Quasi-static Tensile and Compressive Behavior of Nanocrystalline Tantalum based on Miniature Specimen Testing—Part I: Materials Processing and Microstructure

Grain size reduction of metals into ultrafine-grained (UFG, grain size 100 nm < d < 1000 nm) and nanocrystalline (NC, d < 100 nm) regimes results in considerable increase in strength along with other changes in mechanical behavior such as vanishing strain hardening and limited ductility. Severe plastic deformation (SPD) has been among the favored technologies for the fabrication of UFG/NC metals. Primary past research efforts on SPD UFG/NC metals have been focused on easy-to-work metals, especially face-centered cubic metals such as copper, nickel, etc., and the limited efforts on body-centered cubic metals have mainly focused on high strain rate behavior where these metals are shown to deform via adiabatic shear bands. Except for the work on Fe, only a few papers can be found associated with UFG/NC refractory metals. In the first part of the present work (Part I), high-pressure torsion (HPT) is used to process UFG/NC tantalum, a typical refractory metal. The microstructure of the HPT disk as a function of radial location as well as orientation will be examined. In the subsequent part (Part II), the location-specific mechanical behavior will be presented and discussed. It is suggested that refractory metals such as Ta are ideal to employ SPD technology for microstructure refinement because of the extremely high melting point and relatively good workability.     

Deformation behavior of an ultrafine-grained Al–Mg alloy produced by equal-channel angular pressing

Presented here is the deformation behavior of Al–1.5 wt% Mg alloy severely plastically deformed to equivalent pre-strains of 8 and 13 using the equal-channel angular pressing technique. The average subgrain size after severe plastic deformation was 280 and 230 nm respectively. Strain rate change and stress relaxation tests in the range 10⁻⁴–10⁻² s⁻¹ and 298–523 K were performed. The strain rate sensitivity of ultrafine-grained (UFG) Al–1.5Mg was enhanced and the peak strain rate sensitivity shifted to lower temperatures as compared with the coarse-grained (CG) alloy. The increased strain rate sensitivity is a direct consequence of the reduced activation volume. The increase in pre-strain from 8 to 13 has a small effect on both the microstructural refinement and the subsequent deformation behavior. With increasing temperature the UFG material softens compared with the CG material. This demarcation has been clearly shown on a strain rate by temperature plot. Refinement of grain results in an enhanced solute drag regime, primarily due to the decreased activation energy of diffusion.     

Studies of deformation mechanisms in ultra-fine-grained and nanostructured Zn

The temperature, strain rate, grain size and grain size distribution effects on plastic deformation in ultra-fine-grained (UFG) and nanocrystalline Zn are systematically studied. The decrease of ductility with the decrease of average grain size could be an inherent effect in nanocrystalline materials, that is, not determined by processing artifacts. The superior ductility observed in UFG Zn may originate from both dislocation creep within large grains and grain boundary sliding of small nanograins. The stress exponent for dislocation creep is about 6.6. The activation energy for plastic deformation in UFG Zn is close to the activation energy for grain boundary self diffusion in pure Zn.     

Microstructural evolution and its influence on creep and stress relaxation in nanocrystalline Ni

Indentation creep and stress relaxation tests were performed on rolled and annealed nanocrystalline (NC) Ni to study the influence of microstructure evolution on plastic deformation behavior. Dislocation density (ρ) increases with increasing rolling strain, reaching a maximum at 20% strain, followed by a decrease at larger strain. The ρ of Ni decreases significantly with increasing annealing temperature. Softening behavior is observed in NC Ni with grain size <40 nm, i.e., an inverse-like Hall–Petch effect. For rolling NC Ni, both creep strain rate and rate sensitivity first increase and then decrease, while those of annealed Ni continuously decrease. With increasing grain size, creep activation volume unusually decreases first, then starts to rise, which is different from that of coarse-grained metal. A model involving dislocation annihilation and emission at grain boundaries under indenters is used to explain the anomalous behavior of rolled and annealed Ni, respectively.     

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.     

