Low Homologous Temperatures Research Articles

Repulsion leads to coupled dislocation motion and extended work hardening in bcc metals

Work hardening in bcc single crystals at low homologous temperature shows a strong orientation-dependent hardening for high symmetry loading, which is not captured by classical dislocation density based models. We demonstrate here that the high activation barrier for screw dislocation glide motion in tungsten results in repulsive interactions between screw dislocations, and triggers dislocation motion at applied loading conditions where it is not expected. In situ transmission electron microscopy and atomistically informed discrete dislocation dynamics simulations confirm coupled dislocation motion and vanishing obstacle strength for repulsive screw dislocations, compatible with the kink pair mechanism of dislocation motion in the thermally activated (low temperature) regime. We implement this additional contribution to plastic strain in a modified crystal plasticity framework and show that it can explain the extended work hardening regime observed for [100] oriented tungsten single crystal. This may contribute to better understanding the increase in ductility of highly deformed bcc metals.

Simultaneous improvement in strength and ductility of severely deformed niobium alloy

The present study is on the improved monotonic response of a severely deformed niobium-based alloy. Nb-1Zr alloy was processed at room temperature using equal channel angular extrusion and cold rolling. After 16 passes, a more than 2.5 times increase in yield strength is accompanied by a ductility level exceeding the as-received state. Cold rolling of the processed billets resulted in tensile strengths exceeding 1 GPa. The enhancement in both strength and ductility is explained by the microstructural evolution and the low homologous processing temperature.

Internal oxidation of Ag-xIn alloys at low homologous temperature

Internal oxidation of Ag-xIn alloys (x = 10, 13, 16 at%) was studied in an air atmosphere at a relatively low homologous temperature (240 °C), where lattice diffusion is negligible. After exposure, metallic Ag nodules were present on the surfaces of all alloys, expelled to relieve stress due to internal oxidation, confirming that such coarse morphologies can be produced by short-circuit transport. Crystallographic orientation and grain boundary misorientation were found to influence nodule expulsion tendency, and the propensity for internal or external oxidation. Further analytical TEM characterization was used to study the nanoscale morphology and chemistry of nodules and internal oxide(s).

Grain growth at fragmentation stage in commercial purity aluminium deformed by high pressure torsion

The mechanism for grain boundary motion at the fragmentation stage of high pressure torsion still remains uncertain as measuring grain growth velocity in such stage is challenging. A new approach is proposed in this study to calculate the grain growth velocity in the fragmentation stage. The high grain boundary motion rate and the low apparent activation energy indicate the mechanism for coarse grain growth in commercial purity aluminium in the fragmentation stage at low homologous temperature is attributed to stress-induced boundary motion rather than discontinuous dynamic recrystallization.

Influence of ion irradiation on the nanomechanical properties of thin alumina coatings deposited on 316L SS by PLD

In terms of nuclear applications, ceramics are seen as a particularly promising class of materials due to their chemical inertness and relatively high radiation resistance. However, since ceramics exhibit high brittleness at low homologous temperatures, the application of monolithic ceramics as structural components of nuclear power plants is rather limited. On the other hand, deposition of ceramic coatings on a metallic substrate may result in an excellent combination of mechanical, corrosion and radiation properties (especially at high temperature). In this work, Al2O3 coatings deposited by Pulsed Laser Deposition (PLD) on 316L SS substrate at room temperature were investigated. In order to simulate the influence of neutrons, alumina-coated and 316L SS samples were irradiated at room temperature with 250 keV Au+ and 150 keV Fe2+ ions, respectively. The influence of ion irradiation on nanomechanical properties of the studied materials was investigated by means of Nanoindentation (NI) technique. Based on the obtained results, nanomechanical properties as a function of radiation damage level were determined and linked to the results of Grazing Incidence X-Ray Diffraction (GIXRD) analysis and other structural data available in the literature. Irradiation-induced softening and hardening were observed, respectively, in the alumina coating and 316L SS. Reported differences, which are induced by the irradiation effects, are considered to be due to the different microstructures of the pristine materials.

On the relative accuracy of power law analyses for uniaxial low stress steady state high homologous temperature deformation

