钢中fcc/bcc(bct)马氏体形核与长大的一种晶体学模型

将不变线理论和O点阵理论应用于fcc／bcc(bct)马氏体相变的可滑移生长界面的设计,建立了马氏体形核与长大的晶体学模型．通过模型分析表明:fcc／bcc(bct)马氏体形核与长大过程是通过(121)fcc型择优界面推移进行的,界面上的错配位错可以完成马氏体晶体学唯象理论(PTMC)要求的点阵不变变形(LID),但LID要稍滞后于界面迁移,即在马氏体形核与长大过程中推移界面新相一侧存在一未发生LID的新相薄区;当相变温度达到马氏体相变点Ms时,母相奥氏体与这一薄区的晶格常数比为、√3/2,这一几何条件和Olson-Cohen形核模型中要求扩展位错层错区界面能γ≤0是等价的．     

O点阵模型及其在界面位错计算中的应用

介绍了描述O点阵特征的基本参量：主O点陈基矢，主O点阵面和O胞壁的定义和计算公式，提供了一般界面上周期位错结构的矩阵方法和计算特殊界面位错结构的简易矢量方法：还分析了O点阵描述界面位错的条件和局限，讨论了一般界面和大错配界面上位错结构的计算。     

Mullite短纤维/Al-Cu-Mg复合材料界面微结构研究

研究用挤压铸造方法制备Mullite/Al-Cu-Mg复合材料,用透射电镜(TEM)观察了淬火态及时效态复合材料的微观组织.结果表明,莫来石(Mullite)短纤维组织致密但分布不均;在淬火态复合材料纤维/基体界面和Si晶体/基体的界面附近基体一侧中发现存在高密度位错;Mullite/Al-Cu-Si复合材料的界面以非平衡共晶MgAl2O4沉淀相为主.     

相变产物与母相晶粒间取向关系的理论分析

根据0点阵理论计算了fcc/bcc和fcc/bct在三种取向关系时的0点阵单胞体积V~(0).按V~(0)越大,该取向越稳定的判据,得知不同取向关系有各自合理存在的γ值范围。当γ值取某些特定值,就出现能量最低的二维0线点阵。业已提出,溶质原子可能局部偏析或贫化,使界面处γ值发生变化而降低界面的位错密度。α/β黄铜中Zn的偏析可以用这个模型加以解释。业已表明,四方畸变的大小和方向对V~(0)影响显著。如果已知fcc/bct的取向关系,就可以预示合理的畸变方向。但是,要说明Fe-C合金中取向关系由K-S转变成N-W的事实,得假定碳原子偏析到界面内的<11(?)>_f方向。     

镍基单晶高温合金中温稳态蠕变期间的变形机制

通过蠕变性能测试、组织形貌观察及位错组态的衍射衬度分析,研究了镍基单晶高温合金在中温/高应力稳态蠕变期间的变形机制.结果表明,在760℃,760 MPa和800℃,650 MPa蠕变期间,剪切g′相的位错可发生分解,分解后领先的a/3112超点阵Shockley不全位错切入g′相,拖曳的a/6112Shockley不全位错滞留在g′/g相界面,2个不全位错之间形成超点阵内禀堆垛层错(SISF);此外,剪切进入g′相的超点阵位错可由{111}面交滑移至{100}面,形成具有非平面位错芯结构的K-W锁,可抑制位错的滑移和交滑移,提高合金的蠕变抗力.在850℃,500 MPa蠕变期间,合金中的层错消失,部分剪切进入筏状g′相的a110超点阵位错可分解形成"2个a/2110不全位错加反相畴界(APB)"的组态,而合金中K-W锁的消失是由高温热激活致使立方体滑移的位错重新交滑移至八面体所致.     

