Effect of microstructure on plastic deformation of Cu at low homologous temperatures

The mechanical properties of polycrystalline Cu (purity 99.95%) prepared by severe plastic deformation were studied at low homologous temperatures from 0.5 K to room temperature. Material with three different microstructures was prepared by annealing of ultrafine-grained Cu. At cryogenic temperatures (0.5 and 4.2 K) the material exhibited an inverse temperature dependence of the yield stress and unstable plastic deformation accompanied by serrations on the stress–strain curves. These low-temperature anomalies were accentuated with grain refinement. At cryogenic temperatures, enhanced ductility was observed and the Hall–Petch relation was found to hold. Microhardness and yield stress were much more temperature dependent in fine-grained than in coarse-grained material, and there is a correlation between the flow stress at a fixed strain and the microhardness. This study has demonstrated that, apart from enhanced discontinuous plastic flow, severe plastic deformation improves the strength of copper at cryogenic temperatures without sacrificing ductility.     

7A52铝合金热加工过程中高温压缩变形行为

采用圆柱试样在Gleeble-1500热模拟机上进行高温压缩变形模拟实验,研究了7A52铝合金在高温塑性变形过程中流变行为。实验结果表明,合金高温压缩变形时的流变应力随变形温度的升高而减小,随变形速率的提高而增大。热变形条件下流变应力σ、应变速率ε.和变形温度T之间满足一定的关系式。研究指出,合金适宜的热加工温度为400℃~420℃。     

The characterization of plastic deformation in Ce-based bulk metallic glasses

The plastic deformation behavior of Ce₆₈Al₁₀Cu₂₀Nb₂ and Ce₇₀Al₁₀Cu₂₀ bulk metallic glasses (BMGs) at room temperature was studied by depth-sensing nanoindentation and microindentation. It is shown that the two BMGs exhibit a continuous plastic deformation without distinct serration at the all of the studied loading rates during nanoindentation. An obvious creep displacement was observed during the holding-load segment at the maximum load for the two alloys, and the magnitude of creep during holding-load increases with loading rate. The subsurface plastic deformation zone of the two BMGs after indentation at various loading rates was investigated through bonded interface technique using depth-sensing microindentation. A highly developed shear banding pattern can be observed in the plastic deformation region, though the global load–depth curves illuminate a “homogeneous flow”. The plastic deformation behavior of the Ce-based BMGs during indentation measurements is discussed in terms of localized viscous flow.     

Accumulative channel-die compression bonding (ACCB): A new severe plastic deformation process to produce bulk nanostructured metals

This paper introduces a new severe plastic deformation process to produce bulk nanostructured metals: accumulative channel-die compression bonding (ACCB). In the ACCB process, which can be applied to thick billets, the procedure of cutting, stacking and compression bonding in a channel-die is repeated to provide an ultrahigh plastic strain. This process was trialed with high purity aluminum. A fully recrystallized aluminum sample was deformed by ACCB at room temperature for up to 10 cycles, corresponding to an equivalent strain of 8.0. The initially coarse grains were subdivided by deformation-induced high-angle boundaries, and the fraction of such high-angle boundaries increased with increasing strain. Several cycles of ACCB led to a quite uniform ultrafine structure dominated by high-angle grain boundaries. The average boundary spacing of the 10-cycles ACCB sample was as small as 690 nm. The maximum ultimate tensile strength of the ACCB samples was 130 MPa after 5 cycles. Further ACCB cycles, however, led to a slight decrease in strength due to enhanced recovery and boundary migration during the deformation process. It has been demonstrated that the ACCB process can be used to produce bulk nanostructured metals of relatively large dimensions. The results suggest that the ACCB process is equivalent to conventional rolling deformation at high strains.     

Elevated Temperature Mechanical Behavior of Severely Deformed Titanium

In this investigation, compression tests were performed at a strain rate of 0.001-0.1 s^?1 in the range of 600-900 °C to study the high temperature deformation behavior and flow stress model of commercial purity (CP) titanium after severe plastic deformation (SPD). It was observed that SPD via equal channel angular extrusion can considerably enhance the flow strength of CP titanium deformed at 600 and 700 °C. Post-compression microstructures showed that, a fine grained structure can be retained at a deformation temperature of 600 °C. Based on the kinematics of dynamic recovery and recrystallization, the flow stress constitutive equations were established. The validity of the model was demonstrated with reasonable agreement by comparing the experimental data with the numerical results. The error values were less than 5% at all deformation temperatures except 600 °C.     

