Effect of Temper Condition on Stress Relaxation Behavior of an Aluminum Copper Lithium Alloy

Deformation behavior of an Al-Cu-Li alloy in different temper conditions (solutionized and T8) is investigated using stress relaxation tests. Fundamental parameters such as the apparent and physical activation volume, strain rate sensitivity, effective stress, and exhaustion rate of mobile dislocation density are determined from single and multiple relaxation tests. It was found that dislocation–dislocation interaction controls the kinetics of plastic deformation in the solutionized sample, whereas dislocation–precipitate interaction is the overriding factor in the presence of T₁ precipitates. The apparent activation volume was found to be significantly lower in the presence of T₁ precipitates compared with solutionized samples. Strain rate sensitivity and effective stress were found to be higher in the presence of T₁ precipitates. In addition, multiple relaxation tests showed that irrespective of microstructural features (solutes, semi-coherent precipitates), the mobile dislocation density reduces during the relaxation period. Further evidence regarding reduction in mobile dislocation density is obtained from uniaxial tensile tests carried out after stress relaxation tests, where both solutionized and T8 samples show an increase in strength. Additional discussion on relaxation strain is included to provide a complete overview regarding the time-dependent deformation behavior of the Al-Cu-Li alloy in different temper conditions.     

A Revised Criterion for the Portevin–Le Chatelier Effect Based on the Strain-Rate Sensitivity of the Work-Hardening Rate

An improved analysis is presented of the stability of plastic deformation under conditions where dynamic strain aging (DSA) occurs, which leads to instabilities known as the Portevin–Le Chatelier (PLC) effect. It is shown that PLC instabilities can occur for conditions that are not covered by the currently prevailing criterion presented by Estrin and Kubin (1991), which focuses on a negative strain rate sensitivity of the flow stress, caused by interactions of solutes with thermally activated glide of mobile dislocations. The current analysis recognizes that the strain-rate sensitivity of the flow stress consists of two contributions, one associated with glide of mobile dislocations and the second with work hardening, related to storage of immobile dislocations. In this paper, an instability criterion is proposed that takes into account the possibility of a negative strain-rate sensitivity of the work-hardening rate, which is caused by diffusion of solutes to immobile dislocations. The latter contribution leads to an extended instability criterion. This criterion also provides an explanation for the existence of a critical strain above which instabilities occur. In this article, previously published tensile test data are used to show that a negative strain-rate sensitivity of the work-hardening rate, which influences significantly the occurrence of the PLC effect, can indeed occur under DSA conditions.     

The separate roles of subgrains and forest dislocations in the isotropic hardening of type 304 stainless steel

Tests on 304 stainless steel were conducted involving first warm working in torsion, then cold working in torsion, and finally measurement of the elevated-temperature yield strength in compression. These tests permitted separation of the effects of subgrain size and forest dislocation density on the isotropic part of the flow stress. Forest dislocation strengthening appears to dominate in this material. The results are best fitted by a root-mean-square summation of strength terms representing the contributions of solutes, forest dislocations, and subgrain boundaries. The same equation successfully predicts the flow stress during elevated-temperature transient deformation (under both constant strain rate and variable strain rate) from the transient dislocation substructure. formerly Research Assistant, Department of Materials Science and Engineering, Stanford University.     

Anomalous strain rate dependence of the serrated flow in NiH and NiCH alloys

Serrated flow in NiH, NiCH, and NiC alloys was studied over a wide range of temperature and strain rates. The results for C related serrated flow in NiC or NiCH alloys were in excellent agreement with previous results and were consistent with dislocation pinning by C solutes at the dislocation cores. Hydrogen related serrated yielding was observed in NiH and NiHC alloys. Solute C had only a small effect on the temperature range of this H related serrated flow. The results could be interpreted on the basis of hydride formation at the dislocation cores and diffusion of H in these hydrides.     

