Plastic flow instabilities of L12 Co3Ti alloys at intermediate temperatures

The serrated flow of L1₂ Co₃Ti alloys was investigated by tensile tests in terms of the effects of temperature, strain rate, alloy stoichiometry and cooling rate after annealing. The serrated flow was most strongly observed at 673 K and at a strain rate of 3.2×10⁻⁴ s⁻¹. Correspondingly, the maximum stress amplitude and the lowest (negative) strain-rate sensitivity were observed at 673 K. The serrated flow was more significant as the alloy departs from a stoichiometric composition, and also less significant in a water-quenched sample than in a furnace-cooled sample. The static aging at 673 K resulted in a reduced flow stress. The activation energy of the serrated flow was estimated to be about 84 kJ/mol and almost one-half of that of lattice diffusion of the excess Co atoms. The serrated-flow behavior is explained qualitatively on the basis of consideration of the dynamics of the dislocation core structure, and is also discussed in association with the strength anomaly.     

Flow Behavior and Mechanical Properties of a High Silicon Steel Associated with Dynamic Strain Aging

Flow behavior of two grades of steel including a high silicon (HS) steel and a plain low carbon steel as the reference were considered in this work. Tensile testing at temperatures varying between 25 and 550?°C and different strain rates in the range of 4?×?10^?5 to 0.1?s^?1 were conducted and the mechanical properties, such as elongation at fracture point and strain rate sensitivity were then determined. It is observed that for both steels, dynamic strain aging occurs in the employed deformation conditions, however, the region of serrated flow and the type of the serration were somehow different. For the case of the HS steel, the serrated flow region is shifted to the higher temperatures and also, the activation energy for appearance of dynamic strain aging increases as well.     

DYNAMIC STRAIN AGING BEHAVIOR OF K40S ALLOY

The dynamic strain aging behavior during tensile tests of K40S alloy has been investigated in the temperature range of 25-1100℃ with the strain rate range from 10-4 to 10-3s-1. The results show that four different types of serration, identified as A, B, C and E type serration were observed in the temperature range of 300-600℃. The strain exponents for onset of the serrated flow were calculated as 1.21, 2.19 and 1.61, and the activation energies as 121, 40 and 67kJ/mol for E, B and C type serration respectively. The main mechanism for dynamic strain aging discussed in light of the strain exponent and the activation energy.     

LA41镁合金的PLC效应及其解释

通过单向拉伸实验在LA41镁合金中观察到了PLC(Portevin—Le Chatelier)效应,在整个实验的应变速率范围(3．33×10~(-4)—6．66×10~(-3)s~(-1))内这种塑性不稳定性始终存在,合金的抗拉强度和屈服强度随应变速率的增加而减小,其应变速率敏感系数始终为负值,且该不稳定性总是在一个临界应变之后出现,基于以上特征,运用动态应变时效(dynamic strain aging,DSA)机制,即形变过程中溶质原子与可动位错的交互作用,解释了所观察到的力学现象。     

AL6XN超级奥氏体钢高温拉伸时的动态应变时效

研究了固溶处理AL6XN超级奥氏体钢在温度773～973K以及3.3×10-5～3.3×10-3s-1应变速率范围的高温拉伸力学行为。结果表明,AL6XN奥氏体钢在以上实验条件下(不包括873K,3.3×10-5s-1)均存在动态应变时效,具体表现为在真应力-真应变曲线上出现锯齿形,同时屈服强度在773～973K范围呈现平台区;临界应变量随着温度从773K升高到923K逐渐降低,并且在此温度范围激活能为304kJ/mol,说明动态应变时效的机制为AL6XN奥氏体钢中Cr和Mo等置换原子与位错的交互作用。透射电镜分析同时表明AL6XN奥氏体钢形变后位错结构呈平面滑移特征。     

Atomistic study of pipe diffusion in Al–Mg alloys

Solute diffusion in an Al-rich binary Al–Mg alloy is studied by means of atomistic simulations. The activation energy for diffusion of Mg in the bulk is evaluated in the dilute solution limit for the nearest neighbor and the ring mechanisms. It is concluded that bulk diffusion at low and moderate temperatures must be assisted by vacancies. Further, diffusion of Mg along the core of edge, 60° and screw dislocations is studied. The activation energy for vacancy formation in the core and for vacancy-assisted Mg migration is evaluated for a large number of diffusion paths in the core region. It is observed that, similar to the bulk, Mg diffusion in absence of vacancies is energetically prohibitive. The paths of minimum activation energy are identified for vacancy-assisted diffusion, for all three types of dislocations. The lowest energy path is found in the core of the 60° dislocation, its activation energy being 60% of the activation energy in the bulk. Most diffusion paths have activation energies larger than 75% of the equivalent bulk quantity. This analysis is relevant for the discussion on the mechanism of dynamic strain aging in these alloys. The data presented here show that pipe diffusion, which is currently considered as the leading mechanism responsible for dynamic strain aging is too slow in absence of excess vacancies.     