Deformation twinning in bulk nanocrystalline metals: Experimental observations

Deformation twins have been observed in nanocrystalline (nc) fcc metals with medium-to-high stacking fault energies such as aluminum, copper, and nickel. These metals in their coarse-grained states rarely deform by twining at room temperature and low strain rates. Several twinning mechanisms have been reported that are unique to nc metals. This paper reviews experimental evidences on deformation twinning and partial dislocation emissions from grain boundaries, twinning mechanisms, and twins with zero-macro-strain. Factors that affect the twinning propensity and recent analytical models on the critical grain sizes for twinning are also discussed. The current issues on deformation twinning in nanocrystalline metals are listed.     

A new constitutive equation for strain hardening and softening of fcc metals during severe plastic deformation

The stress–strain relationship for strain hardening and softening of high-purity aluminum and copper, which were deformed by equal channel angular pressing (ECAP) at ambient temperature, was analyzed by combining the Estrin and Mecking (EM) model and an Avrami-type equation with experimental data during severe plastic deformation. The initial strain hardening can be described by the EM model, while the flow stress arrives at the peak stress after it was saturated. However, strain softening similar to plastic deformation at high temperatures is observed after the peak stress. Moreover, the peak strain at the maximum flow stress is ∼4 for copper and ∼2 for aluminum. A new constitutive equation was developed to describe strain softening at high strain levels, which was supported well by tensile, compression and microhardness tests at room temperature and low strain rate. It was observed that dynamic recovery and recrystallization occurs in copper, and recrystallized grains and their growth in aluminum. The results indicate that dynamic recovery and recrystallization was the dominant softening mechanism, which was confirmed by scanning electron microscopy–electron channeling contrast observations and the abnormal relationship between the imposed strain during ECAP and subsequent recrystallization temperature after ECAP.     

Effect of room-temperature compression on microstructure of ductile cast iron subjected to hot plastic deformation

The change in the microstructure of ductile cast iron subjected to hot plastic deformation has been investigated after the fracture of the samples induced by compression (upset forging) at room temperature. It has been shown that compression-induced tangential stresses cause shear deformation, which results in the shear fracture of test samples at an angle of 40°–50° to the longitudinal axis of a sample. It has been established that the fracture is accompanied by the formation of a narrow zone of severe plastic deformation of ductile cast iron, which is located on both sides of the major fracture. In this zone, the initial microstructure undergoes significant changes due to the plastic flow of the matrix and graphite inclusions.     

Investigation of Strain Rate Effect by Three-Dimensional Discrete Dislocation Dynamics for fcc Single Crystal During Compression ProcessEI北大核心CSCD

Microelectromechanical systems (MEMS) have become increasingly prevalent in engineering applications. In these MEMS, a lot of micro-components, such as thin films, nanowires, micro-beams and micropillars, are utilized. The characteristic geometrical size of those components is at the same scale as that of grain, the mechanical behavior of crystal materials exhibits significant size effect and discontinuous deformation. In addition, those MEMS are often subjected to high strain rate at work, such collision and impact loading. The coupling deformation characteristics of small scale crystals and high strain rate makes their mechanical behavior more complicated. Accordingly, investigation of the effect of the strain rate on crystal materials at micron scale is significant for both the academia and industry. In this work, a plastic deformation model of fcc crystal under axial compression was developed based on three-dimensional discrete dislocation dynamics (3D-DDD), which considered the influence of externally applied stress, interaction force between dislocation segments, dislocation line tension and image force from free surface on dislocation movement during the process of plastic deformation. It was applied to simulate the plastic deformation process of a Ni single crystal micropillar during compression under different loading strain rates. 3D-DDD and theoretical analysis are carried out to extensively investigate the effect of strain rate on flow stress and deformation mechanisms during plastic deformation process of crystal materials. The results show that the flow stress and the dislocation density increased with the loading strain rate. In the case of low strain rate, the flow stress was dominated by the activation stress of FreakRead (FR) source in plastic deformation. With the increase of strain rate, the contribution of activation stress of FR source to the flow stress decreases and the effective stress gradually dominated the flow stress. Under high strain rate loading, with the increase of the initial FR source, the dislocation density also increased at the same strain correspondingly, which makes it easier to meet the requirement of the loading strain rate, so the flow stress is smaller. In addition, under the low strain rate loading, a few activated FR sources can meet the requirement of the plastic deformation, a single slip deformation come up as a result. While, as the loading strain rate increases, more and more activated FR sources would be needed to coordinate the plastic deformation, the deformation mechanisms of the single crystal micropillar transformed from single slip to multiple slip.     