Dislocation dynamics in strain aging alloys

The problem of dynamic strain aging is treated using a new approach to the aging kinetics. The moving dislocations are considered as an ensemble of double loops which overcome the forest dislocations by thermal activation. This implies an exponential distribution of waiting times in front of the obstacles during which additional pinning agents arrive at the dislocation. The temporal development of the mean number of pinning points per loop can be calculated from an appropriate master equation which is based on the dynamic transition rates for pinning and depinning of the dislocation segments. It is shown that the two pinning mechanisms, lineal pinning and forest strengthening discussed in the literature, can be treated in a unified way. This leads to the definition of a single parameter, the mobility factor, which completely describes the effect of dynamic aging on dislocation velocity and plastic strain rate. The role of local instabilities and the problem of strain rate sensitivity are discussed.     

Impulse Excitation Internal Friction Study of Dislocation and Point Defect Interactions in Ultra-Low Carbon Bake-Hardenable Steel

The simultaneous presence of interstitial solutes and dislocations in an ultra-low carbon bake-hardenable steel gives rise to two characteristic peaks in the internal friction (IF) spectrum: the dislocation-enhanced Snoek peak and the Snoek–Kê–Köster peak. These IF peaks were used to study the dislocation structure developed by the pre-straining and the static strain aging effect of C during the bake-hardening process. A Ti-stabilized interstitial-free steel was used to ascertain the absence of a γ-peak in the IF spectrum of the deformed ultra-low carbon steel. The analysis of the IF data shows clearly that the bake-hardening effect in ultra-low carbon steel is entirely due to atmosphere formation, with the dislocation segment length being the main parameter affecting the IF peak amplitude. Recovery annealing experiments showed that the rearrangement of the dislocation structure lead to the elimination of the C atmosphere.     

The effect of dislocation drag on the stress-strain behavior of F.C.C. metals

The introduction of viscous drag into a simple thermally activated dislocation model for the low temperature plastic deformation of f.c.c. metals leads to some surprising predictions about their stress-strain behavior. One effect of dislocation drag is to produce a region of tensile instability at small strains for any strain rate. At sufficiently high strain rates there is no region of tensile stability. However, computation of a decreased strain for tensile instability at strain rates greater than 10³ s⁻¹, in opposition to experimental measurements, provides evidence that the strong upturn reported for the flow stress of copper is not caused by dislocation drag. The likely explanation is an enhanced rate of dislocation generation.     

Cold work hardening in stages IV and V of F.C.C. metals—I. Experiments and interpretation

The paper presents numerous measurements which confirm stages IV and V to be general ranges of cold work deformation. Analogous to stage II, stage IV exhibits a linear athermal hardening with constant strain rate sensitivity and activation enthalpy. In stage IV the dislocation cell size is constant, while the dislocation density growth rate is markedly reduced compared with stages II and III. Features of stage V are analogous to stage III, the increase of strain rate sensitivity (decrease of activation enthalpy) indicating the onset of thermally activated dislocation annihilation. In stage V, the mechanism is identified as dislocation climb from observing subgrain formation and saturation in density of deformation induced vacancies. Comparisons with recent investigations of stage IV and V at high temperatures suggest a common picture of low and high temperature deformation which only requires principles of storage and annihilation for both screw and edge dislocations.     

Investigation of Portevin–Le Chatelier Effect in Al-2.5 pct Mg Alloy with Different Microstructure

An experimental study is presented on the effect of microstructure change on the Portevin–Le Chatelier (PLC) effect of Al-2.5 pct Mg alloy. Tensile tests are performed on as-received and heat-treated [at 673 K (400 °C) for 16 hours] samples for a wide range of strain rates. The serrations observed in the stress–time curves are investigated on statistical analysis point of view. The microstructures of the samples are characterized by optical metallography, X-ray diffraction, and electrical resistivity measurements. It is found that the excess vacancies generated due to heat treatment have an effect on the PLC phenomenon, which leads to a decrease in the strain rate sensitivity and an increase in the number of stress drop occurrences per unit time. The microstructural parameters like domain size and dislocation density have no appreciable effect on the PLC effect as far as the statistical behaviors of the serrations are considered.     