两相界面几何错配的高分辨电子显微象分析

从界面两相的几何参数出发,计算界面两侧点阵平面的错配,并用实验观测的界面位错排布加以证实,这是相间界面分析的常用方法。本文进一步分析了那些从几何失配来预言界面会出现失配位错,但高分辨电子显微象却显示良好匹配的情况。说明在一定条件下,两相的结构相似性会容纳较大的几何失配。     

ON THE ORIENTATION RELATIONSHIP BETWEEN TRANSFORMATION PRODUCT AND PARENT PHASE

根据0点阵理论计算了fcc/bcc和fcc/bct在三种取向关系时的0点阵单胞体积V~(0).按V~(0)越大,该取向越稳定的判据,得知不同取向关系有各自合理存在的γ值范围。当γ值取某些特定值,就出现能量最低的二维0线点阵。业已提出,溶质原子可能局部偏析或贫化,使界面处γ值发生变化而降低界面的位错密度。α/β黄铜中Zn的偏析可以用这个模型加以解释。 业已表明,四方畸变的大小和方向对V~(0)影响显著。如果已知fcc/bct的取向关系,就可以预示合理的畸变方向。但是,要说明Fe-C合金中取向关系由K-S转变成N-W的事实,得假定碳原子偏析到界面内的<11(?)>_f方向。     

Cu-Ni-Sn三元系相平衡的热力学计算

基于Cu-Ni-Sn三元系的相平衡和热力学的实验信息,采用亚正规溶体模型描述液相和fcc相的Gibbs自由能,为了预测该体系中bcc相的A2-B2有序-无序转变,bcc相的Gibbs自由能采用双亚点阵模型进行描述.利用CALPHAD(相图计算)方法评估了Cu-Ni-Sn三元系各相的热力学参数,计算的富Cu侧相图和热力学性质与实验数据比较一致.并对该三元系中bcc相的A2-B2有序-无序转变及fcc相的溶解度间隙进行了计算.这些计算结果对利用析出强化以及Spinodal分解开发高强度和高导电性的新型Cu基合金的组织设计具有一定的指导意义.     

{112}<111>孪生的形核和长大及终止的ω点阵机制

针对体心立方(bcc)结构金属及合金{112}111孪生的w点阵机制,利用点阵模型详解了bcc结构金属及合金{112}111孪晶形核、长大和终止全过程.模型揭示了孪晶可以通过ω→bcc转变过程形成孪晶核胚,再通过孪晶核胚生长或合并的方式长大,最终与特殊位向ω相作用受阻而停止.该机制说明了{112}111类型孪晶是一种相变孪晶.     

疲劳软化的低能位错结构

用电子衍衬技术研究了预变形6％的1018钢在疲劳过程中位错组态的演变。晶体中位错十分可动,变形态的松散胞迅速变化为主胞壁大致沿{100}的棋盘结构,再逐渐演变为位错偶墙及迷宫结构的低能位错组态。bcc和fcc的迷宫特征以及偶墙取向相似,说明位错线的滑移几何特性及相应的位错细节不是软化过程的控制因素,位错组态的能量状态对软化有重要影响。讨论了周期载荷下胞间位向差的消失以及内应力下降的过程。     

Quantization of the Frank–Bilby equation for misfit dislocation arrays in interfaces

A quantization procedure for the Frank–Bilby equation to calculate the periodic structure of interphase interfaces and grain boundaries with one, two and three sets of discrete misfit dislocation arrays is presented mainly in terms of the basic O-lattice equation. The line directions and spacings of misfit dislocation arrays are derived to account for the full relaxation of the long-range strains in the interface without the ad hoc assumption proposed by Bollmann who assumed that misfit dislocation arrays are the lines of intersection between the O-cell walls and the interface. It has been found that the discrete dislocation arrangement derived from the above Bollmann’s assumption cannot always ensure that the interface is free of long-range strains.     

Slip transmission across fcc/bcc interfaces with varying interface shear strengths

Interfaces with relatively low shear strengths can be strong barriers to slip transmission because of core spreading of the dislocation within the interface. Using atomistic modeling and elasticity-based calculations we have studied the influence of interface shear strength on the slip transmission of a single dislocation across fcc/bcc interfaces. “Tunable” interatomic potentials were employed to vary the interface shear strength for the same interface crystallography. The results show that the slip transmission barrier increases with decreasing shear strength of the interface. The results reveal a mechanism for the variations in ultra-high strengths observed in nanoscale fcc/bcc multilayers.     

