强变形过程中铁镍合金的微观结构演化机制

 采用透射电镜观察了铁镍(Fe 32%Ni)合金在形变温度500 ℃(＜05Tm)、形变速率10-2 s-1的变形条件下多轴锻造变形过程中的微观结构演变。结果表明,低温多轴锻造强变形可明显细化晶粒,细化过程为：首先,位错墙、位错缠绕等结构通过大量位错滑移运动在原始晶粒内形成;其次,不同方向的变形导致不同方向的滑移系开动,从而致使不同方向的位错墙互相交叉,将原始粗晶粒细分成小尺寸的胞块结构,当变形量达到一定程度时,位错墙和位错缠绕结构内的位错开始重新排列,形成小角度晶界,导致亚晶粒形成;由于变形量不断增加强迫大量的位错在亚晶界处积聚、重排,同时不同方向的变形造成亚晶发生转动,位错重新规则排列及亚晶转动使小角度的亚晶界转变为大角度晶界,从而形成细小的新晶粒。     

脉冲电流处理对拉伸过程中纯铜位错组态的影响

通过对退火后的纯铜样品在拉伸过程中施加不同强度的脉冲电流,比较纯铜在不同应变速率下的微观结构的演变,借助透射电子显微镜系统观察不同条件下位错组态的演变.研究结果表明脉冲电流处理后纯铜试样的强度和塑性均降低,透射电镜观察分析发现拉伸过程中施加电流仅是引起位错的增殖和攀移,并没有观察到大量位错的湮灭;拉伸过程中位错的交互作用和交滑移形成大量胞状结构和高密度平面扩展界面.     

0Cr21Ni6Mn9N奥氏体不锈钢的应变强化行为

研究了不同氮含量的0Cr21Ni6Mn9N奥氏体不锈钢的塑性流变行为。结果表明,其形变强化特性可用Ludwigson模型来表示。钢在不同的应变下表现出不同的塑性流变行为,存在一个瞬变应变。当应变量低于它时,流变行为与Ludwik方程存在一个正偏差;而应变量高于它时,则符合Ludwik模型。造成这一差异的主要原因是位错滑移模式发生了改变,低于瞬变应变时为单系滑移,高于瞬变应变时为多系滑移。氮对位错滑移模式的影响主要表现为对瞬变应变的影响。随氮含量的增加,瞬变应变被推向更高的水平,这意味着氮原子使位错在更大的应变下才产生多系滑移和交滑移。     

回火索氏体钢轨钢循环应变行为与位错结构关系的研究

本文用增量步进试验方法，研究了具有不同组织参数的回火索氏体共析钢轨钢的循环应力－应变行为。并通过透射电镜对位错结构的分析，探讨了回火索氏体循环软化机制。试验及分析结果表明，在本试验条件下，回火索氏体组织均发生循环软化。循环应变过程中组织中位错间的交互作用形成位错胞。回火索氏体强烈的循环软化主要是由于位错组态再分布，产生胞状亚结构，使内应力降低所致。     

应变速率对高氮奥氏体不锈钢塑性流变行为的影响

研究了室温拉伸时应变速率对高氮奥氏体不锈钢18%Cr-18%Mn-0.65%N力学性能和塑性流变行为的影响。结果表明,随应变速率的升高,试验钢的屈服强度Rp0.2升高,而抗拉强度Rm及塑性略有降低;在各应变速率下,试验钢的塑性流变行为均可以用Ludwigson模型进行描述;应变速率的升高对试验钢流变方程参数的影响如下：1）强度系数K1、应变硬化指数n1和n2减小,试验钢的加工硬化能力降低;2）真实屈服强度TYS降低;3）瞬变应变εL减小,表明升高应变速率能够促进位错多系滑移和交滑移。     

铸态304L奥氏体不锈钢等径角挤压变形研究

 研究了铸态304L奥氏体不锈钢在等径角挤压(ECAP)变形过程中显微组织的演变过程。结果表明,经4道次剪切变形后树枝晶破碎、原始粗大晶粒碎化。显微组织的变化过程可归纳为：原始粗晶粒→晶粒被滑移带分割→位错发展形成高密度位错墙,与滑移带共同作用形成胞块结构→应变增加形成层片状界面→形成大角度晶界的细小晶粒。表明铸态304L奥氏体不锈钢经ECAP变形后塑性变形机制主要由滑移完成。     

冷变形对Inconel 690合金力学行为与组织的影响

 本文研究了Inconel 690合金在冷轧变形过程中的组织演变和形变强化规律。结果表明,在15%和20%之间存在一个临界应变εL,小于临界变形量时,加工硬化能力随着变形量的增加递减,真应力-真应变曲线可用Ludwigson模型描述,位错运动主要是单系滑移,加工硬化主要来自位错的长程应力场。临界应变到40%变形量之间,加工硬化能力随着变形量的增加又增强,真应力-真应变曲线也可用Hollomon方程描述,位错运动出现了多滑移和交滑移,加工硬化主要是位错滑移和林位错交割的短程交互作用。     

