Comparison between high and low strain-rate deformation of tantalum

To understand the constitutive behavior of tantalum, compression tests are performed over the range of strain rates from 0.0001/s to 3000/s, and at temperatures from 296 to 1000 K. The flow stress is seen to be representable as the sum of a thermal, an athermal, and a viscous drag component. At high strain rates (3000/s), the thermal component is observed to be expressible in terms of the temperature and the strain rate, whereas the athermal component is independent of these variables. At lower strain rates, however, such a separation of the effects of the strain, strain rate, and temperature on the flow stress is not easily achieved. At high enough temperatures, i.e., temperatures above which the thermal component is essentially zero, viscous drag appears to have a significant effect on the flow stress. This article is based on a presentation given in the symposium entitled “Dynamic Behavior of Materials—Part II,” held during the 1998 Fall TMS/ASM Meeting and Materials Week, October 11–15, 1998, in Rosemont, Illinois, under the auspices of the TMS Mechanical Metallurgy and the ASM Flow and Fracture Committees.     

An analysis of the low temperature,low and high strain-rate deformation of Ti−6Al−4V

The deformation behavior of Ti−6Al−4V at temperatures between 76 and 495 K, strain rates between 0.001 and 3000 s⁻¹, and compressive strains to 0.3 has been investigated. Measurements of yield stress as a function of test temperature, strain rate, and prestrain history are analyzed according to the model proposed by Kocks and Mecking. The mechanical threshold stress (flow stress at 0 K) is used as an internal state variable, and the contributions to the mechanical threshold stress from the various strengthening mechanisms present in this alloy are analyzed. Transmission electron microscopy (TEM) is used to correlate deformation substructure evolution with the constitutive behavior. The deformation substructure of Ti-6-4 is observed to consist of planar slip in the α grains at quasistatic strain rates. At high strain rates, deformation twinning is observed in addition to planar slip. Increasing the temperature to 495 K is seen to alter the deformation mode to more random slip; the effect of this on the proposed deformation model is discussed.     

Effects of combined high and low temperature deformation processing of β III titanium

The structure and properties of β III titanium alloy (nominal composition: 11.5 pct Mo, 6 pct Zr, 4.5 pct Sn, bal Ti) were studied as a function of combined high and low temperature ther-momechanical processing. A water quenched extrusion was deformed various amounts by swaging at room temperature prior to the aging treatment. No re-solution heat treatment was employed. The swaging introduced mechanical twinning and a small amount of stress induced orthorhombic martensite. Following a 900°F, 8 hr aging treatment, substantial increases in yield and tensile strength were observed, combined with a severe decrease in tensile ductility in samples with small amounts of swaging. The orientation and morphology of the deformation products have a critical influence on tensile ductility. A decrease in the plane strain fracture toughness accompanied the large increase in tensile strength.     

Damage leading to ductile fracture under high strain-rate conditions

Quantitative metallographic studies of damage evolution leading to ductile fracture under high strain-rate loading conditions are presented. A model material is considered, namely, leaded brass, which contains a dispersed globular lead phase that acts as void nucleation sites. Interrupted tensile split Hopkinson bar tests have been performed to capture the evolution of porosity and void aspect ratio with deformation at strain rates up to 3000 s⁻¹. Both uniaxial and notched specimen geometries were considered to allow the effects of remote stress triaxiality to be investigated. Plate impact testing has also been performed to investigate the evolution of damage under the intense tensile triaxiality and extremely high rates of deformation (10⁵ s⁻¹) occurring within a spall layer. Quantitative metallographic measurements of damage within deformed specimens are used to assess predictions from a Gurson-based constitutive model implemented within an explicit dynamic finite element code. A stress-controlled void nucleation treatment is shown to capture the effect of triaxiality on damage initiation for the range of experiments considered. This article is based on a presentation given in the symposium entitled “Dynamic Behavior of Materials—Part II,” held during the 1998 Fall TMS/ASM Meeting and Materials Week, October 11–15, 1998, in Rosemont, Illinois, under the auspices of the TMS Mechanical Metallurgy and the ASM Flow and Fracture Committees.     

Camage leading to ductile fracture under high strain-rate conditions

Quantitative metallographic studies of damage evolution leading to ductile fracture under high strainrate loading conditions are presented. A model material is considered, namely, leaded brass, which contains a dispersed globular lead phase that acts as void nucleation sites. Interrupted tensile split Hopkinson bar tests have been performed to capture the evolution of porosity and void aspect ratio with deformation at strain rates up to 3000 s⁻¹. Both uniaxial and notched specimen geometries were considered to allow the effects of remote stress triaxiality to be investigated. Plate impact testing has also been performed to investigate the evolution of damage under the intense tensile triaxiality and extremely high rates of deformation (10⁵ s⁻¹) occurring within a spall layer. Quantitative metallographic measurements of damage within deformed specimens are used to assess predictions from a Gursonbased constitutive model implemented within an explicit dynamic finite element code. A stresscontrolled void nucleation treatment is shown to capture the effect of triaxiality on damage initiation for the range of experiments considered. J.P. FOWLER, formerly Research Associate with the Mechanical and Aerospace Engineering Department, Carleton University This article is based on a presentation given in the symposium entiled “Dynamic Behavior of Materials—Part II,” held during the 1998 Fall TMS/ASM Meeting and Materials Week, October 11–15, 1998, in Rosemont, Illinois, under the auspices of the TMS Mechanical Metallurgy and the ASM Flow and Fracture Committees.     