Deformation mechanisms in nanocrystalline palladium at large strains

The mechanical behaviour and microstructure evolution of nanocrystalline palladium was investigated. Material with an initial grain size ～10 nm was prepared by inert gas condensation. Instrumented high-pressure torsion straining was used to characterize the flow stress during plastic deformation to shear strains up to 300. A change in primary deformation mechanism was induced by stress-induced grain growth. For grain sizes <40 nm, grain boundary mediated processes (shear banding, grain boundary sliding and grain rotation) controlled the deformation, with dislocation slip, twinning, and grain boundary diffusion providing the accommodation. For larger grain sizes, the operative deformation mechanism was dislocation slip.     

Mechanical behaviour and in situ observation of shear bands in ultrafine grained Pd and Pd–Ag alloys

An in situ tensile test with grey scale correlation has been performed to study the deformation process in ultrafine grained (UFG) Pd and Pd–x at.% Ag (x = 5 or 20) alloys produced by high-pressure torsion. Shear band nucleation and propagation was found to be an important deformation mechanism after strain localization during the tensile test. The underlying microscopic mechanism is related to cooperative grain boundary sliding. Moreover, the additional influence of stacking fault energy was found to change the nature of the deformation mechanism from localized strain in Pd to more homogeneous deformation in Pd–20% Ag. In situ analysis and the findings are new and give innovative insight into the basics of deformation in UFG face-centred cubic metals.     

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.     

超细晶工业纯钛的变形、应变速率敏感性和激活体积

在温度为250~450 ℃、应变速率为1×10-4-1 s-1的条件下，对超细晶工业纯钛进行变速率压缩实验，计算超细晶工业纯钛的应变速率敏感性因子和激活体积，并研究超细晶工业纯钛的变形行为。研究结果表明：超细晶工业纯钛在稳态变形阶段存在流变软化效应，这是受变形过程中大角度晶界和位错活动所控制的。超细晶工业纯钛的应变速率敏感性因子和激活体积在数值上都相对较低，应变速率敏感性随着变形温度的升高而增加，但激活体积独立于变形温度。应变速率敏感性和激活体积的数值表明晶粒内部位错之间的交互作用几乎不发生，而位错与晶界之间的交互作用显著影响超细晶工业纯钛的塑性变形。     

Multiscale modeling of plastic deformation of molybdenum and tungsten: I. Atomistic studies of the core structure and glide of 1/2〈1 1 1〉 screw dislocations at 0 K

Owing to their non-planar cores, 1/2〈1 1 1〉 screw dislocations govern the plastic deformation of body-centered cubic (bcc) metals. Atomistic studies of the glide of these dislocations at 0 K have been performed using Bond Order Potentials for molybdenum and tungsten that account for the mixed metallic and covalent bonding in transition metals. When applying pure shear stress in the slip direction significant twinning–antitwinning asymmetry is displayed for molybdenum but not for tungsten. However, for tensile/compressive loading the Schmid law breaks down in both metals, principally due to the effect of shear stresses perpendicular to the slip direction that alter the dislocation core. Recognition of this phenomenon forms a basis for the development of physically based yield criteria that capture the breakdown of the Schmid law in bcc metals. Moreover, dislocation glide may be preferred on {1 1 0} planes other than the most highly stressed one, which is reminiscent of the anomalous slip observed in many bcc metals.     

New experimental insight into the mechanisms of nanoplasticity

The evolution of microstructure and texture of a nanocrystalline Pd–10 at.% Au alloy (initial grain size 16 nm) subjected to severe plastic deformation by high-pressure torsion (HPT) at room temperature is investigated by X-ray line profile analysis and X-ray microdiffraction, respectively. In addition, changes in the microhardness are measured and the texture is modeled. During HPT the microstructure changes: the crystallite size goes over the maximum, the dislocation density goes through a minimum and the density of stacking faults decreases at/up to a shear strain of ～1, corresponding to a grain size of 20 nm. Starting with a random texture, typical brass-type shear components develop at a shear strain above ～1. The microhardness with decreasing crystallite size goes over a maximum at ～20 nm. The correlated changes in microstructure, texture and strength strongly suggest the transition from a dislocation slip to a grain boundary sliding (GBS)-dominated deformation mechanism. The unexpected brass-type texture and its deviation from the ideal position can be simulated with the Taylor model assuming dominant partial dislocation slip and a certain contribution of GBS, respectively. Taken together, the results of many techniques applied to the same material, in particular those of the texture investigations, provide a more comprehensive and consistent picture of nanoplasticity than reported before for face-centered cubic metals.     

