Indium doped ZnO films prepared by RF magnetron sputtering: effect of substrate temperature on the strain-induced band gap

Indium doped zinc oxide (InZnO) thin films were deposited onto corning glass substrates by RF magnetron sputtering. The dependence of crystal structure, surface morphology, optical properties and electrical conductivity on substrate temperature was investigated using XRD, AFM, UV-vis Spectrophotometer, Fluorescence Spectrophotometer and four-point probe. The films were prepared at different substrate temperatures viz, room temperature (RT), 473 K and 673 K at RF power 200 W. All the films showed preferred orientation along (002) direction. Crystallite size increased from 14 to 19 nm as the substrate temperature was increased to 473 K. With increase in substrate temperature the crystallites did not show any further growth. AFM analysis showed that the rms roughness value decreased from 60 nm to 23 nm when the substrate temperature was increased to 673 K. Optical measurements revealed maximum band gap and minimum refractive index for the film prepared at 473 K. A strong correlation between the band gap variation and the strain developed at different substrate temperatures is established.     

Correlation between cyclic deformation behaviour and microstructure in a duplex steel between 300 and 773 K

Cyclic tests performed in the temperature range 300–773 K on duplex stainless steel DIN 1.4460 show that the cyclic stress–strain behaviour of this steel is strongly temperature dependent. At 300 and 473 K an almost constant peak tensile stress stage, is followed by a slight softening that continues up to failure in the case of 300 K, but by a secondary hardening at 473 K. Pronounced initial cyclic hardening followed by secondary hardening was the main feature of the temperature range between 573 and 723 K. At 773 K, after a weak hardening stage, a strong softening continues up to failure. The mechanical behaviour and the evolution of the microstructure were analysed, and the internal and the effective stresses were studied. It was found that the internal stress is responsible for the strong hardening that occurs in the intermediate temperature range and for the softening at 773 K.     

Dynamic compressive behavior of ultrafine-grained pure Ti at elevated temperatures after processing by ECAP

Commercial purity titanium was processed by equal-channel angular pressing (ECAP) for 8 passes and then subjected to dynamic compressive testing using a split-Hopkinson pressure bar (SHPB) facility with an imposed strain rate of ~4000 s^?1 and testing temperatures from 288 to 673 K. The results show that ECAP produces an average grain size of ~0.3 μm in transverse sections, but grains which are elongated in longitudinal sections. During dynamic compressive testing at temperatures ranging from 288 to 473 K, the grain shapes and sizes remain unchanged in the transverse sections, but the elongated shapes in the longitudinal sections evolve into polygons due to cell dislocation evolution. At 673 K, the grains become equiaxed with an average size of ~1.8 μm thereby demonstrating the occurrence of dynamic recrystallization. It is shown that the flow stresses decrease with increasing temperature from 288 to 673 K, and there is also a reduction in the rate of strain hardening.     

BAl7-7-2-2铜合金冷变形加工硬化行为

采用室温压缩试验研究了BAl7-7-2-2铜合金冷的变形加工硬化特性。结果表明,修正后的Ludwik模型可以准确地对BAl7-7-2-2铜合金的真应力-真应变数据进行拟合,相关材料常数K=1 001.736、m=0.295、n=0.044,拟合方程的相关系数可以达到0.96。当真应变ε＜0.02时,加工硬化率很高且迅速增大,此阶段为线性硬化阶段;当0.02≤ε≤0.2时,真应力随着真应变的增加而增大,加工硬化率则逐渐减小,这一阶段为抛物线硬化阶段;ε＞0.2时,随着应变的增加,加工硬化率趋于常数,为弱硬化阶段。结合微观组织和硬度测试表明,材料在不同变形阶段的变形机制有所不同。     

Mechanical behavior and shape memory effect of an aged NiAl–Fe alloy

The mechanical behavior and shape memory effect of an aged NiAl–Fe alloy has been investigated. It was found that the first yielding stress of NiAl–Fe alloy in a compression test was decreased with the precipitation of a Ni₅Al₃ phase after aging at 473–673 K, and increased as the aging temperature increased higher. The one-way shape recovery of NiAl–Fe alloy increased as the aging temperature increased from 473 to 673 K, and decreased as the aging temperature increased higher than 673 K. The morphology of precipitates in the NiAl–Fe alloy aged at 473–873 K was investigated, and the mechanism of its effect on the mechanical properties is discussed.     

