Effect of grain boundary character on the martensitic transformation in Fe–32at.%Ni bicrystals

The effect of grain boundary character on the martensitic transformation was examined in two types of Fe–32at.%Ni bicrystals containing a 90°<211> tilt or a 90°{211} twist boundary focusing on the martensite-start temperature (Ms) during cooling, the morphology of the martensite and the variant selection. The Ms of bicrystals with a tilt boundary was higher than that of single crystals, while bicrystals with a twist boundary showed no significant difference from that of single crystals. Coarse lenticular martensites formed symmetrically in neighbouring grains around the tilt boundary. In contrast, tiny martensites were uniformly distributed in bicrystals with a twist boundary, and in single crystals. In the vicinity of the tilt and twist boundaries, some variants with the habit plane almost parallel to the boundaries were preferentially selected among 24 variants. Moreover, since the equivalent variants in neighbouring grains at the tilt boundary are selected, strain compatibility of the shape strain of martensite across the tilt boundary is satisfied, resulting in an increased Ms. The effect of pre-strain on the heterogeneous nucleation of martensite at the boundaries was also investigated.     

Stress-induced martensitic transformation in Fe–Ni bicrystals

The effect of applied stress on martensitic transformation behaviour near the grain boundary was examined using Fe–32 at.% Ni bicrystals containing a 90°2 1 1 tilt or a 90°{2 1 1} twist grain boundary focusing on the martensite-start stress (σ_M), the morphology of martensites and the variant selection. The σ_M for single crystals and tilt boundary bicrystals increases almost linearly with increasing temperature (T), and the σ_M−T relation for tilt boundary bicrystals is situated in the higher temperature side than that for single crystals. In contrast, the σ_M of the twist boundary bicrystals exists between the correlated σ_M−T relations for single crystals and tilt boundary bicrystals. The variant selection near the tilt boundary is not sensitive to applied stress; some variants with the habit plane almost parallel to the boundary were symmetrically formed similar to those in the thermal transformation. The effect of loading axis to grain boundary on the martensitic transformation behaviour was also investigated.     

Fe-C-Mn-Si钢中奥氏体共格孪晶界对贝氏体铁素体变体选择的影响

将C含量(质量分数)分别为0.05%和0.4%的Fe-C-Mn-Si钢进行等温处理得到贝氏体组织,采用EBSD技术对奥氏体共格孪晶界上形成的贝氏体铁素体变体进行分析.结果表明,2种钢中的贝氏体铁素体与母相奥氏体均成近似K-S取向关系.奥氏体孪晶界两侧形成取向相同的变体对.此变体对形成后,孪晶界基本不再显现.晶体学分析表明,共格孪晶界两侧可能出现的变体对最多不超过3组,且这3组变体对的惯习面均与孪晶界平行,因此,贝氏体铁素体变体都将沿孪晶界生长.含C量为0.05%的Fe-C-Mn-Si钢中奥氏体孪晶界上只观察到一组贝氏体铁素体变体对的形成,这是因为C含量较低,贝氏体铁素体生长速度较快,消除了其它变体对的形核机会,先形核的变体对一旦形核就迅速覆盖整个孪晶面.而在含C量为0.4%的Fe C Mn-Si钢中,由于C含量较高,贝氏体铁素体生长速度较慢,3组变体对均有机会形核,因此,在孪晶界上可以观察到这3组变体对同时出现.     

Fe-C-Mn-Si钢中奥氏体共格孪晶界对贝氏体铁素体变体选择的影响

将C含量(质量分数)分别为0.05%和0.4%的Fe-C-Mn-Si钢进行等温处理得到贝氏体组织,采用EBSD技术对奥氏体共格孪晶界上形成的贝氏体铁素体变体进行分析.结果表明,2种钢中的贝氏体铁素体与母相奥氏体均成近似K-S取向关系.奥氏体孪晶界两侧形成取向相同的变体对.此变体对形成后,孪晶界基本不再显现.晶体学分析表明,共格孪晶界两侧可能出现的变体对最多不超过3组,且这3组变体对的惯习面均与孪晶界平行,因此,贝氏体铁素体变体都将沿孪晶界生长.含C量为0.05%的Fe-C-Mn-Si钢中奥氏体孪晶界上只观察到一组贝氏体铁素体变体对的形成,这是因为C含量较低,贝氏体铁素体生长速度较快,消除了其它变体对的形核机会,先形核的变体对一旦形核就迅速覆盖整个孪晶面.而在含C量为0.4%的Fe-C-Mn-Si钢中,由于C含量较高,贝氏体铁素体生长速度较慢,3组变体对均有机会形核,因此,在孪晶界上可以观察到这3组变体对同时出现.     