Tensile and compressive behavior of gold and molybdenum single crystals at the nano-scale

In situ mechanical tests were carried out to measure the tensile behavior of single-crystalline face-centered cubic (fcc) gold (Au) and body-centered cubic (bcc) molybdenum (Mo) nano-pillars with diameters between 250 and 1 μm, and to compare this with the compression results of these materials at the equivalent sizes. In Au, we observed similar tensile and compressive flow stresses at ～10% strain although strain-hardening in tension is somewhat more pronounced than it is in compression. In Mo, the amount of strain-hardening in tension is significantly lower than that in compression, leading to a distinct tension–compression asymmetry in the flow stress at ～5% strain. The dissimilarities between tensile and compressive behavior in both crystals are discussed in terms of sample geometry constraints and dislocation behavior in bcc crystals.     

On the measure of dislocation densities from diffraction line profiles: A comparison with discrete dislocation methods

In this work the accuracy and range of applicability of peak broadening models, from which dislocation densities can be extracted, is studied. For that purpose dislocation microstructures are generated via a discrete dislocation dynamics method and the internal elastic strains within the simulated volume are calculated. Diffraction peaks are generated from the simulations and a whole pattern line profile analysis method based on the Wilkens model is used to quantify the dislocation densities associated with the simulated microstructures. The work is applied to the case of face-centered cubic crystals deforming in coplanar slip. The accuracy of the analytical models is quantified by considering realistic three-dimensional microstructures containing curved dislocations with a specified distribution. The dependence and sensitivity of the analytical models upon dislocation density and long-range order are investigated. It was found that, provided the distribution of dislocations is rather homogeneous, line profile analysis provides fairly accurate predictions of the dislocation density.     

Atomistic modeling of the interaction of glide dislocations with “weak” interfaces

Using atomistic modeling and anisotropic elastic theory, the interaction of glide dislocations with interfaces in a model Cu–Nb system was explored. The incoherent Cu–Nb interfaces have relatively low shear strength and are referred to as “weak” interfaces. This work shows that such interfaces are very strong traps for glide dislocations and, thus, effective barriers for slip transmission. The key aspects of the glide dislocation–interface interactions are as follows. (i) The weak interface is readily sheared under the stress field of an impinging glide dislocation. (ii) The sheared interface generates an attractive force on the glide dislocation, leading to the absorption of dislocation in the interface. (iii) Upon entering the interface, the glide dislocation core readily spreads into an intricate pattern within the interface. Consequently, the glide dislocations in both Cu and Nb crystals are energetically favored to enter the interface when they are located within 1.5 nm from the interface. In addition to the trapping of dislocations in weak interfaces, this paper also discusses geometric factors such as the crystallographic discontinuity of slip systems across the Cu/Nb interfaces, which contribute to the difficulty of dislocation transmission across an interface. The implications of these findings to the unusually high strengths experimentally measured in Cu/Nb nanolayered composites are discussed.     

CYCLC DEFORMATION OF Nb SINGLE CRYSTALS

The Nb single crystals of both[321]and[110]orientations have been cyclicly deformed intension-compression at constant strain rate 8×10~(-4)s~(-1)over a range of plastic strainamplitudes between 10~(-3)and 10~(-4).The cyclic hardening,the changes in shape of crystals andthe asymmetry of stress have been studied.The hardening curve can be divided into threestages,i.e.first,rapid-hardening and saturated stage.In the first stage of cyclic hardening curve dominant features of dislocation configura-tions are high density networks and debris loops.In the rapid-hardening stage the main fea-ture is the formation of dislocation bundles.In the saturated a well defined bundle structurefully develops and between them it is filled with only screw dislocations and the imposed strainis accommodated mainly by the motion of screw dislocations travelling to and from betweenthe bundles.Three-dimension cell,two-dimension cell or bundle structures are summarized asthe saturated structures of bcc metals.     

Determining the Burgers vectors and elastic strain energies of interface dislocation arrays using anisotropic elasticity theory

A formalism for describing interface dislocation arrays linking the Frank–Bilby equation and anisotropic elasticity theory under the condition of vanishing far-field stresses is developed. The present approach enables the determination of a unique reference state for interface misfit dislocations, within which the Burgers vectors of individual dislocations are defined and allows for the unequal partitioning of elastic fields between neighboring crystals. The elastic strain energies of interface dislocation arrays are computed using solutions for short-range elastic fields. Examples of applications to simple interfaces are given, namely symmetric tilt and twist grain boundaries, as well as a pure misfit heterophase interface.