If the mathematical operations are correct, uniaxial low stress, steady state high homologous temperature creep and steady state structural Superplasticity phenomena can be well described by a ‘power law’ that relates the applied stress to the strain rate of deformation (Padmanabhan et al., 2015). Isothermal dislocation creep, which exhibits no grain size dependence, displays a linear log strain rate – log stress relationship till there is a change in the rate controlling mechanism. In contrast, during grain-size dependent, isothermal steady state structural superplastic flow, even within narrow strain rate ranges the slope changes, notwithstanding the presence of the same rate controlling mechanism. In both the phenomena the stress exponent, n, decreases with increasing temperature even for a constant rate controlling mechanism (Padmanabhan, 1973). Four analytical methods, differing in details, are based on the power law equation, viz., a procedure used by experimental scientists, its improved variant, a method in vogue in rheology and a procedure due to Padmanabhan et al., 2015. By examining experimental data pertaining to many systems, it is demonstrated that the method of rheology does not follow the tenets of Dimensional Analysis and that the scatter in the predictions is the maximum for this case. The method of experimental scientists ignores the temperature dependence of the stress exponent and this leads to significant discrepancies between the measured and predicted properties. When mathematics is correct and the relevant physical situation is taken into account, the other two methods, i.e. the improved method of the experimental scientists and that of Padmanabhan et al., 2015, lead to similar results, but the latter analysis is more accurate because of its better normalizing procedure. It is also simpler. A missing detail in the earlier paper (Padmanabhan et al., 2015), viz., estimation of the grain-size exponent of the strain rate predicted for isothermal, steady state structural superplastic flow in terms of the Buckingham Pi Theorem is also furnished here.

Survey of grain boundary migration and thermal behavior in Ni at low homologous temperatures

In this study, we report the migration and thermal behavior of 388 Ni grain boundaries that were surveyed by atomistic simulations using a recently proposed ramped synthetic driving force method. With this approach, the critical driving force required to initiate the grain boundary motion rather than the grain boundary mobility was extracted to evaluate the ability of a specific grain boundary to move. It was found that this method was capable of capturing the migration and thermal behavior of almost all of the 388 Ni grain boundaries regardless of their type at low homologous temperatures, e.g., 100−600K (or ∼ 0.06–0.35 the melting point of Ni), which was not possible by using previous atomistic methods. Rich and complex thermal behaviors were found in this temperature regime, many of which were reported for the first time, for example, in grain boundaries that are typically considered to be immobile such as the well-known coherent twin boundary. Furthermore, strong structure-property relationship was found in the tilt and twist boundaries whose migration and thermal behavior was correlated to their tilt/twist axis and angle. In particular, non-thermally activated behavior was commonly observed in most surveyed tilt and twist Ni grain boundaries in this study when the tilt/twist angle was relatively small.

High-Pressure Torsion Deformation Induced Phase Transformations and Formations: New Material Combinations and Advanced Properties

Heavy plastic shear deformation at relatively low homologous temperatures is called high-pressure torsion (HPT) deformation, which is one method of severe plastic deformation (SPD). The aim of the paper is to give an overview of a new processing approach which permits the generation of innovative metastable materials and novel nanocomposites by HPT deformation. Starting materials can be either coarse-grained multi-phase alloys, a mixture of different elemental powders or any other combination of multiphase solid starting materials. After HPT processing, the achievable microstructures are similar to the ones generated by mechanical alloying. Nevertheless, one advantage of the HPT process is that bulk samples of the different types of metastable materials and nanocomposites are obtained directly during HPT deformation. It will be shown that different material combinations can be selected and materials with tailored properties, or in other words, materials designed for specific applications and the thus required properties, can be synthesized. Areas of application for these new materials range from hydrogen storage to materials resistant to harsh radiation environments.

Slip-stimulated grain boundary sliding in Ti-6Al-4 V at room temperature

Grain boundary sliding has a substantial contribution to high temperature deformation of α/β titanium alloys. Despite the low homologous temperature, the operation of grain boundary sliding is presently reported at room temperature using a pre-deformed Ti-6Al-4 V specimen. Scanning electron microscopy and atomic force microscopy observations were combined with electron back-scattered diffraction characterization in order to clarify the underlying mechanisms. Intragranular prismatic slip seemingly triggers grain boundary sliding. The analysis of activated slip systems while accounting for crystallographic aspects unveiled several highly influential parameters. Notably, strain partitioning, as a result of plastic anisotropy and microstructural heterogeneities, promotes grain boundary sliding. The orientation of the incident slip band relative to the boundary plane was also found correlated with the occurrence of grain boundary sliding. These findings lay the foundations for identification of conditions favoring grain boundary sliding in titanium alloys and highlight an enriched panel of possible interactions between slip bands and grain boundaries that is likely to play a critical role in crack initiation processes.

Low-temperature anatase-to-rutile phase transformation and unusual grain coarsening in titanium oxide nanopowders by high-pressure torsion straining

Simultaneous effect of pressure, temperature and plastic strain on structural evolution of titania ceramic nanopowder was investigated using the high-pressure torsion (HPT) method. Unlike static annealing, in which the anatase-to-rutile phase transformation occurred at 1173 K, the transformation was observed at low temperatures as 473–573 K by HPT processing. The formation of rutile phase accompanied with dynamic recrystallization and an unusual grain growth, indicating that severe plastic deformation at low homologous temperatures does not necessarily result in grain refinement of metastable nanoceramics. Besides the coarsened grains of rutile phase, a high-pressure TiO2-II columbite phase was also detected in small nanograins.