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。      

低温强变形奥氏体的马氏体相变

 研究了Fe 32%Ni合金在形变温度550 ℃、形变速率2×10-2 s-1的实验条件下,经过多道多向锻压变形后奥氏体的马氏体相变过程。采用电阻法、XRD分别研究了累积应变量对Ms点以及马氏体生成量的影响;采用TEM对形变位错密度以及马氏体组织进行了分析。结果表明：随着累积应变量的增大,奥氏体组织不断细化,深冷处理后马氏体生成量逐渐减少,Ms点逐渐降低,最终趋于恒定。原因是奥氏体的细晶强化以及形变位错的引入导致母相加工硬化抑制了马氏体形核,随着形变的累积,奥氏体的晶粒细化效果逐渐减弱,使得马氏体生成量减少的趋势也逐渐减弱,最终趋于恒定。     

Structural Superplasticity at High Strain Rates of Super Duplex Stainless Steel Fe‐25Cr‐7Ni‐3Mo‐0.3N

The fine‐grained super duplex stainless steel Fe‐25Cr‐7Ni‐3Mo‐0.3N consisting of two phases (δ‐ferrite/austenite) exhibits structural super‐plasticity at higher strain rates of ? ≈ 10^?2s^?1 in the temperature range between 975 and 1100°C. The equiaxed microstructure with an average grain size of was produced by thermomechanical processing. Maximum strain‐rate‐sensitivity exponents of m ≈ 0.5 and elongations to failure of more than 500% were achieved. From thermal activation analysis an activation energy for superplastic flow of Q = 310 ± 20 kJ/mole was derived. The superplastic behaviour at higher strain rates is quantitatively described by a deformation model where grain or interphase boundary sliding is accommodated by sequential steps of dislocation glide and climb. The high strain‐rate‐sensitivity exponent and the observed dislocation density indicate that dislocation climb in the slightly solid solution strengthened austenite is the rate controlling step for superplastic flow. The deformation mechanism reveals that the investigated super duplex stainless steel exhibits superplastic behaviour that is typical for class II solid solution alloys.     

The effect of temperature on strain rate sensitivity in an AlMgSi alloy

The effect of temperature on strain rate sensitivity has been studied in an AlMgSi alloy in the range of dynamic strain ageing. Two regimes of behaviour were observed which depended on the temperature. At low temperatures (T <- 303 K) it was possible to correlate all strain rate sensitivity measurements with a single strain/strain rate/temperature parameter. In the high temperature regime the temperature and strain rate dependence of the strain rate sensitivity deviated significantly from that measured at low temperatures. The two regimes of behaviour were found to correspond with the normal and inverse Portevin-Le Chatelier effect.     

Superplasticity of Directionally Solidified NiAl-15Cr Alloy at High Temperature

Ni-25Al-15Cr (atomic percent, %) alloy was directionally solidified (DS) under argon atmosphere in an Al2O3-SiO2 ceramic mold by standard Bridgman method. The microstructure of the as-fabricated alloy was studied using optical microscope, X-ray diffractometer, and scanning electron microscope. The alloy consisting of dendritic β-NiAl phase, interdendritic γ/γ′ phase, and transient layer γ′ phase, has been investigated. This alloy exhibits superplastic deformation behavior at 1 273-1 373 K over an initial strain rate range of 8.35×10-4-1.67×10-2 s-1. The maximum elongation of 280% with strain rate sensitivity index m=0.22 was obtained at the temperature of 1 323 K and an initial strain rate of 8.35×10-3 s-1. Transmission electron microscopy (TEM) observations indicate that the superplastic deformation stems from the balance between high resistance (by dislocation sliding) and strain softening (by dynamic recovery and recrystallization).     