Structure and free volume of 〈1 1 0〉 symmetric tilt grain boundaries with the E structural unit

Atomistic simulations are used to investigate the structure and interfacial free volume of 〈1 1 0〉 symmetric tilt grain boundaries in copper containing the E structural unit from the Σ9(2 2 1)θ = 141.1° grain boundary. In this work, a stereologically-based methodology is used to calculate the grain boundary free volume along with the spacing and connectivity of free volume. After generating the minimum energy equilibrium grain boundary, we examine (i) the grain boundary structure, (ii) a measure of free volume associated with the grain boundary, (iii) spatial correlation functions of the distribution of free volume, and (iv) images of grain boundary free volume distribution. Using the results from these calculations, the influence of free volume spatial distribution and grain boundary structure on dislocation dissociation and nucleation is briefly discussed for boundaries with the E structural unit subjected to tensile loading normal to the interface along with the potential implications of free volume connectivity.     

Crystallographic variant selection in α–β brass

The transformation texture of α/β brass with a diffusional Widmanstätten α growth morphology has been investigated. Electron micrographs and electron backscattered diffraction was used to determine that the orientation relationship between the β phase and the α associated with nucleation at β grain boundaries was 44.3° 〈1 1 6〉. Crystallographic variant selection was observed across those prior β/β grain boundaries, but this has little effect on the transformation texture due to the crystal symmetry. The effect of the crystallographic variant selection on texture is further weakened by nucleation of diffusional transformed α in the grain interior.     

γ-TiAl中<110>倾斜晶界断裂行为的分子动力学模拟(英文)

采用分子动力学(MD)方法研究γ-Ti Al合金中<110>对称倾斜界面的断裂行为,模拟在不同温度与应变速率下垂直界面方向的拉伸变形。结果表明:晶粒的相对取向及晶界特定的原子结构是影响位错形核临界应力的两个主要因素。取向差角度大于90°的Σ3(111)109.5°、Σ9(221)141.1°和Σ27(552)148.4°界面,位错在晶界处形核和扩展;取向差角度小于90°的Σ27(115)31.6°和Σ11(113)50.5°界面,无位错在晶界处形核,当应力达到峰值后界面直接断裂。γ-Ti Al双晶的断裂机制为微裂纹在界面处的形核及沿界面扩展;不同取向差界面的区别在于裂纹前端有无塑性区增韧。     

Nucleation and variant selection of secondary α plates in a β Ti alloy

The microtexture of secondary α plates in Ti–4.5Fe–6.8Mo–1.5Al has been investigated by electron backscatter diffraction (EBSD) to obtain more insight in the nucleation and variant selection of these α plates. A statistical analysis of the EBSD data shows that for most β grain boundaries the variant selection of the α plates is in agreement with a commonly used variant selection criterion yielding that the α-{0 0 0 1} pole is nearly parallel to the closest β-{1 1 0} poles of the two adjacent β grains. For a small angle between the β-{1 1 0} poles nucleation is predominantly observed at both sides of the grain boundary, while with increasing angle some β grain boundaries exhibit nucleation of α plates at only one side. In the β grain interior many so-called Type 2 α–α grain boundaries are observed which are thought to originate from autocatalytic nucleation when a new α plate is formed at an existing α–β interface.     