Microscopic description of plasticity in computer generated metallic nanophase samples: a comparison between Cu and Ni

Simulations are reported on the plastic behavior of two model f.c.c. metals, Ni and Cu, with different stacking fault energies, and average grain sizes in the range of 3–12 nm. A change in deformation mechanism is observed: at the smallest grain sizes all deformation is accommodated in the grain boundaries. At higher grain sizes intragrain deformation is observed. Analysis of the atomic configurations shows that intrinsic stacking faults are produced by motion of Shockley partial dislocations generated and absorbed in opposite grain boundaries. In Cu the stacking faults are observed at smaller grain sizes than in Ni (8 nm in Cu, 12 nm in Ni) which is attributed to the lower stacking fault energy. Shockley partial dislocations appear on slip systems that are not necessarily those favored by the Schmid factor. Atomic displacement analysis shows that deformation starts at triple points, with grain boundary sliding followed by the creation of intragrain partial dislocations.     

STRAIN AND FRACTURE OF LIQUID PHASE SINTERED ALLOY 90W-TNi-3Fe

研究了液相烧结的90W-7Ni-3Fe合金的形变和断裂特征。试样由基体相首先开始屈服,承受塑性变形。当界面结合强度较低时,试样首先沿界面裂开,而当界面结合强度增高到高于钨的解理断裂应力时,试样同时发生钨球的穿晶解理开裂和基体相的塑性撕裂。 氢是造成烧结试样界面脆化的重要原因之一。真空热处理能去除界面孔隙中的氢以及钨颗粒和基体相界面之中的氢,从而提高界面的结合强度,使试样的断裂强度和塑性同时得到提高。     

120°模具室温ECAP制备工业纯钛的热压缩变形行为

在Gleeble-1500热模拟机上对室温120°模具等径弯曲通道变形(ECAP)制备的平均晶粒尺寸为200nm的工业纯钛(CP-Ti)进行等温变速压缩实验,研究超细晶(UFG)工业纯钛在变形温度为298～673K和应变速率为10-3～100s-1条件下的流变行为。利用透射电子显微镜分析超细晶工业纯钛在不同变形条件下的组织演化规律。结果表明:流变应力在变形初期随应变的增加而增大,出现峰值后逐渐趋于平稳;峰值应力随温度的升高而减小,随应变速率的增大而增大;随变形温度的升高和应变速率的降低,应变速率敏感性指数m增加,晶粒粗化,亚晶尺寸增大,再结晶晶粒数量逐渐增加;超细晶工业纯钛热压缩变形的主要软化机制随变形温度的升高和应变速率的降低由动态回复逐步转变为动态再结晶。     

Cyclic deformation of ultrafine-grained aluminum

Cyclic deformation of ultrafine-grained (UFG) Al with different grain sizes has been studied. It was found that UFG Al had shorter fatigue life than its coarse-grained counterparts. For UFG Al, the fatigue life decreases with decreasing grain size. Shear bands (SBs) shorten fatigue life. SBs are always inclined at 45° to the loading axis, and extend across the whole specimen. A SB is a thin sheet of tangled dislocations that have different Burgers vectors; its thickness is much less than the grain size. The strain–stress field inside a SB is very high. SBs produce shear steps, but not surface extrusions/intrusions, on the specimen surface. Thick shear bands (TSBs), about 200–300 μm, were found in the 6.36 μm grain size specimens, which also inclined 45° to the loading axis. TSBs consist of dislocation cells. The formation of TSBs does not reduce the fatigue life.     

Grain-size effect on plastic flow in nanocrystalline cobalt by atomistic simulation

Molecular dynamics simulation is employed to investigate the plastic flows in nanocrystalline (nc) hexagonal close-packed cobalt under uniaxial tensile deformation. In nc-Co samples modeled by a semi-empirical tight-binding potential, different deformation behaviors such as nucleation and growth of disordered atom segments (DAS) inside grains, deformation-induced hexagonal close-packed to faced-centered cubic transformation, partial dislocation activities are identified at different grain sizes (4–12 nm). At high stresses (1.2–3.2 GPa) and low temperatures (77–470 K), growth of DAS and their interaction with stacking faults are found to dominate the deformation process, even when the grain size is as small as 4 nm. A model for plastic flow generated by DAS inside grains is proposed. The strain rates and the inverse Hall–Petch-like behaviors in nc-Co with sub-10 nm grain sizes can be well described by the DAS plastic-flow model.     