Dynamic strain aging effects in niobium microalloyed steel

Abstract

The dynamic strain aging behaviour of a niobium microalloyed steel has been examined. Hot tensile testing was carried out on heat treated and as received specimens. Heat treated specimens were austenitised at 1000°C for 1 h, and then cooled in air or in a stainless steel cylinder to obtain various amounts of free or uncombined interstitial solutes in solid solution, to examine the effect on the dynamic strain aging behaviour of the steel. It was found that dynamic strain aging takes place in niobium microalloyed steel during tensile testing at temperatures ranging from ambient temperature to 450°C at a crosshead displacement rate of 2 mm min^-1. As a result, the ultimate tensile strength and initial work hardening rate exhibit maximum values at temperatures between 200 and 350°C. Also, load-extension graphs for tested specimens show serrated behaviour and yield points at 200, 250, and 300°C. It is believed that dynamic strain aging in niobium steel is caused by interaction between dislocations and interstitial solutes (nitrogen and carbon) or solute pairs consisting of one interstitial and one substitutional solute atom (for example Mn-C and Mn-N).     

Serrated yielding in AISI 316 stainless steel

Serrated yielding behaviour of a type AISI 316 stainless steel is investigated in the temperature range 300–923 K. Detailed observations have been made on the effect of prior ageing in the temperature range (823–1323 K) for different periods of time on the serrated yielding behaviour at 923 K. Two temperature regions with different activation energies have been identified for serrated flow. The mechanism for serrated flow in the low temperature region (∼ 523–623 K) is the diffusion of interstitial solute to dislocation while substitutional solutes like Cr is responsible for serrated yielding at high temperature region. Certain time-temperature combination of ageing is found to eliminate serrated yielding at 923 K. Results also support the role of grain boundaries as preferred sites for dynamic strain ageing in austenitic stainless steels as previously suggested.     

经验原子势下铝镁合金中溶质原子向位错芯迁移的最低能量路径

铝镁合金在制造业中应用广泛, 但其在特定应变率下的塑性失稳不利于加工应用. 溶质原子与位错的交互作用是塑性失稳的微观机理. 本文采用势能曲面过渡态搜索技术计算了铝镁合金中替代型溶质镁原子向位错芯迁移的过渡态, 确认了溶质原子与位错芯的交互作用范围, 并采用过渡态理论估算了迁移扩散所需的时间, 且区分了无空位及有空位参与迁移两种情况. 结果表明, 位错压应力区内的溶质原子迁移无明显规律, 而在位错拉应力区内, 随着溶质原子与位错间距的缩短, 迁移势能垒和系统总能量均逐渐降低. 说明目前广泛采用的经验原子势可以很好地反映溶质原子易朝位错拉应力区偏聚这一现象. 溶质原子迁移的过渡态证实迁移过程中的微观结构变化因溶质原子所处位置不同而各异, 而交互作用范围不超过约2 nm. 空位参与对迁移的辅助作用被量化为迁移热激活时间的缩短, 并得出其可在微秒量级. 当溶质原子完成迁移稳定至位错芯附近, 并不倾向于沿位错线密集分布.      

The portevin-le chatelier effect in carburized nickel alloys

From a study of serrated yielding (the Portevin-Le Chatelier Effect) in carburized nickel, it is shown that the expected diffusion coefficient does not fit the approximate Cottrell condition relating strain rate (?e), mobile dislocation density (p), and diffusion coefficient(D). Since serrations begin immediately after yielding, vacancy-enhanced diffusion caused by plastic strain is not responsible for this. Generally, it is proposed that carbide stability controls the temperature at which serrated yielding disappears while carbon diffusion may play a part in the complex process responsible for the appearance of serrated yielding. Cobalt additions to carburized nickel quickly attenuate the effect while copper additions at first accentuate the effect and then remove it. The driving force for carbon to the dislocations responsible for the mobile solute-mobile dislocation interactions is most probably due to the elastic misfit of the carbon atom in solution.     

多向高速冲击下LZ91镁合金的力学性能及组织

常温下采用霍普金森压杆在冲击气压为0.2MPa～0.7MPa的条件下对LZ91镁合金进行多向高速冲击,结合变形后的微观组织及应力应变曲线,分析应变速率和不同冲击方向对微观组织演变的影响.结果表明,冲击过程中β-Li和α-Mg的滑板效应和形变温升产生的软化作用和加工硬化存在竞争关系,导致随着应变速率增加和材料初始组织中位...