于在合金化时Ta原子优先占据在Al2位置，导致了γ′相强度升高，2Ta合金在进行压缩变形时γ′相形貌可以保持为立方状，位错对γ′相的切割破坏较为有限，而2Mo合金由于Mo原子优先占据W6位置导致γ′相强度降低，进行压缩变形时γ′相形貌由立方状转变为筏排状，位错对γ′相的破坏较为严重。     

Evolution of interfacial dislocation networks during long term thermal aging in Ni-based single crystal superalloy DD5

Interfacial dislocations found in single crystal superalloys after long term thermal aging have an important effect on mechanical properties. Long term thermal aging tests for DD5 single crystal superalloy were carried out at 1,100 ℃ for 20, 100, 200, 500 and 1000 h, and then cooled by air. The effect of long term thermal aging on the dislocation networks at the γ/γ' interfaces was investigated by FE-SEM. Results showed that during the long term thermal aging at 1,100 ℃, misfit dislocations formed firstly and then reorientation in the(001) interfacial planes occurred. Different types of square or rectangular dislocation network form by dislocation reaction. Square dislocation networks consisting of four groups of dislocations can transform into octagonal dislocation networks, and then form another square dislocation network by dislocation reaction. Rectangular dislocation networks can also transform into hexagonal dislocation networks. The interfacial dislocation networks promote the γ' phase rafting process. The dislocation networks spacings become smaller and smaller, leading to the effective lattice misfit increasing from-0.10% to-0.32%.     

Simulation of dislocation penetration through a general low-angle grain boundary

The interaction of dislocations with low-angle grain boundaries (LAGBs) is considered one important contribution to the mechanical strength of metals. Although LAGBs have been frequently observed in metals, little is known about how they interact with free dislocations that mainly carry the plastic deformation. Using discrete dislocation dynamics simulations, we are able to quantify the resistance of a LAGB—idealized as three sets of dislocations that form a hexagonal dislocation network—against lattice dislocation penetration, and examine the associated dislocation processes. Our results reveal that such a coherent internal boundary can massively obstruct and even terminate dislocation transmission and thus make a substantial contribution to material strength.     

Dislocation interactions and low-angle grain boundary strengthening

The transmission of an incoming dislocation through a symmetrical low-angle tilt grain boundary (GB) is studied for {1 1 0}〈1 1 1〉 slip systems in body-centered cubic metals using discrete dislocation dynamics (DD) simulations. The transmission resistance is quantified in terms of the different types of interactions between the incoming and GB dislocations. Five different dislocation interaction types are considered: collinear, mixed-symmetrical junction, mixed-asymmetrical junction, edge junction, and coplanar. Mixed-symmetrical junction formation events are found not only to cause a strong resistance against the incident dislocation penetration, but also to transform the symmetrical low-angle tilt GB into a hexagonal network (a general low-angle GB). The interactions between the incident dislocation and the GB dislocations can form an array of 〈1 0 0〉 dislocations (binary junctions) in non-coplanar interactions, or a single 〈1 0 0〉 dislocation in coplanar interaction. We study how the transmission resistance depends on the mobility of 〈1 0 0〉 dislocations. 〈1 0 0〉 dislocations have usually been treated as immobile in DD simulations. In this work, we discuss and implement the mobility law for 〈1 0 0〉 dislocations. As an example, we report how the mobility of 〈1 0 0〉 dislocations affects the equilibrium configuration of a ternary dislocation interaction.     

MoS2中的位错

给出了具有层状结构的硫化钼(MoS_2)中位错的透射电镜(TEM)观察结果在基面上,位错和位错网络常分解成不全位错和不全位错网络利用衍衬效应分析了这些位错的Burgers矢量、材料的层错能可由扩展结估算出     

VARIATION OF SUBSTRUCTURES OF PEARLITIC HEAT RESISTANT STEEL AFTER HIGH TEMPERATURE AGING

The observations of dislocations， substructures and other-microstructural details were conducted mainly by means of transmission electron microscope (TEM) and scanning electron microscope (SEM) for 12Cr1MoV pearlitic heat-resistant steel. It is shown that during the high temperature long-term aging, the disordered and jumbled phasetransformed dislocations caused by normalized cooling are recovered and rearranged into cell substructures, and then the dislocation density is reduced gradually. Finally a low density linear dislocation configuration and a stabler dislocation network are formed and ferritic grains grow considerably.     