10Ni5CrMoV船体钢拉伸性能的高应变速率效应

在1.6×10-3～1.2×103s-1的应变速率范围内,研究了应变速率对10Ni5CrMoV钢室温拉伸性能的影响.结果表明,应变速率对10Ni5CrMoV钢的断裂方式没有影响,拉伸试样均为塑性断裂,断裂微观机制主要是位错滑移;抗拉强度随应变速率增加而略有增加;伸长率随应变速率的增加先下降后上升,但总体无降低;高应变速率并未造成该钢的脆化.根据位错滑移机制对此进行了说明.     

热作工具钢中温脆性的探讨

经不同温度下的拉伸试验,考察了几种热作工具钢的中温脆性。中温脆性随应变速度的下降和晶内强度的提高而趋于严重。本文分别考察了晶内位错运动速度和晶界滑移速度随变形温度的变化,进而证实中温脆性是在某一临界温度下,晶界滑移速度大于晶内位错运动速度,使变形高度集中于晶界而产生。     

铝合金疲劳裂纹形成机制

较详细地研究了LY12CZ铝合金的应变疲劳性能、疲劳裂纹生成区断口特征及疲劳位错组态。在高应力(变),尤其高拉伸脉冲应力条件,表面夹杂物开裂成为疲劳源;在低应变条件,裂纹在滑移线上形成并沿滑移带Ⅰ阶段扩展。提出了疲劳裂纹形成方式转变的唯象理论,解释了平均应力,夹杂物尺寸等因素对转变的影响。并以位错同微观组织相互作用的观点阐明了疲劳裂纹形成的原因。     

An analysis of equilibrium dislocation distributions

Equilibrium distributions of collections of discrete dislocations are analyzed, with the dislocations modelled as line defects in a linear elastic medium. The dislocated equilibrium configuration is determined by finding a minimum potential energy configuration, with respect to variations in the dislocation positions, for a fixed number and type of dislocations. Numerical results are presented for finite and infinite bodies with distributions of edge dislocations under plane strain conditions. Calculations involving doubly periodic arrays of cells, within which there is a single set of parallel slip planes, show a strong tendency for sharp dislocation walls to form. Perturbations of the wall structure due to the presence of pinned dislocations, vacant slip planes and free surfaces are illustrated. The stress fields due to the dislocation walls are calculated and large shear stress values are found away from any dislocation core. Pileups involving dislocations on two sets of intersecting slip planes are found to give rise to equilibrium configurations involving dislocation free regions. The response of dislocation patterns in an infinite medium to an imposed shear stress is also analyzed.     

Fatigue deformation of silver single crystals

The defect microstructure at saturation of silver single crystals subjected to constant plastic strain amplitude, γ_p, fatigue tests in the range γ_p = ±0.0025 to ±0.025 was studied by transmission electron microscopy. The rate of fatigue hardening and the saturation stress increase with increasing plastic strain amplitude. The dislocation microstructure at saturation changes from an open band structure at small plastic strain amplitudes to a closed cell structure at large plastic strain amplitude. The defect microstructure at saturation can be characterized by the density and distribution of point defect clusters and the dislocation wall spacing along the primary Burgers vector direction. The saturation stress is inversely proportional to the dislocation wall spacing along the primary Burgers vector direction and to the spacing of the point defect clusters on the primary slip plane. There is no significant difference between the fatigue hardening, the cyclic stress-strain curve and the dislocation microstructure at saturation of silver and copper single crystals.     

Elastic strain energy analysis of the dislocation structures in fatigue

The dislocation structures which evolve during fatigue have been investigated in terms of the elastic strain energy. Mathematical models for the configuration of dislocation dipoles such as the ladder structure and vein structure are introduced. The elastic strain energy for these models is calculated and evaluated using the eigenstrain method. This analysis may provide insight as to why persistent slip bands in fatigued copper are created after the vein structure reaches a certain stage and why persistent slip bands consist of ladder rungs with an equal spacing of about 1.3 μm at room temperature.     

The orientations of dipolar dislocation structures produced by the cyclic deformation of B.C.C. metals

The orientations of dipolar walls produced by cyclic deformation in a renitrogenized mild steel and in literature observations on b.c.c. metals are shown generally to agree well with the predictions of a geometrical model of the dipolar wall, based on dipole loop stacking and on the sweeping of such loops into the walls by edge dislocations. Ladder-like walls within persistent slip bands are found to be perpendicular to the primary Burgers vector, as predicted for the dislocation activity of dislocations of a single Burgers vector. Literature observations of slip-bands containing rung-like walls not perpendicular to the primary Burgers vector are shown to be compatible with the two slip systems reported. Evidence for the occurrence of mutually perpendicular {100}-{210} and {311}-{110} combinations of labyrinth-like walls is also presented, with each combination predicted for a pair of {110}〈111〉 slip systems, the occurrence of which is compatible with the observations. In contrast to observations previously analyzed in f.c.c. metals, consideration of the cross-slip systems generally does not appear necessary to explain the observed wall orientations. It is suggested that the role of cross slip on influencing the orientations of the dipolar walls produced should depend on the test temperature and strain rate.     

