应变速率对18Cr-12Mn-0.55N高氮奥氏体不锈钢塑性流变行为的影响

研究了拉伸应变速率对高氮奥氏体不锈钢18Cr-12Mn-0.55N(质量分数/%)室温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,实验钢的屈服强度R0.2增大,断后延伸率A减小,抗拉强度Rm略有降低,断面收缩率Z变化不大;在各应变速率下,实验钢的塑性流变行为均可用Ludwigson模型进行描述;随应变速率的升高,实验钢的加工硬化能力和发生屈服时第一根位错开动所需的短程作用力降低;增大应变速率促进多系滑移和交滑移,降低瞬变应变,使实验钢的塑性流变行为在更低的应变水平符合Ludwik模型.     

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。     

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。     

单晶铜塑性变形的二维离散位错动力学模拟研究

针对亚微米尺度晶体元器件在加工和服役中出现的反常力学行为和动态变形等问题,基于离散位错动力学理论建立了单晶铜塑性变形过程的二维离散位错动力学模型。该模型考虑外加载荷、位错间相互力和自由表面镜像力对位错的作用机制,引入了截断位错速度准则。与微压缩实验对比验证了模型的正确性,并且能够描述力加载描述的位错雪崩现象。应用该模型分析了不同加载方式和应变率下位错演化及力学行为,结果表明:当外部约束为力加载和位移加载时,应力应变曲线分别呈现出台阶状的应变突增和锯齿状的应力陡降,位错雪崩效应的内在机制则分别归结为位错速度的随机性和位错源开动的间歇性;应变率在102～4×104 s?1范围内,单晶铜屈服应力的应变率敏感性发生改变,位错演化特征由单滑移转变为多滑移面激活的均匀变形,位错增殖逐渐代替位错源激活作为流动应力的主导机制。      

耦合位错密度的6111铝合金热变形本构模型

利用Gleeble-1500热模拟试验机对6111铝合金进行高温拉伸试验,研究了其在变形温度为350、450和550℃以及应变速率为0.1、1和10 s-1时的热变形行为.6111铝合金的流变应力随温度升高而减小,随应变速率增大而增大,其热变形从应变硬化阶段过渡到稳态变形阶段.建立了综合考虑应变、温度和应变速率对流变应力的影响以及耦合位错密度的统一黏塑性本构模型,并通过遗传优化算法求解出本构模型中的材料常数.模型计算得到的真应力-真应变曲线与试验数据吻合较好.      

Si-Mn系相变诱导塑性钢热轧过程中组织演变的预测

 建立了热轧相变诱导塑性(TRIP)钢奥氏体动态和静态再结晶、晶粒尺寸和流变应力预测模型,定量分析了不同硅含量对其再结晶的影响。结果表明,预测值与实测值符合较好;硅含量增加可抑制、减缓动态和静态再结晶发生,抑制奥氏体晶粒长大,提高流变应力、残余应变和位错密度,从而有利于铁素体相变的发生。     

预变形条件下纳观裂纹扩展的微观机制研究

采用晶体相场方法研究预变形条件下样品在单轴拉应变作用下的纳观裂纹扩展行为。通过观察裂纹演化,分析裂纹扩展过程的体系自由能变化、应力-应变曲线、裂纹周长曲线和裂纹面积分数曲线,发现:无预变形的样品,在拉应变作用下,当应变量达到临界值时,在裂口附近萌生出位错,使得应变集中的应变能得到释放。虽然位错伴随着裂尖扩展,没有观察到裂尖发射位错现象,裂纹呈解理扩展模式。预剪切变形为1%和2%的样品,在裂纹扩展的过程,左右裂尖各存在一个位错对,这两个位错沿着各自的滑移面交替滑移,使得裂尖沿着两个滑移方向交替扩展,裂纹呈锯齿状。预剪切变形为3%的样品,在裂口处发射位错,在变形滑移带上诱发生成孤立的空穴串,随后发展成空洞并连通成为主裂纹。在主裂纹之外,还出现空洞萌生二次裂纹,整个裂纹扩展呈韧性扩展模式。预剪切变形量的增加,有助于裂纹由脆性扩展转化成韧性扩展。     