Strain Softening and Hardening Behavior in AZ61 Magnesium Alloy

The deformation behavior of AZ61 Mg alloy during hot deformation has been investigated in wide temperature and strain rate range by a Gleeble simulator. Specimens are deformed in compression in the temperature range of 523～673 K and at strain rates of 0.001～1 s-1. It is found that the flow curves exhibit a peak and then decrease towards steady-state of classical DRX, which decrease with rising temperature and decreasing strain rate. The deformation behavior of the specimens can be attributed to the occurrence of strain hardening and softening. As stress decreases, the strain hardening rate declines at a fast rate when temperature rises or strain rate decreases. The shapes of θ-σ curves indicate some important features such as subgrain formation, the critical stress, the peak stress and steady stress. The onset of DRX can be determined by the point of inflection on θ-σ or Inθ-σ curves.     

Elevated-temperature strengthening in carbide (Al4C3)-dispersed aluminium

The effect of strain rate on the compressive flow behaviour of DISPAL 2 is investigated in the temperature range 473–823 K. The stress exponent, n, was 28 in the temperature range 473–623 K, while it increased to 59 above 673 K. The activation volume and energy for deformation were 70 b ³ and 100–200 kJ mol^–1, respectively, in the temperature range 473–623 K. In the higher range, 673–823 K, the observed activation volume of 300–500 b ³ and the activation energy of ~ 1086 kJ mol^–1 cannot be reconciled with any of the deformation mechanisms. A new model-based creep equation for dispersion-strengthened materials proposed by Rosler and Arzt has been applied to the flow data from 673–823 K. Its predictions are in agreement with the experimental data in the temperature range 673–723 K. The predictions of the model, however, differ from the experimental data at 773 and 823 K.     

Dislocation slip distance during compression of Al–Si–Cu–Mg alloy with additions of Ti–Zr–V

The plastic deformation behaviour of the Al–Si–Cu–Mg alloy with micro-additions of Zr–V–Ti was measured in the temperature range of 298–673 K and the true stress–true strain compression curves were used to calculate the dislocation slip distance (DSD). A new constitutive equation for the temperature dependent DSD was developed, based on Mott’s theory of strain hardening. The DSD predicted by the model was in good agreement not only with values achieved for the Al–Si–Cu–Mg alloy tested but also for other Al based and Pb–Sb alloys with deformation data available in the literature. A comparison of deformation and microstructure suggests that the grain refinement during hot compression deformation occurring due to continuous dynamic recrystallisation is responsible for a drastic growth of the DSD.     

Mechanical properties of Co3Ti polycrystals alloyed with various additions

Third elements including vanadium, tantalum, chromium, molybdenum, tungsten, iron, nickel, aluminium and germanium, were added to L1₂-type Co₃Ti polycrystals with a composition of 23 at % Ti. Tensile tests from 77 to 1273 K were carried out to investigate the mechanical properties of these alloys. Anomalous increases of the yield stress with increasing temperature were observed from 473 to 973 K (or 1100 K depending on the strain rates) for all tested alloys. It was also observed that the yield stress increased with decreasing temperature below 473 K and decreased with increasing temperature above 973 K (or 1100 K). The yield stresses above the peak temperature were very sensitive to the strain rate and grain size. Microstructural observation showed that gram-boundary sliding had a considerable contribution to the fall in yield stress at high temperatures. The elongation showed a maximum around 673 K and a minimum around 1073 K for all alloys. Microstructural and fractographic observation showed that most alloys tested suffered from hydrogen-related embrittlement at ambient temperatures and grain-boundary cavitation-related embrittlement around 1073 K. Addition of aluminium and iron was found to produce a significant improvement in hydrogen-related embrittlement at room temperature.     