时效对Al-Cu合金中锯齿形流动的影响

研究了不同热处理条件下,时效对Al-Cu合金中锯齿形流动的影响．实验发现,随着自然时效时间的延长,PLC（Portevin—Le Chatelier）效应出现的时间推迟．随着时效的进一步强化（自然时效24h,或在353K下保温20min的人工时效）,应力-时间曲线上锯齿幅值减小,出现的频率迅速降低,直至消失．同时,还研究了变形和时效相耦合对锯齿形应力流动的影响．针对所得的实验结果,肯定了基于可动位错和障碍之间的相互作用的动态应变时效微观机制假设,并探讨了出现锯齿形应力流动而无PLC变形带产生的现象．     

GRAIN SIZE AND ACTIVATION ENERGY OF SERRATED YIELDING

The apparent activation energy of serrated yielding was measured by two different methodsfor a commercial brass H68.The results showed that the apparent activation energy of serrat-ed yielding measured by a method involving(m+β)values increases with the grain size.Incontrast,those measured by another method without involving(m+β)values are all the samefor specimens with different grain sizes.Combining with the observation of themicrostructures,the above phenomena have been explained.The method to measure the trueactivation energy of serrated yielding has also been proposed.     

变形温度对形变强化相变完成时临界应变量的影响

利用“形变强化相变”机制研究了低碳钢过冷奥氏体在740℃和780℃,10s-1变形时的变形温度对相变完成时临界应变量εc的影响。结果表明,变形温度对εc和组织演变的影响很大。在740℃和780℃变形时,εc分别为0.96和1.39,变形温度降低明显促进了相变。变形温度对εc的影响在组织演变上主要表现为铁素体形核地点的不同。740℃变形时,铁素体由奥氏体晶界形核过渡到以形变带形核为主,形核速率极高;780℃变形时,铁素体由奥氏体晶界形核过渡到在铁素体/奥氏体相界面前沿高畸变区快速形核。     

EFFECT OF PRECIPITATION HARDENING ON SERRATED FLOW CHARACTERISTICS IN AN Al－Li ALLOY

ＥＦＦＥＣＴＯＦＰＲＥＣＩＰＩＴＡＴＩＯＮＨＡＲＤＥＮＩＮＧＯＮＳＥＲＲＡＴＥＤＦＬＯＷＣＨＡＲＡＣＴＥＲＩＳＴＩＣＳＩＮＡＮＡｌ－ＬｉＡＬＬＯＹＥＦＦＥＣＴＯＦＰＲＥＣＩＰＩＴＡＴＩＯＮＨＡＲＤＥＮＩＮＧＯＮＳＥＲＲＡＴＥＤＦＬＯＷＣＨＡＲＡＣＴＥ...     

The influence of serrated flow on necking in tensile specimens

Three-dimensional finite element simulations are used to investigate the role of serrated flow on the strain at the onset of necking in a cylindrical uniaxial tension specimen. The material is idealized using a modified form of the McCormick constitutive equation, which has an additional material parameter that allows the rate of transient aging to be varied without affecting its steady-state response. Stability calculations and direct simulations show that, if the transient response is sufficiently slow, serrated flow can be suppressed, even though the material has negative steady-state strain rate sensitivity. This result is then used to determine the effect of suppressing serrated flow on the strain to localization. We find that negative steady-state sensitivity significantly reduces the strain required to initiate necking failure in a tensile specimen. However, the strain to failure is largely unaffected by the transient response of the material, and suppressing the serrated flow in particular has a negligible effect on the localization strain. We conclude that, while both serrated flow and reduced ductility are observed in materials with negative rate sensitivity, the reduction in ductility is not a direct consequence of serrated flow.     