Al-20Cu-4.5Si-3Ni-0.25RE合金的高温流变本构方程

在Gleeble-1500热模拟机上进行高温等温圆柱体压缩试验,研究Al-20Cu-4．5Si-3Ni-0.25RE合金在高温塑性变形过程中流变应力的变化规律。结果表明：应变速率和变形温度的变化强烈地影响Al-20Cu-4．5Si-3Ni-0．25RE合金的流变应力,流变应力随变形温度升高而降低,随应变速率提高而增大。可用Zener-Hollomon参数的双曲正弦形式来描述Al-20Cu-4．5Si-3Ni-0．25RE合金热压缩变形时的流变应力行为。     

Deformation in metals after low-temperature irradiation: Part I – Mapping macroscopic deformation modes on true stress–dose plane

Macroscopic deformation modes, elastic, uniform plastic, and unstable plastic deformation modes, are mapped in tensile true stress–dose space for more than two dozen metallic materials consisting of 13 body-centered cubic (bcc), 11 face-centered cubic (fcc), and two hexagonal closed packed (hcp) metals. The boundaries between different deformation zones are set by the true stress versus dose curves: the yield stress (YS), plastic instability stress (PIS), and true fracture stress (FS) plotted as functions of dose. Values for these true stresses are obtained from uniaxial tensile tests or calculated from engineering tensile data using a linear strain-hardening model for necking deformation. The relatively low-strength annealed fcc metals display large uniform plasticity regions, while unstable deformation regions are dominant in the harder bcc and hcp metals. PIS values for all materials are independent of dose except for the precipitation-hardened IN718 alloy, where a decrease of PIS occurs due to an irradiation-induced change in second phases. In the bcc materials for high-temperature application, such as 9Cr ferritic/martensitic steels, sintered molybdenum, vanadium, and tantalum, the radiation-induced embrittlement is characterized in terms of FS decreasing with dose at relatively high doses. FS is nearly dose-independent below the critical dose for embrittlement. It is concluded that the tensile stress-based deformation mode maps effectively integrate mechanical property information and characterize differences in radiation effects between crystalline structures or material groups.     

Ductile fracture behavior of 5052 aluminum alloy sheet under cyclic plastic deformation at room temperature

Ductile fracture behavior of a 5052 aluminum alloy sheet undergoing cyclic plastic deformation is investigated in order to clarify the effect of cyclic plastic deformation on formability enhancement in incremental stretch sheet forming at room temperature. In the incremental forming, formability markedly increases owing to strain distribution and accumulation effects. The former effect is activated when the deformation region expands along tool paths. Thus, localization of deformation, which leads to necking or fracturing, can be prevented. On the other hand, local strain is accumulated without fracturing when a blank sheet is repeatedly subjected to out-of-plane deformation at the same position. In this paper, the effect of the strain accumulation due to cyclic deformation generated by bending and unbending is primarily focused on to discuss the effect on deformability. To apply cyclic plastic straining to the specimen, a cyclic stretch-bending test was adopted. A cyclic tensile test was also conducted for larger bending curvature. The experimental results show that cyclic bending–unbending affects the ductility of sheet metals. The fractography obtained by scanning electric microscopy also indicates that fewer and smaller voids are observed particularly on bending the inner side than on the outer side.     

On the reverse mode of fcc deformation twinning

The reverse mode of face-centered cubic (fcc) deformation twinning is found to be responsible for the formation of kink bands during compression of pre-twinned Cu–8.5 at.% Al single crystals. The formation of kink bands is described, and a very high level of activity of the reverse fcc twinning is established. It is also found that the reverse twinning is driven at a critical resolved shear stress lower by well over factor of 2 in comparison with the direct twinning mode operating in Cu–8.5 at.% Al single crystals during primary tensile deformation. This discovery of the reversible plastic mode of fcc twinning should find important application in the theory and modeling of large plastic deformations of fcc materials, including cyclic deformation phenomena.     

Plastic instability in polycrystalline metals after low temperature irradiation

The dose dependence of plastic instability behavior has been investigated for polycrystalline metals after neutron irradiation at low temperatures (<200 °C). The analyzed materials consist of 10 body-centered cubic (bcc), 7 face-centered cubic (fcc), and 2 hexagonal close packed (hcp) metals. It was found that the metals after irradiation showed necking at yield when the yield stress exceeded the true plastic instability stress, σ_IS, for the unirradiated material. It was also shown that σ_IS was almost independent of dose below a critical dose. The critical dose is called the dose to plastic instability at yield, D_C, because at higher doses the material shows necking at yield. The D_C values ranged from 0.002 to 0.2 dpa for bcc and hcp metals, except for a high purity iron, that had a D_C value of 6 dpa; whereas the fcc metals gave generally high values ranging from 0.1 to 40 dpa. It is attempted to explain the dose independence of the plastic instability stress by a straightforward shifting of tensile curves by the appropriate strain corresponding to the radiation-induced increase in yield stress. The dose independence of strain-hardening behavior suggests that radiation-induced defects and deformation-produced dislocations give similar net strain-hardening effects.     