Size dependent phase transformation and mechanical behaviors in nanocrystalline Ta thin films

Nanocrystalline (NC) Ta thin films with various thicknesses (t = 600, 1200, and 2200 nm) were grown on Si (100) substrate by using magnetron sputtering system. A phase transformation from β-Ta to α-Ta was observed when the thickness reduced to 600 nm, which is rationalized by employing a thermodynamic model. It was interesting to find that the α-Ta phase exhibited tetrahedral pyramidal grain morphology, while the β-Ta had equiaxial grain. Hardness and strain rate sensitivity (SRS, m) were measured as a function of film thickness t. Compared with the reduction in SRS as the grain size decreased in the submicron bcc metals, the NC Ta thin films with average grain size smaller than ~72 nm showed an opposite trend, both in the α-Ta and in the β-Ta phase. Improved m was observed as the grain size reduced but the increasing trend was not continuous between different phases. This unusual variation trend of m in the NC Ta was explained by a model based on the traditional double-kink mechanism coupled with a GB-mediated dislocation process, which agrees well with the experiment results.     

Deformation in metals after low-temperature irradiation: Part II – Irradiation hardening,strain hardening,and stress ratios

Effects of irradiation at temperatures ⩽200 °C on tensile stress parameters are analyzed for dozens of body-centered cubic (bcc), face-centered cubic (fcc), and hexagonal close packed (hcp) pure metals and alloys, focusing on irradiation hardening, strain hardening, and relationships between the true stress parameters. Similar irradiation-hardening rates are observed for all the metals irrespective of crystal type. Typically, irradiation-hardening rates are large, in the range 100–1000 GPa/dpa, at the lowest dose of <0.0001 dpa and decrease with dose to a few tens of MPa/dpa or less at about 10 dpa. However, average irradiation-hardening rates over the dose range of 0 dpa−D_C (the dose to plastic instability at yield) are considerably lower for stainless steels due to their high uniform ductility. It is shown that whereas low-temperature irradiation increases the yield stress, it does not significantly change the strain-hardening rate of metallic materials; it decreases the fracture stress only when non-ductile failure occurs. Such dose independence in strain-hardening behavior results in strong linear relationships between the true stress parameters. Average ratios of plastic instability stress to unirradiated yield stress are about 1.4, 3.9, and 1.3 for bcc metals (and precipitation hardened IN718 alloy), annealed fcc metals (and pure Zr), and Zr-4 alloy, respectively. Ratios of fracture stress to plastic instability stress are calculated to be 2.2, 1.7, and 2.1, respectively. Comparison of these values confirms that the annealed fcc metals and other soft metals have larger uniform ductility but smaller necking ductility when compared to other materials.     

Evolution of dislocation cells during plastic deformation

In recent years, materials with ultrafine grain size(UFG) have attracted much attention. By using severe plastic deformation(SPD) techniques, materials with fine grain size as small as 200 - 250 nm have been obtained.However, the nature of the grain boundaries has not been theoretically understood. It is still an unsolved question whether or not finer grain sizes down to 100 nm could he reached. A semi-quantitative model for the evolution of dislocation cells in plastic deformation was proposed. The linear stability analysis of this model leads to some interesting results, which facilitate the understanding of the formation of cell structures and of the factors determining the lower limit of the cell size of SPD materials.     

Cyclic deformation of nanocrystalline and ultrafine-grained nickel

The cyclic deformation behavior of ultrafine-grained (UFG) Ni samples synthesized by the electrodeposition method was studied. Different from those made by severely plastic deformation, the UFG samples used in this study are characterized by large-angle grain boundaries. Behaviors from nanocrystalline Ni and coarse-grained Ni samples were compared with that of ultrafine-grained Ni. With in situ neutron diffraction, unusual evolutions of residual lattice strains as well as cyclic hardening and softening behavior were demonstrated during the cyclic deformation. The microstructural changes investigated by TEM are discussed with respect to the unusual lattice strain and cyclic hardening/softening.