温度和应变速率对FeCrNi合金位错组态的影响

 利用透射电镜观察了不同温度及不同应变速率下FeCrNi合金的位错组态,分析了位错组态与温度及应变速率的关系。结果表明：温度使交滑移频率提高,位错组态从位错墙向位错胞方向发展,而应变速率升高使交滑移频率降低,位错组态从位错胞向位错墙、高密度层错方向发展。     

冷变形对0Cr25Ni35AlTi组织与力学行为的影响

采用室温拉伸和硬度测试研究了不同冷变形量对0Cr25Ni35AlTi室温力学性能和硬度的影响。通过OM、TEM对冷变形后的组织进行观察,分析不同冷变形后力学性能的变化机制。结果表明,随着变形量的增加,合金的抗拉强度、屈服强度和硬度增加。当变形量为20%时,合金的屈服强度提高了1.5倍,抗拉强度提高了1.2倍,分别达到了679和762 MPa。合金加工硬化指数随着冷变形量的增加而减小。在10%和15%形变量之间存在一个临界值,小于临界值时,位错运动主要是单滑移,真应力-真应变曲线可用Ludwigson模型描述;大于临界变形量,位错运动出现了多滑移和交滑移,真应力-真应变曲线可以用Hollomon方程描述。     

碳含量对淬火碳钢温变形流变行为的影响

目前关于变形温度、应变速率等因素对流变应力的影响有大量的研究报道,而对温变形条件下碳含量的增加对流变应力的影响规律还缺乏系统研究.基于此开展起始组织为马氏体的不同淬火碳钢温变形流变行为研究.结果表明,在较低温度和较高应变速率(600℃,0.1～1s-1)下,流变应力随碳含量的增加而增大;在较高温度和较低应变速率下(700℃,0.01～0.001 s-1),流变应力在高碳范围内呈下降趋势.碳的质量分数低于0.78％时,淬火碳钢温变形激活能随碳含量的增加而降低;碳的质量分数高于0.78％时,淬火碳钢温变形激活能随碳含量的增加而增加.在相同变形条件下,能量耗散效率最大值随着碳含量的增加呈增大趋势.     

Hydrogen induced ductility losses in austenitic stainless steel welds

The effect of hydrogen on the tensile behavior of austenitic stainless steel welds was studied in two AISI 300 series alloys and two nitrogen strengthened alloys. The microstructure of these welds typically contained several percent ferrite in an austenite matrix. Hydrogen was found to reduce the ductility of all welds; however, the severity of ductility loss increased with increasing tendency to deform via a planar slip mode. In materials exhibiting large degrees of slip planarity, 304L and 308L, hydrogen changed the fracture mode from dimple rupture to a mixed mode of ductile and brittle fracture associated with the austenite-ferrite interface. The two alloys, 22-13-5 and 309S, which tend to deform by cross slip mechanisms, showed smaller losses in ductility even though hydrogen assisted the ductile rupture process by aiding void growth and coalescence, without changing the fracture mode. Varying the amount of ferrite from approximately one to 10 pct had no significant effect on performance in hydrogen.     

Impact Response and Microstructural Evolution of 316L Stainless Steel under Ambient and Elevated Temperature Conditions

The impact response and microstructural evolution of 316L stainless steel are examined at strain rates ranging from 1?×?10³ to 5?×?10³?s^?1 and temperatures between 298?K and 1073?K (25?°C and 800?°C) using a split Hopkinson pressure bar and transmission electron microscopy (TEM). The results show that the flow behavior, mechanical strength, and work-hardening properties of 316L stainless steel are significantly dependent on the strain rate and temperature. The TEM observations reveal that the dislocation density increases with increasing strain rate but decreases with increasing temperature. Moreover, twinning occurs only in the specimens deformed at 298?K (25?°C), which suggests that the threshold stress for twinning is higher than that for slip under impact loading. Finally, it is found that the volume fraction of transformed ???? martensite increases with increasing strain rate or decreasing temperature. Overall, the results suggest that the increased flow stress observed in 316L stainless steel under higher strain rates and lower temperatures is determined by the combined effects of dislocation multiplication, twin nucleation and growth, and martensite transformation.     