Three-dimensional investigation of grain boundary–twin interactions in a Mg AZ31 alloy by electron backscatter diffraction and continuum modeling

The effect of grain boundary (GB) misorientation (θ) on twinning in a Mg AZ31 alloy is investigated using a three-dimensional (3-D) experimental and modeling approach, in which 3-D electron backscattered diffraction is performed in a volume consisting of a central grain, favorably oriented for twinning, and surrounded by three boundaries, with θ ranging from 15° to 64°. This study corroborates previous observations that twin nucleation and propagation are favored at low θ. Furthermore, it reveals that non-Schmid effects, such as the activation of low Schmid factor (SF) variants or of double tensile twins, are absent in the vicinity of low misorientation boundaries and that they become more abundant as θ increases. The 3-D morphology of individual twin variants is found to be related to their SF. High SF variants have well-established plate morphology, while low SF variants adopt irregular shapes. A crystal plasticity continuum model recently proposed by the authors is used in a very high intragrain resolution and large-scale finite element polycrystalline aggregate model of the experimental specimen. This model is shown to successfully capture the influence of θ on twin propagation and variant selection. It ultimately predicts (i) a rise in local non-basal slip with increasing θ, (ii) that low θ GB favor twin nucleation by non-Schmid stress concentrations, but that propagation is immediately accommodated by the macroscopic stress, and (iii) that high θ GB are not favorable twin nucleation sites, despite having high von Mises stress concentrations.     

Dislocation–grain boundary interaction in 〈1 1 1〉 textured thin metal films

The interaction of lattice dislocations with symmetrical and asymmetrical tilt grain boundaries in 〈1 1 1〉 textured thin nickel films was investigated using atomistic simulation methods. It was found that the misorientation angle of the grain boundary, the sign of the Burgers vector of the incoming dislocation and the exact site where the dislocation meets the grain boundary are all important parameters determining the ability of the dislocation to penetrate the boundary. Inclination angle, however, does not make an important difference on the transmission scenario of full dislocations. Only limited partial dislocation nucleation was observed for the investigated high-angle grain boundary. The peculiarities of nucleation of embryonic dislocations and their emission from tilt grain boundaries are discussed.     

Characteristic features of diffusion induced grain boundary migration for Σ9 [110] asymmetric tilt boundaries in the Cu(Zn) system

The characteristic features of diffusion induced grain boundary migration (DIGM) in the Cu(Zn) system were experimentally studied for [110] asymmetric tilt boundaries using Cu bicrystals annealed at 693 K for various times by a capsule zincification technique. The experiment was carried out for the boundaries with inclination angles of φ=0, 20, 35, 55, 65 and 90° and with a constant misorientation angle of θ=39° (Σ9). During annealing, the grain boundary migrates towards a crystal grain of larger coherency strain energy with higher probability due to DIGM. Taking elastic anisotropy of each crystal grain into consideration, the difference between the probabilities of grain boundary migration towards both side grains can be accounted for by the coherency strain model proposed by Hillert. The migration rate v of the moving boundary is almost constant regardless of the annealing time t between t=72 and 384 h (2.59×10⁵ and 1.38×10⁶ s). The value of v monotonically decreases with increasing inclination angle. This implies that the boundary diffusion coefficient of Zn in Cu is a monotone decreasing function of the inclination angle. The experimental results on the kinetics at the steady state stage were theoretically analyzed using the energy balance model proposed by Kajihara and Gust (Scripta mater., 1998, 38, 1621. The analysis indicates that the effective driving force Δ^efG for the grain boundary migration is merely one-thirtieth of the chemical driving force, whereas it still remains three times greater than the minimum value corresponding to the coherency strain energy during DIGM under the present experimental conditions. The mobility of the moving boundary was evaluated to be M=3.77×10⁻¹⁷ m⁴/Js from the values of v and Δ^efG according to the relationship M=v/Δ^efG. This value of M is close to the results estimated by Yamamoto et al. (Acta mater., 1999, 47, 1757) for the [100] twist and random boundaries in the Cu(Zn) system.     

Aluminum Σ3 grain boundary sliding enhanced by vacancy diffusion

Grain boundary sliding is an important deformation mechanism for elevated temperature forming processes. Molecular dynamics simulations are used to investigate the effect of vacancies in the grain boundary vicinity on the sliding of Al bi-crystals at 750 K. The threshold stress for grain boundary sliding was computed for a variety of grain boundaries with different structures and energies. These structures included one symmetrical tilt grain boundary and five asymmetrical tilt grain boundaries. Without vacancies, low energy Σ3 grain boundaries exhibited significantly less sliding than other high energy grain boundaries. The addition of vacancies to Σ3 grain boundaries decreased the threshold stress for grain boundary sliding by increasing the grain boundary diffusivity. A higher concentration of vacancies enhanced this effect. The influence of vacancies on grain boundary diffusivity and grain boundary sliding was negligible for high energy grain boundaries, due to the already high atom mobility in these boundaries.     