20vol%体积分数纳米Al2O3颗粒增强铝基复合材料的高温压缩性能

为提高铝基材料的高温力学性能以满足其在573 K以上用于航空航天装备结构件的性能需求,采用高能球磨结合真空热压烧结工艺制备了体积分数高达20vol%的纳米Al₂O₃颗粒(146 nm)增强铝基复合材料,对其微观结构和高温压缩性能进行了研究。结果表明：纳米Al₂O₃颗粒均匀分散于超细晶铝基体中,且复合材料完全致密;该复合材料具有优异的高温压缩性能：应变速率为0.001/s时,473 K时压缩强度高达380 MPa,即使673 K时依然高达250 MPa,比其他传统铝基材料提高至少1倍;通过对其流变应力进行基于热激活的本构模型拟合可以发现,该复合材料具有高的应力指数(30)和表观激活能(204.02 kJ/mol)。这是由于高体积分数纳米颗粒能够有效钉扎晶界,并与铝基体形成热稳定的界面结合,显著提高复合材料的组织热稳定性,而且在变形过程中与晶界有效阻碍位错运动,显著提高复合材料的热变形门槛应力(在473～673 K时为190.6～328.4 MPa),其热变形过程可以由亚结构不变模型进行解释。     

温度和应变率对原位合成钛基复合材料动态压缩性能及破坏机制的影响

使用分离式Hopkinson压杆（SHPB）系统,在温度293~973 K、应变率6 000~10 000 s-1下,对原位合成TiC颗粒和TiB晶须混合增强钛基复合材料（TMCs）的动态压缩性能进行了研究。试验结果表明:在373~573 K、673~773 K和873~973 K范围内TMCs流变应力随温度的增加而显著减小;在较低温度（低于373 K）和较低应变率（6 000~8 000 s-1）下,TMCs呈现小幅的应变率硬化特征,而在较高温度（573 K及以上）时各应变率下TMCs均存在应变率软化特征,且温度越高材料应变率软化效应越明显。材料失效/断裂机制分析表明:应变率软化机制主要是绝热软化及其产生的绝热剪切带（ABS）中微裂纹的形成和扩展的综合作用;在较高的应变率和较大应变下ABS中会产生微裂纹,温度较低时TMCs塑性不足以抑制或阻碍微裂纹的扩展,从而导致TMCs在宏观上迅速破坏;材料破坏时以钛合金基体塑性断裂为主,但在局部伴随部分增强相脆性断裂。      

Mg-9Gd-3Y-0.3Zr合金高温塑性变形行为研究

以Mg-9Gd-3Y-0.3Zr合金为研究对象,采用热模拟等温压缩的试验方法,利用Arrhenius关系式,分析了合金应力与应变的关系,同时利用金相显微镜和扫描电镜,观察合金在热变形过程中组织的演变。结果表明,合金高温等温压缩变形的真应力-真应变曲线属于动态再结晶型,并且当应变速率ε一定时,温度升高,峰值流变应力下降,当温度一定时,应变速率ε增大,峰值流变应力和它所对应的应变值均提高;变形过程中随着温度的升高,合金发生不同程度的再结晶。当变形温度为623K时,组织变化以动态回复为主,变形温度提高到673K,开始出现再结晶现象,温度达到773K时得到完全再结晶组织。     