钒微合金化低碳钢高温变形动态再结晶

利用热模拟压缩试验测定了不同钒含量的钒微合金化低碳钢在900～1000℃温度区间和0.1～1s-1变形速率范围内的真应力-真应变曲线.对曲线的分析表明:随钢中钒含量的增加,低碳钢的动态再结晶开始时间延长,变形奥氏体的动态再结晶名义激活能提高.实验钢薄膜试样的TEM观察表明,钢中的微量钒以固溶态存在于奥氏体中,微量的固溶钒对奥氏体动态再结晶起到抑制作用.     

The problem of the negative strain-rate sensitivity of metals under the portevin-lechatelier deformation conditions

Experimental data are used to show that the ‘negative’ strain rate sensitivity of metals, under conditions producing ‘jerky flow’, is only an apparent value. Taking into account the mode of propagation of slip-bands, a simple model of the phenomenon is postulated. Also discussed are the most important features of non-uniform deformation in Cu-base and Al-base alloys.     

Q235低碳钢高温变形过程的动态组织演化分析

通过Gleeble 2000上的热模拟压缩实验,分析了Q235低碳钢在不同热加工参数下的动态组织演化特征.结果表明:应变速率和温度对Q235钢的奥氏体形变特征影响强烈.在相同变形温度下,应变速率的提高可以明显推迟动态再结晶的发生:应变速率较低时,降低温度同样可以延迟动态再结晶的发生.利用定量金相技术及线性、非线性拟合算法,建立了 Q235钢热变形过程的唯像本构关系及组织演化动力学模型,并将其应用于Autoforge3.1有限元软件平台.压缩过程有限元模拟分析表明,分别采用Arrhenius双曲正弦方程描述Q235钢的唯像本构关系及Yada模型表征Q235钢变形过程的平均晶粒尺寸,可以满足预测精度,与实际变形过程基本吻合.     

Comparison of cyclic deformation behavior between copper bicrystals and their component crystals

In this article, the cyclic deformation behavior of a combined copper bicrystal (CB), a naturally grown copper bicrystal (RB), and their component crystals (G1 and G2) was investigated under total strain amplitude control at room temperature in air. The bicrystal CB without grain boundary (GB) was prepared by joining the two single slip-oriented component crystals G1 and G2 in the gage. The results showed that the cyclic hardening rates and the axial stresses increased, in order, in the component crystal G2, the bicrystal CB, the bicrystal RB, and the component crystal G1. The cyclic stress-strain curves (CSSCs) of the component crystals G1 and G2 and the bicrystal CB exhibited a plateau behavior under the selected strain range. However, the cyclic saturation stress of the bicrystal RB increased with strain amplitude and did not show a plateau in its CSSC. Surface observations of the copper bicrystals and the component crystals revealed that the cyclic plastic strains were carried by the primary slip (B4) within the component crystals in the bicrystal CB and its component crystal G1 and G2. However, an additional slip (A3) within the component crystal G2 was observed in the vicinity of the GB in the bicrystal RB. The width of the additional slip region increased with strain amplitude and varied in the range from 265 to 650 μm. In order to compare the CSSCs of the copper bicrystals and their component crystals, an “orientation factor” for the copper bicrystal was introduced and the effect of the GB on the bicrystal RB is quantified during cyclic deformation.     

Interaction between recrystallization and precipitation during the high temperature deformation of HSLA steels

A new mechanical method is described for following the progress of precipitation in niobium-modified steels. The technique is based on the determination of the strain to the peak stress in high temperature, constant strain rate compression tests. The peak strain is sensitive to holding or aging time prior to testing, thus permitting the kinetics ofstatic precipitation to be determined in either the re crystallized or the predeformed condition. A modification of this technique permits the determination of the kinetics ofdynamic pre-cipitation. The rates of static and dynamic precipitation measured in this way are generally ‘faster’ than the kinetics determined by other methods. The results indicate that the addition of niobium to austenite retards recrystallization in two distinct ways. There is a significant delay introduced by what appears to be a solute effect. In addition, under conditions where precipitation is more rapid thansolute- retarded recrystallization, the operation of the recrystallization process is prevented or retarded until precipitation is complete or nearly complete. This paper is based on a presentation made at a symposium on “Recovery Recrystallization and Grain Growth in Materials” held at the Chicago meeting of The Metallurgical Society of AIME, October 1977, under the sponsorship of the Physical Metallurgy Committee.     