The Effect of Electric Current and Strain Rate on Serrated Flow of Sheet Aluminum Alloy 5754

Electrically assisted tensile tests are carried out on sheet aluminum alloy AA5754 at electric current densities ranging from 0 to 30.4 A/mm² and strain rates ranging from 10^?3 to 10^?1 s^?1. The strain rate sensitivity and the serrated flow behavior are investigated in accordance with dynamic strain aging mechanism. The strain rate sensitivity changes from negative to positive and keeps increasing with current density. The tendency toward serrated flow is characterized by the onset of Portevin-Le Chatelier (PLC) instabilities, which are influenced by strain rate, temperature, and electric current. The evolutions of three types of serrated flow are observed and analyzed with respect to strain rate and current density. The magnitude of serration varies with strain rate and current density. The serrated flow can be suppressed by a high strain rate, a high temperature, or a strong electric current. The threshold values of these parameters are determined and discussed. Conventional oven-heated tensile tests are conducted to distinguish the electroplasticity. The flow stress reduces more in electrically assisted tension compared to oven-heated tension at the same temperature level. The electric current helps suppress the serrated flow at the similar temperature level of oven-heating.     

Shear striations and deformation kinetics in highly deformed Zr-based bulk metallic glasses

Detailed microscopic observations of the shear surfaces of a deformed, but unfractured, bulk metallic glass sample reveal a wealth of information on the deformation characteristics, kinetics and influence of temperature during serrated flow. The shear surfaces exhibit shear striations, which are similar to those resulting from viscous-like flow in rock-forming minerals. On the shear surface only a few areas show typical vein patterns, the thicknesses of which are less than those known from fracture surfaces. Combined with estimates for adiabatic heating, this indicates that sufficiently high temperatures are already present during shear banding before fracture, though instigated by non-purely adiabatic effects. A kinetic model based on an energy variable which reflects the structural relaxation ability is proposed that accounts for the occurrence of serrated flow combined with negative strain rate sensitivity, and the transition to non-serrated flow, i.e. positive strain rate sensitivity, below a critical temperature and strain rate.     

Inconel625合金高速热变形动态再结晶的临界条件

通过等温热压缩试验获得Inconel625合金在变形温度为1000～1200℃,应变速率为1～80S^-1条件下的真应力-应变曲线,利用加工硬化率,结合lnθ-ε曲线上的拐点判据及-δ（1nθ）／δε-ε曲线上的最小值,来研究Inconel625合金动态再结晶的临界条件。结果表明,在该实验条件下,Inconel625合金的lnθε曲线均出现拐点特征,对应的-δ（lnθ）／δε-ε曲线出现最小值,该最小值处对应的应变即为临界应变;临界应变随应变速率的增大和变形温度的降低而增加,并且临界应变和峰值应变之间有一定的关系,即εc=0．69εp;动态再结晶时临界应变的预测模型可以表示为εc=4．41×10^-4Z^0.14261。     

T45Al10Nb附带 金全片层组织韧脆转变机制的研究

研究了应变速率和温度对Ｔｉ４５Ａｌ１０Ｎｂ合金的屈服强度和延伸率的影响。结果表明：随着应经升高,合金的屈服强度升高而延伸率下降,由此得到韧脆温度ＴＢＤＴ随应变束率升高而升工计算出Ｔｉ４５Ａｌ１０Ｎｂ合金韧脆转变的激活能为３３０ＫＪ／ｍｏｌ。这一数值与ｒ－ＴｉＡｌ合金中原子的自扩散激活能（２９０ＫＪ／ｍｏｌ）相当,说明Ｔｉ４５Ａｌ１０Ｎｂ合金韧脆转变过程受扩散控制的形迹机制,即位错攀移控制,ＴＥＭ形     

EFFECT OF MARTENSITE ORDERING ON SME IN A Cu-Zn-Al ALLOY

The correlation between the shape memory effect(SME)and the ordering degree inmartensite formed through various heat-treatment processes,e.g.ice water quenching,step-quenching and aging etc.,has been studied in a Cu-26Zn-4Al alloy.The martensite or-dering degree is estimated by △d,the spacing difference of some pairs of diffracting planeswith indices satisfying a relation of(h_1~2-h_2~2)/3=(k_2~2-k_1~2)/n(n=1 for 9R martensite,n=4for 18R martensite).M 18R martensite is obtained from step-quenching,in which the valueof △d increases with the holding duration of step-quenching,and the shape recoveryrate η increases synchronouslly.9R martensite is obtained from direct water-quenching,thevalue of △d is quite large and SME is also good at just quenching state.But both △dand η decrease continuously with aging time at room temperature.This stabilization phe-nomenon of martensite is explained by the observation of TEM lattice fringe image,and itmay be attributed to the clustering of quenched-in supersaturated vacancies at(001)_M close-packed plane in martensite and decreasing the ordering degree.A reduction in△d and η occuring in the specimens on step-quenching or aging at higher temperature,which may be related to the precipitation of the α-phase and the enrichment of solute atoms,decreases the ordering degree.     