7055铝合金高温塑性变形的热模拟研究

采用Gleeble-1500D热模拟机研究了7055铝合金在应变速率为0.01、0.1和1s-1、变形温度为300～450℃,最大真应变为0.7条件下的高温塑性变形行为,分析了合金流变应力与应变速率、变形温度之间的关系,计算了合金高温塑性变形时的变形激活能,并观察了合金变形过程中显微组织变化情况。结果表明:合金在热变形过程中流变应力随温度的升高而减小,随应变速率的增加而增大,7055铝合金的高温塑性变形行为可以用包含Zener-Hollomon参数的流变应力方程进行描述。该合金在实验条件范围内热变形以动态回复为主要软化机制并伴随极少量的再结晶发生。     

Strain rate-dependent deformation in bulk metallic glasses

Metallic glasses have metastable structures. As a result, their plastic deformation is dependent upon structural dynamics. In the present paper, we present data obtained from Zr-base and La-base metallic glasses and discuss the kinetic aspects of plastic deformation, including both homogeneous and heterogeneous deformation. In the case of homogeneous deformation (typically occurring in the supercooled liquid region), Newtonian behavior is not universally observed and usually takes place only at low strain rates. At high strain rates, non-Newtonian behavior is usually observed. It is demonstrated that this non-Newtonian behavior is associated with in situ crystallization of the amorphous structure. In the case of heterogeneous deformation (occurring at room temperature), deformation is controlled by localized shear banding. The plastic deformation of a La-base metallic glass is also investigated using instrumented nanoindentation experiments over a broad range of indentation strain rates. At low rates, the load-displacement curves during indentation exhibit numerous serrations or pop-ins, but these serrations become less prominent as the indentation rate is increased. Using the tip velocity during pop-in as a gauge of serration activity, we find that serrated flow is only significant at indentation strain rates below a certain critical value.     

块状非晶态合金Zr65Al7.5Ni10Cu17.5的室温单轴压缩断裂行为

利用IUTM和SEM研究了Zr65Al7 5Ni10Cu17.5块状非晶合金的室温单轴压缩变形和断裂行为.该合金的室温压缩变形过程主要表现为弹性和塑性变形,并且塑性变形阶段没有加工硬化现象.宏观形貌观察发现样品形成与压缩方向呈约45°的变形带.塑性变形以剪切带粘性流层相对滑动的方式进行,在变形过程中形成脉络状断面形貌.裂纹于塑性变形积累到一定程度之后,在切变变形较大的区域表面形成.裂纹从萌生、扩展到断裂,时间极短.流变积累和应力集中综合作用的结果导致断裂.     

Effects of nanocrystalline and ultrafine grain sizes on constitutive behavior and shear bands in iron

The mechanical behaviors of consolidated iron with average grain sizes from tens of nanometers to tens of microns have been systematically studied under uniaxial compression over a wide range of strain rates. In addition to the well-known strengthening due to grain size refinement, grain size dependence is observed for several other key properties of plastic deformation. In contrast with conventional coarse-grained Fe, high-strength nanocrystalline and submicron-grained Fe exhibit diminished effective strain rate sensitivity of the flow stress. The observed reduction in effective rate sensitivity is shown to be a natural consequence of low-temperature plastic deformation mechanisms in bcc metals through the application of a constitutive model for the behavior of bcc Fe in this strain rate and temperature regime. The deformation mode also changes, with shear localization replacing uniform deformation as the dominant deformation mode from the onset of plastic deformation at both low and high strain rates. The evolution and multiplication of shear bands have been monitored as a function of plastic strain. The grain size dependence is discussed with respect to possible enhanced propensity for plastic instabilities at small grain sizes.     

T92/HR3C异种钢焊接接头高温拉伸变形及断裂行为

采用500℃和625℃拉伸试验,研究T92/HR3C异种钢管接头的高温变形及其断裂行为。结果表明,在高温拉伸过程中,焊缝、T92侧热影响区(HAZ)及母材(不包含颈缩段)均未发生明显的塑性变形及组织结构的变化,而HR3C侧母材晶粒明显被拉长,HR3C侧HAZ的拉伸变形不明显。HR3C母材塑性变形量随温度升高而明显降低,孪晶回复越少。高温拉伸断口位于T92侧HAZ的细晶区(FGHAZ),呈正断加剪切断的混合断裂方式,均与室温状态下该焊接接头的拉伸变形及断裂行为不同。应力三轴度理论可很好地解释该接头高温短时拉伸变形及断裂特征。     

Anomalies of the plastic yield stress in the intermetallic compound Fe–30 at.% Al

The plastic properties of Fe–30 at.% Al were investigated in compression, cyclic tension/compression and shear tests between room temperature and 870 K. At elevated temperatures in all tests a positive temperature dependence of the yield stress was observed. At room temperature an asymmetry of the flow stress with respect to the deformation direction (tension or compression) was found, which disappeared for temperatures T⩾570 K. The results are discussed on the basis of the present state of knowledge about the decoupling of D0₃-superdislocations at elevated temperatures and the deformation behaviour of b.c.c. metals at low temperatures.