Constitutive modeling for elevated temperature flow behavior of 42CrMo steel

In order to study the workability and establish the optimum hot formation processing parameters for 42CrMo steel, the compressive deformation behavior of 42CrMo steel was investigated at the temperatures from 850 to 1150 °C and strain rates from 0.01 to 50 s⁻¹ on Gleeble-1500 thermo-simulation machine. The results show that the true stress–true strain curves exhibit a peak stress at a small strain, after which the flow stresses decrease monotonically until high strains, showing a dynamic flow softening. The flow stress obtained from experiments consists of four different stage, i.e., Stage I (Work hardening stage), Stage II (Stable stage), Stage III (Softening stage) and Stage IV (Steady stage). The stress level decreases with increasing deformation temperature and decreasing strain rate, which can be represented by a Zener–Hollomon parameter in an exponent-type equation. A revised model describing the relationships of the flow stress, strain rate and temperature of 42CrMo steel at elevated temperatures is proposed by compensation of strain and strain rate. The stress–strain values of 42CrMo steel predicted by the proposed model well agree with experimental results, which confirmed that the revised deformation constitutive equation gives an accurate and precise estimate for the flow stress of 42CrMo steel.     

Modified Johnson-Cook description of wide temperature and strain rate measurements made on a nickel-base superalloy

In this study, uniaxial compression experiments of a Nickel-base superalloy is conducted over a wide range of temperatures (298–1073 K) and strain rates (0.1–5200/s) to obtain further understandings of the plastic flow behaviours. The temperature and strain rate effects on the plastic flow behaviour are analysed. The flow stress decreases with increasing temperature below 673 K. Within the temperature range of about 673–873 K, the flow stress varies indistinctively, and even increases slightly with increasing temperature. As the temperature further increases, the flow stress decreases again. The flow stress of the Nickel-base superalloy displays insensitive to strain rate below 800/s and an enormous increase with increasing strain rate in excess of 800/s. Then the effects of temperature and strain rate on the microstructure are discussed. The result shows that high strain rate and high temperature may make the grain boundary of Nickel-base superalloy frail. Taking into account the anomalous temperature and strain rate dependences of flow stress, modified J–C constitutive model is developed. The model is shown to be able to accurately predict the plastic flow behaviour of Nickel-base superalloy over a wide range of temperatures and strain rates.     

Influence of dynamic strain ageing on mixed mode I/III fracture toughness of Armco iron

Fracture toughness tests under mode I and mixed mode I/III loading were carried out at different test temperatures ranging from ambient to 673 K. The dynamic strain ageing (DSA) range in Armco iron was identified to be between 383 and 573 K. A marked increase in fracture toughness was observed in the DSA regime and this correlated with the increase in the strain hardening exponent. The magnitude of fracture toughness, however, decreased with increasing loading angle. The extent of decrease was high at temperatures below the DSA regime (≤383 K) which can be understood in terms of the nature of the stress field ahead of a mixed mode I/III as well as the operative fracture mechanism. However, at higher temperatures, the effect of mode III in this respect diminished in the DSA regime (383–573 K) due to DSA causing the opposite effect, that is fracture toughness to increase.     

Deformation and fracture behaviour of porous FVS0812 aluminium alloy prepared by spray deposition

Abstract

The deformation and fracture behaviour of a porous FVS0812 aluminium alloy prepared by spray deposition was investigated using compression testing. Rapid densification of the porous alloy was observed before a height reduction of ~50%. The optimum strain rate and deformation temperature to achieve maximum density were determined. Microstructural evolution involved the following stages: breakup of boundaries between adjacent particles; plastic deformation of particles; and collapse of large pores into various smaller pores and their disappearance. A strain hardening phenomenon occurred at all strains when compression was carried out at at 573 and 673 K, while a recovery mechanism came into operation at 773 K and a true strain of 0.2. The critical strain for the occurrence of strain softening in the porous alloy was much higher than that for the fully dense alloy. The fracture criteria curve of the porous alloy was a straight line, with a slope of ~0.45268 at high temperature and ~0.47636 at room temperature, almost parallel to that of the fully dense alloy in homogeneous compression.     