高氮不锈轴承钢碳化物分布与高温断裂机制

 通过高温拉伸试验研究高氮不锈轴承钢高温断裂行为,探究了170 ℃和470 ℃回火态钢中碳化物分布特征,分析了高温拉伸断裂及组织演变和碳化物分布规律。研究发现,回火温度从170 ℃升高至470 ℃,高氮钢中大于0.8 μm的碳化物明显增加,高氮钢中M₂₃C₆强化增量提高了2.59 MPa,固溶强化增量下降了118.82 MPa,470 ℃回火态钢的室温抗拉强度降低、拉伸断口表现为准解理和少量撕裂韧窝;拉伸温度升高至300 ℃,试样断口表现为等轴型韧窝特征,170 ℃和470 ℃回火态试样起裂源断裂碳化物尺寸分别为2.8~3.6 μm和5.5~6.7 μm;450 ℃拉伸断口表现为塑孔韧窝特征,170 ℃和470 ℃回火态试样起裂源断裂碳化物尺寸分别为2.7~3.4 μm和5.8~6.4 μm。拉伸温度从300 ℃提高至450 ℃,钢的固溶强化和位错强化作用减弱,金属原子间结合能下降,碳化物与基体不连续应力分布加剧变形不协调性,碳化物承担较高应力而发生断裂。单纯热作用下钢中0.5~0.8 μm尺寸碳化物数量比例增加;在热力耦合作用下,钢中应力所导致的位错增殖为碳元素扩散提供通道,钢中碳化物在晶界和位错线上形核析出0.2~0.8 μm碳化物。裂纹沿着与拉伸方向45°角的最大剪力方向快速扩展而断裂,最终形成锯齿状的断口,小尺寸碳化物增多阻碍位错滑移导致塑性降低;钢中大尺寸碳化物不均匀分布在碳化物间形成大变形塑孔而增加钢的塑性。     

Low Cycle Fatigue Behavior of 316LN Stainless Steel Alloyed with Varying Nitrogen Content. Part I: Cyclic Deformation Behavior

In this study, the influence of cyclic strain amplitude on the evolution of cyclic stress–strain response and the associated cyclic deformation mechanisms in 316LN stainless steel with varying nitrogen content (0.07 to 0.22 wt pct) is reported in the temperature range 773 K to 873 K (500 °C to 600 °C). Two mechanisms, namely dynamic strain aging and secondary cyclic hardening, are found to strongly influence the cyclic stress response. Deformation substructures associated with both the mechanisms showed planar mode of deformation. These mechanisms are observed to be operative over certain combinations of temperature and strain amplitude. For strain amplitudes >0.6 pct, wavy or mixed mode of deformation is noticed to suppress both the mechanisms. Cyclic stress–strain curves revealed both single and dual-slope behavior depending on the test temperature. Increase in nitrogen content is found to increase the tendency toward planar mode of deformation, while increase in strain amplitude leads to transition from planar slip bands to dislocation cell/wall structure formation, irrespective of the nitrogen content in 316LN stainless steel.     

Influences of cell walls and grain boundaries on transient responses of an if steel to changes in strain path

The effects of changes in strain path on plastic behaviour in sheets of an interstitial-free steel with two widely different grain sizes were investigated. The sheets were prestrained in rolling and, apart from supplementary tests, they were tested in uniaxial tension at 90° to the rolling direction. The results support the following conclusions. The magnitude of the increase in reloading yield stress and amplitude of the subsequent reduction in work hardening depend on the strength of dislocation walls generated in the prestrain rather than the grain size. The walls are more effective barriers to dislocation glide in freshly activated slip systems than to glide in the original slip systems operating in the prestrain. The primary cause of the subsequent reduction in hardening rate is disruption and partial dissolution of the original dislocation substructure. The final recovery in hardening rate is caused by generation of a new substructure compatible with the new deformation mode.     