Plastic flow of Co3 Ti single crystals

The flow stress of Co₃Ti single crystals (Ll₂ structure) by compression test was measured as a function of temperature, orientation, chemical composition and strain rate. The critical resolved shear stress (CRSS) showed a rapid increase with decreasing temperature below about 500 K, a remarkable increase with increasing temperature above about 500 K, and then a sharp decrease above the maximum temperature (about 900–1100 K). It was found that in all samples octahedral {111} slip occurs over the entire range of test temperatures. An exceptional slip, i.e. a cube {100} slip was found in sample having orientation axis near [1̄11] and tested above the maximum temperature. The CRSS depended on neither orientation nor strain rate below the minimum temperature but depended on both orientation and strain rate above the minimum temperature. The maximum temperature was dependent of orientation, chemical composition but almost independent of strain rate. It was suggested that the plastic flow of Co₃Ti single crystals below the minimum temperature is responsible for the dislocation movement of the superpartials dissociated on the (111) plane with the SISF. The plastic flow between the minimum and maximum temperatures is due to thermally activated cross-slip from (111) to (001) plane. It was also suggested that the decrease of the CRSS above the maximum temperature is probably due to the intrusion of diffusive process, i.e. unlocking process on cross-slipped (111) dislocation motion.     

Temperature and strain rate dependence of cyclic deformation response and damage accumulation in ofhc copper and 304 stainless steel

The effects of temperature and strain rate on deformation behavior and dislocation structure were investigated for OFHC copper and type 304 stainless steel. It is shown that the cyclic stress response is inversely related to the cell size for copper cycled at different temperatures ranging from -75 to 650°C. Type 304 stainless steel underwent a change from a planar to a wavy slip character as the temperature was changed from room temperature to 760°C. At elevated temperatures, cells were observed and the size of the cells tended to increase with increase in temperature. The effects of temperature on the cyclic stress-strain parameters were investigated for copper, type 304 stainless steel and Ferrovac “E” iron. On studying the effects of temperature and strain rate on the fracture mechanisms it was found that a time dependent fracture mode was dominant at high temperature levels and low strain rates. However, at high strain rates the life was insensitive to temperature. The role of grain boundary migration on the fracture process was investigated. Grain boundary migration was found to be dependent on strain rate for copper. However, for type 304 stainless steel, the grain boundary migration was inhibited at high temperature (760°C) due to the presence of precipitates at the grain boundaries. In strain cycling of OFHC copper and type 304 stainless steel, it was found that the addition of creep-type damage to fatigue damage resulted in a total damage which was not equal to unity for failure when these different modes were imposed sequentially. The sense of the damage accumulation appeared to have no effect on this summation.     

Ductile fracture initiation in pure α-Fe: Part I. Macroscopic observations of the deformation history and failure of crystals

Single crystalα-Fe whiskers, grown by the reduction of ferrous chloride by hydrogen have been strained to fracture in an Instron tensile testing machine and in a bench straining device at various elongation rates at room temperature. Whiskers were found to exhibit macroscopic slip behavior strongly dependent upon elongation rate while the geometric reduction in area and the fracture mode remained in all cases identical. Ductile rupture of iron whiskers produces a characteristically shaped chisel-edge fracture whose geometry is sensitive to crystal orientation, due to the geometry of active slip systems, but which isnot a function of strain rate. The micromechanisms of ductile rupture of these single crystals are strongly affected by dislocation dynamics. The development of dislocations necessary to accomodate an extensive reduction in area appears to be independent of the nature of surface slip observed. Dislocation structures form small volume elements which are separated from one another by dislocation cell walls. The accommodation of large strains as well as the reduction in area is determined by the movement of dislocations on the order of a distance equal to that of the dislocation cell size. The boundaries of the cell and/or the cell volume could then be expected to be specifically related to the site where the initiation of fracture occurs.     

Stacking fault energy and mechanical properties

Several deformation mechanisms are discussed which may be influenced by the stacking fault energy: forest cutting at the beginning of plastic deformation in fcc and hex metals, cross slip in fee metals, prismatic slip in hexagonal metals, and slip in bee metals. Existing experimental data for the temperature and strain rate dependence of the shear stress at which the relevant deformation mechanism begins are analyzed. It is shown that the data for cross slip in fee metals, for prismatic slip, and for slip in bee metals can be described by an Arrhenius equation with an activation energy that is proportional to the logarithm of the stress. Stacking fault energies are determined from the strain rate sensitivity of the shear stress.