Stress relaxation in ultrafine-grained copper at low homologous temperatures

The temperature and rate sensitivity of the flow stress in ultrafine-grained (UFG) copper prepared by equal-channel angular hydroextrusion was studied. Tensile tests and stress relaxation tests were carried out in the temperature range of 77 to 295 K. It was established that, with increasing temperature, the flow stress decreases monotonously, and the activation volume reaches a maximum of ∼190b3 at a temperature of ∼(200 ± 20) K. The results of the experiment are discussed in terms of two thermally activated plastic deformation mechanisms, namely the intersection of “forest” dislocations and dynamic recovery, which are capable of significantly distorting stress relaxation kinetics in UFG copper.

In-situ quantification of intra and intergranular deformation in pure magnesium using full-field measurements at low and high strain rates

Due to the presence of weak grain boundaries as well as significantly coarse grain structure with minimal deformability, grain boundary cracking is almost an inevitable source of failure when a cast magnesium-based alloy is deformed at low homologous temperatures. The main objective in this work is to quantitatively investigate the contribution of inter- and intragranular deformation to the macroscale deformability and ductility of nominally pure as-cast magnesium subjected to quasi-static and dynamic loading. The proposed mesoscale full-field measurement approach is first presented and verified through measurements carried out to investigate deformation and grain boundary cracking in cast magnesium subjected to quasi-static loading. The method is then extended into the analysis of mesoscale deformation and failure under dynamic loading conditions using an experimental setup consisting of a split Hopkinson pressure bar (SHPB) and a high-speed imaging system. In both cases, the effect of the initial grain configuration on the local deformation response of Mg is investigated. The results indicate that the contribution of grain boundary region deformation to the total deformation exerted on the Mg samples is significant and depends on the initial grain configuration. Also, the strain rate sensitivity of the material is found to be dominated by the material deformation in the vicinity of grain boundaries.

Void growth and coalescence in irradiated copper under deformation

A decrease of fracture toughness of irradiated materials is usually observed, as reported for austenitic stainless steels in Light Water Reactors (LWRs) or copper alloys for fusion applications. For a wide range of applications (e.g. structural steels irradiated at low homologous temperature), void growth and coalescence fracture mechanism has been shown to be still predominant. As a consequence, a comprehensive study of the effects of irradiation-induced hardening mechanisms on void growth and coalescence in irradiated materials is required. The effects of irradiation on ductile fracture mechanisms - void growth to coalescence - are assessed in this study based on model experiments. Pure copper thin tensile samples have been irradiated with protons up to 0.01 dpa. Micron-scale holes drilled through the thickness of these samples subjected to uniaxial loading conditions allow a detailed description of void growth and coalescence. In this study, experimental data show that physical mechanisms of micron-scale void growth and coalescence are similar between the unirradiated and irradiated copper. However, an acceleration of void growth is observed in the later case, resulting in earlier coalescence, which is consistent with the decrease of fracture toughness reported in irradiated materials. These results are qualitatively reproduced with numerical simulations accounting for irradiation macroscopic hardening and decrease of strain-hardening capability.

Strong ductile bulk tungsten

Tungsten has long faced the problem of limited to no ductility in bulk polycrystalline form, due in part to the low energy needed for intergranular fracture. This limited ductility can be overcome through processing by dramatically reducing the cross sectional area following a conventional temperature step-down method that improves ductility through elongation of grain boundaries and development of a {110} fiber texture along the length of the material. While this technology is over 100 years old, little progress has been made to improve the ductility of bulk material. In the current investigation plastically deformed bulk tungsten was observed to have a room temperature tensile ductility between 17% and 23% total elongation at failure in bend specimens, and a flexural yield strength near ~ 3GPa. This improvement in ductility following multipass equal channel angular extrusion processing appears to be caused by a similarity in microstructure and texture to that of ductile tungsten wire. The onset temperature for transition to noticeable ductility is seen to decrease dramatically with the level of plastic strain. The results indicate that severe plastic deformation processing at low homologous temperatures may be an effective way to improve the ductility and toughness of bulk tungsten and other brittle crystalline metals.