Effect of climb on dislocation mechanisms and creep rates in γ′-strengthened Ni base superalloy single crystals: A discrete dislocation dynamics study

Creep of single-crystal superalloys is governed by dislocation glide, climb, reactions and annihilation. Discrete three-dimensional (3D) dislocation dynamics (DDD) simulations are used to study the evolution of the dislocation substructure in a γ/γ′ microstructure of a single-crystal superalloy for different climb rates and loading conditions. A hybrid mobility law for glide and climb is used to map the interactions of dislocations with γ′ cubes. The focus is on the early stages of creep, where dislocation plasticity is confined to narrow γ channels. With enhancing climb mobility, the creep strain increases, even if the applied resolved shear stress is below the critical stress required for squeezing dislocations into the γ channels. The simulated creep microstructure consists of long dislocations and a network near the corners of the γ′ precipitate in the low-stress regime. In the high-stress regime, dislocations squeeze into the γ channels, where they deposit dislocation segments at the γ/γ′ interfaces. These observations are in good agreement with experimentally observed dislocation structures that form during high-temperature and low-stress creep.     

Phase field model of dislocation networks

In phase field model of dislocations, the short-range, non-linear core–core interactions are characterized through the crystalline energy and the gradient energy. In this article we extend and generalize the approximations of these energies employed in previous phase field models to account for dislocation reactions leading to network formation. In order to characterize dislocation activities involving all slip planes, we suggest the crystalline energy to be a function of a general plastic strain tensor produced by arbitrary linear combinations of simple shears associated with each slip system. For the four {111} slip planes in an fcc crystal, a particular form of such a crystalline energy is formulated by simple linear superposition of the interplanar potential of each individual slip plane. A more detail and general form of the gradient energy is derived from the consideration of the total Burgers vector dependence of dislocation line energy. Examples of applications are presented for interactions between two dislocation loops expanding on either a single slip plane or two intersecting slip planes, as well as for more complex reactions taking place in dislocation networks. It is shown that the generalized expressions are able to handle self-consistently reactions among dislocations of all slip systems in accord with Frank’s rule. These extensions are necessary steps toward advanced applications of the phase field method to dislocation substructure formation and coarsening.     

Dislocation–dislocation and dislocation–twin reactions in nanocrystalline Al by molecular dynamics simulation

We use massively parallel molecular dynamics simulations of polycrystal plasticity to elucidate the intricate dislocation dynamics that evolves during the process of deformation of columnar nanocrystalline Al microstructures of grain size between 30 and 100 nm. We analyze in detail the mechanisms of dislocation–dislocation and dislocation–twin boundary reactions that take place under sufficiently high stress. These reactions are shown to lead to the formation of complex twin networks, i.e. structures of coherent twin boundaries connected by stair-rod dislocations. Consistent with recent experimental observations, these twin networks may cause dislocation pile-ups and thus give rise to strain hardening.     

双模晶体相场研究六方四方相变过程晶界和位错演化

对于晶粒,晶界,应力和位错的交互作用的深入理解有助于优化材料组织和提升材料性能。本文采用双模晶体相场法研究六方相向正方相的转变。分别针对倾侧角为0°,15°,30°,和 45°,晶粒取向差为6°的六方相体系做了系统研究。六方晶粒长大、溶合、并形成共格晶界,位错组沿六方晶界均匀分布,并有两种取向。正方相在位错组处形核,并且其取向取决于位错组取向。每一种倾侧角的体系种均形成两个取向正方相的变体。针对倾侧角为0 °,15°,30°,和 45°的六方相体系生成的四方相相变体之间的取向差分别为30°, 30°, 10°, 和5°。不同取向的正方相晶粒长大熟化的方式有差异,位于有利取向的晶粒将会优先生长占据主导地位。以共格晶界形式长大的晶粒,晶界处有位错组生成以松弛晶粒长大的应力集中。     

DISLOCATION BEHAVIOR IN THE SUPERPLASTIC DEFORMATION OF ALLOY Al-Li 2091

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