Cyclic response-electrochemical interaction in mono- and polycrystalline AISI 316L stainless steel in H2SO4 solution—II. Potential dependence of the transient dissolution behavior during corrosion fatigue

The transient current behaviors of an AISI 316L stainless steel, in both mono- and polycrystalline forms, have been studied in corrosion fatigue tests performed in a 1N H₂SO₄ solution, under different potentials. Extreme care in experimental technique and instrumentation have disclosed minute current variations which provide information about mechanisms. The effects of both the cyclic process and the applied potentials on the current behavior have been investigated, in following up a study of the influence of strain on the transient current behavior (Part I). Selective dissolution of iron from slip steps has been demonstrated to have a major influence on the transient current, during the early stage and the cracking stage of cycling, and does not depend on applied potentials. A significant potential dependence of the transient current was found, however, in the stage of stable current (saturation), which is correlated with different electrochemical reactions in the slip bands. Corrosion fatigue lives were measured and analyzed in relation to the current behavior.     

Transient cyclic stress-strain response and cumulative damage in Cu-16at.% A1 single crystals fatigued under variable straining

In order to investigate the transient cyclic stress-strain response and cumulative damage behavior of Cu-16at.% A1 alloy, single crystals with a common single slip orientation were cyclically deformed under variable straining. Studies were made of hardening behavior, strain bursts, hysteresis loops, friction and back stresses, dislocation structures, damage accumulation and life behavior. During ascending step tests, hardening naturally occurred and the strain was found to be accommodated not by increasing the plastic strain of current active slip bands but by increasing the volume fraction of active slip bands. During descending step tests, however, the plastic strain per individual slip band decreases with decrease of strain amplitude. The transient response after a reduction of strain amplitude in Cu-16at.% Al single crystals is not softening as commonly occurs but hardening. Hardening in this case seems to be caused both by hardening of individual slip bands and by the decrease of the volume fraction of active slip bands, which is also an indication of hardening. With regard to damage behavior, in low-high step tests, the number of cracks was found to increase remarkably during high amplitude cycling whereas the number of cracks did not change much after the step to low amplitude cycling during a high-low test. Miner summations of fatigue life for H-L step tests were found to be smaller than unity because the stresses associated with the structure produced at high amplitude were high and the plastic strain localized in active slip bands increased with the decrease of the volume fraction of active slip bands, caused by continuous cycling at the reduced strain amplitude. In L-H step tests, the Miner summations for fatigue life were found to be greater than unity because the hardening and the damage accumulation associated with low amplitude cycling were relatively insignificant as compared to that at high amplitude and fresh damage developed during the high amplitude sequence. The Miner summation behavior in these alloy single crystals was found to be more typical of that in commercial metals, and different from behavior in copper.     

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。     

Influence of interstitial carbon,nitrogen, and hydrogen on the plasticity and brittleness of steel

The introduction of carbon, nitrogen, and hydrogen in steel is analyzed in terms of the electron structure, dislocation properties, hardening, and failure of the steel. The similarity and differences in the mechanical properties of the corresponding solution solutions are discussed in relation to the influence of these elements on the density of electron states at the Fermi level of iron and correspondingly on the concentration of free electrons. Carbon reduces the concentration of free electrons, while nitrogen and hydrogen have the opposite effect. Hence, the atomic interaction is changed: specifically, its covalent or metallic component will be intensified. The dislocation rate in deformation is analyzed in the approximation of mobile and immobile interstitial atoms. In the first case, the interstitial atoms obstruct dislocational slip; the mobility of the dislocations is determined by the binding enthalpy of the dislocations with impurity atoms. If the interstitial atoms may accompany dislocations, the atomic bond is locally changed in dislocational atmospheres. That affects the unit energy of the dislocations and the distance between them in the slip planes. On the basis of the research results, the significant similarity between the hydrogen brittleness of austenitic steel and the ductile–brittle transition in alloying with nitrogen is explained.