Cold rolled tungsten (W) plates and foils: Evolution of the tensile properties and their indication towards deformation mechanisms

This paper is the second part of our series on ultrafine-grained (UFG) tungsten produced by cold rolling. The aim of this paper is to determine tensile properties, with a focus on uniform elongation and flow stress, and to identify the mechanism of plastic deformation in cold rolled tungsten plates and foils. For this purpose, five plates have been rolled out of a single sintered compact with increasing degrees of deformation, which enables the analysis of mechanical properties with changing microstructure without influence from the chemical composition.Uniaxial tensile tests have been performed in the range of room temperature to 800°C. The tensile curves show a clear trend of increased strength for finer microstructures and simultaneously evolving room temperature ductility in the low temperature regime for cold rolled plates. The thinnest foil (100μm) behaves uniquely by forming a plateau in the stress-strain curve, which hints at a change in the deformation mechanism.Analyzing the temperature and grain size dependence of the flow stress reveals that the bulk plasticity is still controlled by screw dislocations and further suggests a change in the dislocation-grain boundary interactions from blocking at low homologous temperatures to absorbing at elevated temperatures.A fracture analysis of cold rolled tungsten foils confirms a plastic deformation prior to fracture and reveals a steady transition from brittle towards lamellar fracture for the highly deformed tungsten foils.

Cyclically induced grain growth within shear bands investigated in UFG Ni by cyclic high pressure torsion

Abstract

Nanocrystalline Al-Mg with extreme strength due to grain boundary doping

Nanocrystalline Al-Mg alloys are used to isolate the effect of grain boundary doping on the strength of nanostructured metals. Mg is added during mechanical milling, followed by low homologous temperature annealing treatments to induce segregation without grain growth. Nanocrystalline Al -7at% Mg that is annealed for 1h at 200°C is the strongest alloy fabricated, with a hardness of 4.56GPa or approximately three times that of pure nanocrystalline Al. Micropillar compression experiments indicate a yield strength of 865MPa and a specific strength of 329kNm/kg, making this one of the strongest lightweight metals reported to date.

Nanoscale structure of submicron-thick sputter-deposited Pd films: Effect of the adatoms diffusivity by the film-substrate interaction

Pd films were deposited, at room temperature, on Ti and SiO2 substrates by normal (0°) and oblique (36° and 48°) incidence sputtering processes. Scanning electron microscopy was used to analyze the surface and cross morphology of the Pd films so to study the effect of the Pd adatoms surface diffusivity on the films nanostructure. In fact, even if the low homologous temperature (0.17) prescribes that the Pd film grows on both substrates in the Zone I of the structure zone model, some differences are observed: in normal incidence condition, the Pd film on the Ti substrate was found to be formed by tapered grains, while on the SiO2 substrate the Pd film exhibited a columnar morphology with the columns showing a cauliflower shape. Also in the oblique deposition configurations, notable differences in the nanoscale morphology of the Pd films grown in the same conditions are observed on the basis of the supporting substrate. These data suggest that, during depositions, the Pd adatoms diffusivity on the substrate, as determined by the film-substrate interaction both in normal and oblique deposition conditions, is crucial in determining the final film nanostructure. This aspect is semi-quantitatively discussed on the basis of the adsorption energy of the Pd atoms on Ti (0.69eV) and on SiO2 (0.27eV).

The influence of long-term annealing at room temperature on creep behaviour of ECAP-processed copper

Present work investigates the effect of long-term annealing at low homologous temperature (~0.22 Tm) on creep behaviour of pure copper (99.99%) processed by equal-channel angular pressing (ECAP). Coarse-grained pure copper was processed by 1 and 8 ECAP passes at room temperature. The ECAP-processed specimens were annealed at room temperature for 3 years. Microstructures of ECAP-processed specimens and their room temperature-annealed states were investigated by electron back scatter diffraction (EBSD) technique. Tensile tests were performed at room temperature under constant rate and at 373K in creep under constant load. It was found that long-term annealing at room temperature led to occurrence of large grains in microstructure which influenced the strength. The influence grew with number of ECAP passes.

Creep and stress relaxation of a FeMnSi-based shape memory alloy at low temperatures

The creep and stress relaxation behavior of a Fe–17Mn–5Si–10Cr–4Ni–1(V,C) (wt%) shape memory alloy at low homologous temperatures (−45°C<T<50°C) was systematically studied in stress and strain controlled tensile tests. At constant stresses at −45°C, the alloy exhibits pronounced creep up to 0.6% at 600MPa after only 30min holding time. If the strain is kept constant, a pronounced stress relaxation of up to 10% of the initial stress was observed. The final creep strains, the creep rates at a constant stress as well as the stress relaxation at a constant strain increase with decreasing temperature. In addition, the change of the recovery stress as a function of time in a restrained sample after 4% elongation and heating to different constant temperatures was monitored. It was observed that the increase of the final recovery stress is more pronounced when the holding temperature is increased. This behavior was explained with the time and temperature dependent formation of stress induced ε-martensite from the parent γ-austenite phase during mechanical loading according to the model of Kajiwara et al. as well as with the increased number of stacking faults at lower temperatures, which serve as nucleation sites for the ε-martensite formation.