The influence of deformation path on the slow strain-rate stress corrosion cracking of admiralty brass sheet

The slow strain-rate stress corrosion cracking (SCC) of Admiralty brass sheet in an aqueous 0.1M CuSO₄ solution has been studied over a range of strain paths from uniaxial to equibiaxial tension. At a constant bulk solution pH and open circuit potential, the brass undergoes transgranular SCC characterized by cleavage-like cracks propagating on a macroscopic plane normal to the maximum principal strain axis for all strain paths. The average crack growth velocity is also independent of deformation path. It is thus concluded that the mechanism of transgranular SCC in this system does not depend on multiaxial strain path for the range of stress states examined. However, the fracture strain data show that the slow strain-rate SCC of the brass sheet results in ductility losses which are much larger in equibiaxial tension than in either uniaxial or plane strain tension. This behavior is attributed to stress corrosion cracks acting as linear imperfections whose presence causes failure of the uncracked ligament by a form of localized necking and whose influence is dependent not only on time but also on strain path.     

Tensile strain-rate sensitivity of

The tensile strain-rate sensitivity of continuous-tungsten-fiber reinforced niobium composites (W/Nb), fabricated by an arc-spray process, was studied in the 1300 to 1600 K temperature range. The tensile properties of the fiber and matrix components, as well as of the composites, were measured and compared to rule of mixtures (ROM) predictions. The deviation from the ROM was found to depend upon the chemistry of the tungsten alloy fibers, with positive deviations for thoria-dispersed W wire (ST300) reinforced Nb composite(i.e., stronger composite strength than the ROM) and negative or zero deviations for lamp-grade W wire (218) reinforced Nb composite. In addition, it was found that the composites tested at higher crosshead speeds exhibited a strain-rate sensitivity greater than that of the free fibers tested at the same crosshead speeds, even though the composite tensile strength is determined mainly by the fiber component.     

The development of strain-rate gradients

The development of nonuniformities in tensile deformation and its dependence on material parameters and external conditions have been reanalyzed. It is found that the major influence of strain rate on neck growth is usually through its effect on the rate of strain hardening, rather than on the flow stress itself. In addition, the curvature of the stress-strain curve plays an important role: its usual sign is responsible for the unavoidability of eventual unbounded neck growth. The new analysis uses the state-parameter formulation of constitutive laws, avoiding the integrated strain as a variable. The result is a second-order differential equation describing the development, at each cross section, of the relative gradients of cross-sectional area and of strain rate. Numerical solutions are presented for small strains, analytical ones for slow neck growth. The application of a strain-rate criterion for catastrophic growth leads to a quantitative formula for the maximum tensile ductility, which is in agreement with observations, and provides guidelines for material development.     

锌精矿常压浸出与加压浸出工艺比较

比较了常压浸出与加压浸出两种工艺的机理、流程、技术经济指标、投资以及存在的问题。试验和生产数据表明,加压浸出在技术上和工艺上都更具有吸引力。     

锌精矿常压浸出与加压浸出工艺比较

比较了常压浸出与加压浸出两种工艺的机理、流程、技术经济指标、投资以及存在的问题。试验和生产数据表明,加压浸出在技术上和工艺上都更具有吸引力。     

Steady-state deformation at intermediate and high temperatures

Experimental data of steady-state deformation obtained either in creep or in constant strain-rate experiments, which extend from high to intermediate temperatures, become increasingly available in the literature. While in many cases the high-temperature regime can be explained by the well-known power-law models, the incorporation of the results at intermediate temperatures is still under discussion. It is the aim of the present work to show by a careful analysis of the above mentioned data, that the jog-dragging model put forward by Barrett and Nix can account for the whole temperature range considered here and that the activation enthalpy of steady-state deformation is that of lattice self-diffusion.     

Affective, cognitive, and behavioral differences between high and low procrastinators.

Examined the relation between academic procrastination and academically related trait measures among 379 university students, including 261 females, 117 males, and 1 S who did not specify gender. 126 of the Ss participated in weekly assessment sessions. Ss completed a procrastination assessment scale, the Test Anxiety Scale, a self-control scale, and a trait measure of attributions of success and failure 3 times when midterm examinations approached. Results indicate that more than 40% of the Ss reported a high level of procrastination. Self-reported procrastination was positively correlated with delay in taking self-paced quizzes and was negatively correlated with grade point average (GPA). High procrastinators, particularly women, were significantly more likely than were low procrastinators to report more test anxiety, weekly state anxiety, and weekly anxiety-related physical symptoms. High procrastinators were significantly more likely than were low procrastinators to attribute success on examinations to external and unstable factors. As the examination deadline approached, both high and low procrastinators perceived examinations to be less difficult, less important, and less anxiety provoking; reported fewer factors to hinder study behavior; increased their study behavior; and decreased delay. Implications for anxiety reduction as a procrastination intervention are discussed. (13 ref) (PsycINFO Database Record (c) 2010 APA, all rights reserved)