SUPER82B硬线钢热变形行为及其数学模型研究

文章利用Gleeble-1500热模拟试验机研究了SUPER82B硬线钢在温度为900℃~1050℃、变形速率为0.10s-1~10s-1条件下的热变形行为。通过奥氏体再结晶动力学回归计算了SUPER82B硬线钢的动态再结晶激活能、峰值应力σm与变形温度、应变速率之间的关系,动态再结晶临界应变εc和动态再结晶完成应变εs与ln(Z)的计算模型,给出了反映该钢动态再结晶进行过程的动态再结晶状态图等,为合理预报和控制SUPER82B硬线钢的组织和性能提供基本依据。     

EFFECTS OF MODIFICATION OF THE CARBIDE CHARACTERISTICS THROUGH GRAIN BOUNDARY SERRATION ON CREEP-FATIGUE LIFE IN AUSTENITIC STAINLESS STEELS

Modification of the carbide characteristics through the grain boundary serration is investigated, using an AISI 316 and 304 stainless steels. In both steels, triangular carbides were observed at straight grain boundaries while planar carbides were observed at the serrated grain boundaries. The serrated grain boundary energy is observed to be much lower than that of the straight one. Therefore, the carbide morphology is found to be changed from triangular to planar along the serrated boundary to reduce the interfacial energy between the carbide and the matrix. The creep-fatigue properties of these steels at 873K have been investigated. The creep-fatigue life of the sample with planar carbide at the serrated grain boundary was found to be much longer than that with triangular carbide at the straight one. These results imply that the planar carbides with lower interfacial energy have higher cavitation resistance, resulting in the retardation of cavity nucleation and growth to increase creep-fatigue life.     

HIGH TEMPERATURE DEFORMATION MECHANISM AND CONSTITUTIVE EQUATION OF Ti-40 BUREN RESISTANT TITANIUM ALLOY

1.IlltroductionAsacriticalmaterialItitaniumanditsalloyshavebeenwidelyusedinaerospaceindustry.Sincetitaniumpossessesbetterperformancesthanconventionalstmcturalmaterials,suchasstrength--to--weightratio,corrosionresistance,heatresistanceetc.,theapplicationsinlarge--scalefanbladesinaircraftjetenginebecomespossible,andthehighthrustengineandthehighmachntllnberaircraftcanberealized.HoWever,titaniumanditsalloyscanbeignitedandburntundersomespecialconditions.Therequirementofaircraftengineswithmuchhighp…     

Influence of carbon content on the microstructure,martensitic transformation and mechanical properties in austenite/ε-martensite dual-phase Fe–Mn–C steels

We report on the effects of carbon content on the martensitic transformation and its contribution to the work-hardening behavior of Fe–Mn–C steels during tensile deformation based on analysis by X-ray diffraction, electron backscatter diffraction and transmission electron microscopy. Austenite/ε-martensite dual-phase Fe–17Mn–C (wt.%) steels containing different carbon contents (0.01, 0.10, 0.20 wt.%) were investigated before, during and after tensile deformation. Before deformation, the transformation of austenite to thermally induced ε-martensite on cooling was suppressed as the carbon content increases. To precisely monitor microstructural changes during deformation, stepwise loading experiments were carried out in combination with electron backscatter diffraction analysis. This approach revealed that with increasing carbon content, the kinetics of transformation of γ phase to deformation stimulated ε-martensite became faster, while that of ε-martensite to α’-martensite was sluggish. We attribute this controversial effect to an increased γ grain size by the microstructural refinement of thermally induced ε-martensite and the reduction of solid solution strengthening effects by the redistribution of solute carbon. In addition, the dependence of deformation-induced ε-martensite on the loading direction differed from that of α’-martensite, and the evolution of α’ morphology was controlled by achieving appropriate levels of strain during stepwise loading. Based on the observations at the surface and inside the bulk after deformation, insights into various deformation-driven displacive phenomena, such as the formation of α’-martensite at the nonintersecting parts of two ε_initial bands, the presence of nanotwinned bundles inside austenite, cementite precipitation inside α’-martensite, and the origin of the serrated flow in strain–stress curves, were obtained. Therefore, the present study is able assist in identifying whether the deformation-induced martensitic transformation varied as a function of carbon content and the resulting fracture behavior, thereby enabling us to understand the work-hardening behavior of these steels.