Atomic motion during the migration of general [0 0 1] tilt grain boundaries in Ni

We generalize a previous study of the atomic motions governing grain boundary migration to consider arbitrary misorientations of [0 0 1] tilt boundaries. Our examination of the nature of atomic motions employed three statistical measures of atomic motion: the non-Gaussian parameter, the “dynamic entropy” and the van Hove correlation function. These metrics were previously shown to provide a useful characterization of atomic motions both in glass-forming liquids and strained polycrystalline materials. As before, we find highly cooperative, string-like motion of atoms, but the grain boundary migration itself is a longer timescale process in which atoms move across the grain boundary. These observations are consistent with our previous results for Σ5 [0 0 1] tilt boundaries. It is evident from our work that the grain boundary structure and misorientation have a significant influence on the rate of grain boundary migration.     

The disconnection mechanism of coupled migration and shear at grain boundaries

The mechanism of coupled migration and shear is studied in a range of [0 0 0 1] tilt boundaries in hexagonal close-packed metal using atomic-scale computer simulation. Symmetrical tilt boundaries spanning the low- and high-angle regimes and comprising regular arrays of grain boundary dislocations are simulated. For each misorientation, θ, the perfect boundary (pristine) is investigated as well as one containing a disconnection. Both types of structures are subjected to incremental applied strains to determine the stress that produces coupled migration and shear. The stress for motion in the pristine case, entailing nucleation, is higher than the Peierls stress for motion when disconnections are present. We conclude that the applied stresses in our simulations exert a Peach-Koehler force on pre-existing disconnections, thereby providing a feasible mechanism with a well-defined driving force that produces coupled migration and shear. This mechanism is feasible for the lower-angle boundaries studied, and facile for the high-angle cases.     

Length-scale effects in the nucleation of extended dislocations in nanocrystalline Al by molecular-dynamics simulation

The nucleation of extended dislocations from the grain boundaries in nanocrystalline aluminum is studied by molecular-dynamics simulation. The length of the stacking fault connecting the two Shockley partials that form the extended dislocation, i.e., the dislocation splitting distance, r_split, depends not only on the stacking-fault energy but also on the resolved nucleation stress. Our simulations for columnar grain microstructures with a grain diameter, d, of up to 70 nm reveal that the magnitude of r_split relative to d represents a critical length scale controlling the low-temperature mechanical behavior of nanocrystalline materials. For r_split>d, the first partials nucleated from the boundaries glide across the grains and become incorporated into the boundaries on the opposite side, leaving behind a grain transected by a stacking fault. By contrast, for r_split<d two Shockley partials connected by a stacking fault are emitted consecutively from the boundary, leading to a deformation microstructure similar to that of coarse-grained aluminum. The mechanical properties of nanocrystalline materials, such as the yield stress, therefore depend critically on the grain size.     

Different Mechanisms of ε-M and α'-M Variant Selection and the Influencing Factors of ε-M Reversion During Dynamic Tension in TRIP Steel

The variant selection of martensites(ε-M and α'-M) and ε-M reversion in dynamic tensile high-manganese TRIP steel were investigated. α'-M variant pairs with a zigzag morphology frequently formed, and the pairs of neighboring α'-M variants were examined in terms of mechanical work and strain energy reduction. The occurrence of a primary α'-M variant is determined by mechanical work, but high products of mechanical work and strain energy reduction are essential for secondary variant selection. In contrast to α'-M variant pair selection, ε-M variant selection can be attributed to the highest mechanical work. During ε-M→α'-M transformation, the mechanical work of ε-M reversion is higher than that of α'-M variant, thereby implying that ε-M reversion in h110 icgrain is possible. e-M plate distribution also affects the feasibility of ε-M reversion.     