Applicability of Kocks–Mecking approach for tensile work hardening in P9 steel

Abstract

Work hardening behaviour of P9 steel in the temperature range 300–873 K has been examined in the framework of Kocks–Mecking (K–M) approach. At all temperatures, P9 steel exhibited two-stage work hardening behaviour characterised by a rapid decrease in instantaneous work hardening rate (i.e. θ?=?dσ/d?, where σ is the true stress and ? is the true plastic strain) with stress at low stresses (transient stage) followed by a gradual decrease at high stresses (stage III). Stage III work hardening of P9 steel was adequately described by K–M approach. The variations of work hardening parameters associated with K–M approach for stage III with temperature indicated three distinct temperature regimes. At all temperatures, good correlations between the respective work hardening parameters evaluated using K–M approach and from Voce equation and its derivative have been obtained for P9 steel.     

GH5188合金板材高温拉伸变形流动行为与本构方程研究

目的 研究GH5188合金板材高温拉伸变形流动行为,为高温合金板材高温成形工艺的制定和优化提供指导。方法 基于GH5188合金板材高温拉伸试验,分析了变形工艺参数对GH5188合金板材高温拉伸变形时真应力、应变速率敏感性指数和应变硬化指数的影响规律,建立了本构模型对其流动行为进行描述和预测。结果 GH5188合金板材高温拉伸变形流动行为受应变硬化、流动软化和应变速率硬化的共同影响,其变形过程分为弹性变形、加工硬化、稳态流动和断裂4个阶段。随变形温度的升高和应变速率的降低,真应力减小。变形温度、应变速率和真应变对GH5188合金板材的应变速率敏感性指数和应变硬化指数具有显著影响。基于Johnson-Cook和Hensel-Spittel模型,建立了考虑应变硬化效应、流动软化效应和应变速率硬化效应耦合影响的GH5188合金板材高温拉伸变形本构模型(JC-HS模型),采用该模型预测的真应力与试验值的平均相对误差为6.02%。结论 建立的JC-HS模型能够较好地描述和预测GH5188合金板材的高温拉伸流动行为。     

Low temperature stress relaxation of nanocrystalline nickel

Stress relaxation in nanocrystalline nickel within the temperature range 523–673 K in a uniaxial compression regime is studied in the present investigation. The results obtained for coarser grained nickel are given for comparison. An average strain rate of nanocrystalline nickel within the investigated range of temperatures is 1.75 × 10^–5–3.03 × 10^–5s^–1. The presence of two types of stress relaxation dependencies are shown. The most likely strain mechanism is grain boundary sliding controlled by grain boundary diffusion for temperatures between 623 and 673 K.     

Key issues in cyclic plastic deformation: Experimentation

Cyclic plastic deformation phenomena include the Bauschinger effect, cyclic hardening/softening, strain range effect, loading history memory, ratcheting, mean stress dependent hardening, mean stress relaxation and non-proportional hardening. In this work, different cyclic plastic deformation responses of piping materials (SA333 C-Mn steel and 304LN stainless steel) are experimentally explored. Cyclic hardening/softening is depends upon loading types (i.e. stress/strain controlled), previous loading history and strain/stress range. Pre-straining followed by LCF and mean stress relaxation shows similar kind of material response. Substantial amount of non proportional hardening is observed in SA333 C-Mn steel during 90° out of phase tension-torsion loading. During ratcheting, large amount of permanent strain is accumulated with progression of cycles. Permanent strain accumulation in a particular direction causes cross-sectional area reduction and which results uncontrollable alteration of true stress in engineering stress controlled ratcheting test. In this work, true stress control ratcheting on piping materials has been carried out in laboratory environment. Effects of stress amplitude and mean stress on the ratcheting behaviors are analyzed. A comparison has also been drawn in between the true and engineering stress controlled tests, and massive difference in ratcheting life and strain accumulation is found.