Effect of β grain growth on variant selection and texture memory effect during α → β → α phase transformation in Ti-6 Al-4 V

In the present study, the texture evolution and the role of β grain growth on variant selection during β → α phase transformation have been investigated in Ti-6 Al-4 V with and without 0.4 wt.% yttrium addition. The aim of adding yttrium was to control β grain growth above the β transus by pinning grain boundaries with yttria. Both materials were first thermomechanically processed to generate similar starting microstructures and crystallographic textures. Subsequently, both materials were solution-heat-treated above the β transus followed by slow cooling to promote growth of the α lath structure from grain boundary α. Additional interrupted slow cooling experiments were carried out to identify the α lamellae that nucleate first from β grain boundaries. Detailed electron backscatter diffraction analysis was carried out and it was found that the β heat treatment did not generate new texture components although the intensities of the individual components changed dramatically depending on the alloy/β grain size. Variant selection was assessed by comparing measured α texture components with predicted α texture components based on the high-temperature β texture assuming equal variant selection. It was found that with increasing β grain size variant selection intensified favouring the {φ1, Φ, φ2} {90°, 30°, 0°} texture component. Interrupted cooling experiments revealed that α nucleates first on β grain boundaries that are formed by two β grains having a common (1 1 0) normal and that these α lamellae display almost exclusively a {φ1, Φ, φ2} {90°, 30°, 0°} orientation. Consequently, the dominance of this variant with increasing β grain size can be related to the relative free growth of this particular α texture component into an “empty” β grain.     

Variant selection during nucleation and growth of γ-massive phase in TiAl-based intermetallic alloys

This work deals with the mechanisms of nucleation and growth of γ-massive phase in TiAl-based intermetallic alloys. In particular, it focuses on the process of variant selection operating at both stages of the transformation. Small γ-massive domains produced by rapid cooling are extensively characterized by high-resolution electron backscatter diffraction. This large data set allows a statistical analysis of nucleation sites, according to different crystallographic configurations. It is established that, whatever the nucleation sites, i.e., grain boundaries or triple junctions, a coherent facet is always found presenting a Blackburn orientation relationship (BOR) between the γ nucleus and the α parent grain. Moreover, some γ nuclei can additionally present another semicoherent facet with an approximate BOR with the α host grain. A new nucleation mechanism, called “co-nucleation”, is highlighted for this type of double-faceted nucleus. Variant selection during nucleation is discussed for both types of nuclei, in terms of minimization of interface energy. In addition, it is shown that growth of γ-massive domains from their initial nucleus always involves successive {1 1 1} twinning. Variant selection also occurs either at the nucleus growth stage or during the development of successive twin generations, and is discussed in terms of interphase boundary mobility.     

Relative grain boundary area and energy distributions in nickel

The three-dimensional interfacial network of grain boundaries in polycrystalline nickel has been characterized using a combination of electron backscatter diffraction mapping and focused ion beam serial sectioning. These data have been used to determine the relative areas of different grain boundary types, categorized on the basis of lattice misorientation and grain boundary plane orientation. Using the geometries of the interfaces at triple lines, relative grain boundary energies have also been determined as a function of lattice misorientation and grain boundary plane orientation. Grain boundaries comprising (1 1 1) planes have, on average, lower energies than other boundaries. Asymmetric tilt grain boundaries with the Σ9 misorientation also have relatively low energies. The grain boundary energies and areas are inversely correlated.     

Stress induced grain boundary motion

We investigated the motion of planar symmetrical and asymmetrical tilt boundaries in high-purity aluminium with <112>- and <111>-tilt axes under the influence of an external mechanical stress field. It was found that the motion of low-angle grain boundaries as well as high-angle grain boundaries can be induced by the imposed external stress. The observed activation enthalpies allow conclusions on the migration mechanism of the grain boundary motion. The motion of planar low- and high-angle grain boundaries under the influence of a mechanical stress field can be attributed to the movement of the grain boundary dislocations which comprise the structure of the boundary. A sharp transition between low-angle grain boundaries and high-angle grain boundaries was observed at 13.6°, which was apparent from a step of the activation enthalpy for the grain boundary motion. For the investigated boundaries the transition angle was independent of tilt axis, impurity content and tilt boundary plane.