Low Homologous Temperatures Research Articles

The role of new particle surfaces in synthesizing bulk nanostructured metallic materials by powder metallurgy

The role of new particle surfaces in synthesizing bulk nanostructured metallic materials by consolidation of nanostructured powders and nanopowders is analysed by developing three simple mathematical equations for calculating the α factor for different thermomechanical powder consolidation processes such as hot pressing, high pressure torsion and extrusion. The α factor is the fraction of the area of the powder particle surfaces newly formed during consolidation over the total particle surface area which includes both pre-existing surface area and the newly formed surface area. It is demonstrated that the values of the α factor calculated using these equations can be reasonably used to predict the level of inter-particle atomic bonding that is likely to be achieved through cold-welding by the above mentioned typical thermomechanical powder consolidation processes which also include high energy mechanical milling. Based on this analysis, it is clear that uniaxial hot pressing of a powder compact in a rigid die at low homologous temperatures (<0.5 T m) is unlikely to be capable of achieving a sufficiently high level of inter-particle atomic bonding for producing a high quality consolidated material, while processes involving a large amount of plastic deformation have such capabilities.

A viscoplastic behavior law for ferritic steels at low homologous temperature

The original viscoplastic framework at low homologous temperature is based on the coupling of the E. Rauch's original approach for mixing athermal and viscous term in the flow stress of ferritic steels, a general viscoplastic potential and a microstructure based quasi-static behavior law. The formalism gives accurate results in excellent agreement with available experimental results with only two adjusted parameters, over a wide range of strain rates and temperatures.

Simulating Time Temperature Transformation Diagram of Steel Using Artificial Neural Network

Design and development of steel is essentially governed by the Time-Temperature-Transformation (TTT) diagram. The diagram predicts the phase evolution during isothermal transformation schedules for a given chemistry. Selection of chemistry for obtaining a desired microstructure in steel under isothermal schedule needs determination of the TTT diagrams either by extensive experimental exercise or by rigorous thermodynamic calculations. Artificial neural network (ANN) technique has recently been employed as a versatile tool to predict the CCT diagrams of steels. The present work aims to identify a favorable composition capable of yielding an ultrafine bainitic microstructure by isothermal holding of austenite at low homologous temperature. To achieve this, TTT diagrams of varying compositions have been predicted a priori to reduce the required experimental trials. The exercise has led to the development of bainitic microstructure of nanoscale dimension in steel having 0.7C-2.0Mn-1.5Si-0.3Mo-1.5Cr (wt%). Experimental trial with the predicted composition of bainitic steel resulted into attractive combination of strength and ductility.

Influence and determinative factors of ion-to-atom arrival ratio in unbalanced magnetron sputtering systems

Low pressure sputtering with a controlled ratio of ion flux to deposited atom flux at the condensing surface is one of the main directions of development of magnetron sputtering methods. Unbalanced magnetron sputtering, by producing dense secondary plasma around the substrate, provides a high ion current density. The closed-field unbalanced magnetron sputtering system (CFUBMS) has been established as a versatile technique for high-rate deposition high-quality metal, alloy, and ceramic thin films. The key factor in the CFUBMS system is the ability to transport high ion currents to the substrate, which can enhance the formation of full dense coatings at relatively low value homologous temperature. The investigation shows that the energy of ions incidenced at the substrate and the ratio of the flux of these ions to the flux of condensing atoms are the fundamental parameters in determining the structure and properties of films produced by ion-assisted deposition processes. Increasing ion bombardment during deposition combined with increasing mobility of the condensing atoms favors the formation of a dense microstructure and a smooth surface.

Texture and High-Angle Boundary Formation during Deformation Processing

Among the phenomena leading to formation of high-angle boundaries during deformation processing at low homologous temperatures is the subdivision of prior grains and formation of deformation bands. Evidence for this phenomenon during processing of AA2004, a superplastic aluminum alloy, is reviewed; fragmentation of deformation bands leads to equiaxed grains and high-angle boundaries that support superplasticity. In addition to subdivision, groups of grains undergo lattice rotation toward one or the other variant of the β orientation fibers during plane-strain deformation of pure aluminum by ambient temperature rolling. The formation of equiaxed grains from banded structures during simple shear by equal channel angular pressing (ECAP) is also considered.

The role of Harper–Dorn creep at high temperatures and very low stresses

Creep experiments were conducted on single crystals of very high purity aluminum to evaluate the validity of the Harper–Dorn region of flow which occurs at very low stresses and high homologous temperatures. The results confirm the existence of a different flow process under these conditions but with a stress exponent closer to ∼3 rather than 1. Measurements show that the dislocation density within this low stress region varies with stress in a manner consistent with the behavior anticipated from an extrapolation of data reported in the regime of conventional power-law creep at high stresses. All of the experimental results are in reasonable agreement with earlier published data, including with the original data of Harper and Dorn when their results are plotted without incorporating a threshold stress.

Influence of Severe Thermomechanical Treatment on Formation of Nanocrystalline Structure in Ni 718 and Ni 718Plus Alloys and their Mechanical Properties

The influence of severe thermomechanical treatment via multiple forging on the formation of a nanocrystalline (NC) structure in bulk samples of Alloy 718 and ATI 718Plus has been investigated. It was observed that a step-wise decrease of processing temperature from 950 down to 575°C allowed the refinement of the initial coarse grain structure to a NC state. Investigations of structural changes in the deformed samples have shown that extending the temperature interval of dynamic recrystallization to low homologous temperatures resulted in the formation of a fine-grained recrystallized structure. The temperature of NC structure formation in ATI Alloy 718Plus was 50-100°С higher than that required for alloy 718. This was due to the presence of the additional γ′-phase, which increased the recrystallization temperature. This decreased the total strain required to produce NC structure, as compared with Alloy 718. It was observed that increasing the total strain and decreasing temperatures step-wise during deformation via multiple forging resulted in a uniform structure across the cross-section of the samples. The room temperature mechanical properties of the investigated alloys with various grain sizes are also compared.

Improvement of the fatigue performance of an ultrafine-grained Nb–Zr alloy by nano-sized precipitates formed by internal oxidation

The formation of nano-sized precipitates in an ultrafine-grained Nb–Zr alloy was investigated. ZrO2 precipitates induced by internal oxidation during heat treatment at low homologous temperatures significantly intensify the hardness of the surface layer without deterioration of the surface quality, and thus significantly improve the fatigue performance.

On the Onset of a Steady State in Body-Centered Cubic Iron during Severe Plastic Deformation at Low Homologous Temperatures

Armco Iron was severely deformed by means of a high-performance, high-pressure torsion (HPT) tool at a hydrostatic pressure of 5.4 GPa at low homologous temperatures, Tm, between 0.08 and 0.40 Tm. The flow stress was estimated from the measured torque during severe plastic deformation (SPD). At all investigated deformation conditions, a saturation in the flow stress at large strains without any decline was obtained. It is shown that this occurrence of a “steady state” at low homologous temperatures is significantly influenced by the deformation parameters, whereby an increase in deformation temperature or a decrease in strain rate results in a decrease in onset strain and an increase in flow stress in both the steady state and the size and aspect ratio of the structural elements. The Zener–Hollomon parameter is used to analyze the mechanical and microstructural properties as a function of the processing parameters; this shows that the investigated range of deformation conditions is divided into two regimes. For comparable high-deformation temperatures, the occurrence of a steady state is ascribed to a process similar to geometric dynamic recrystallization (DRX). Possible reasons for the occurrence of a different behavior in the low homologous temperature regime are discussed; the size of the crystallites formed during straining by the subdivision of the initial grains is considered to play an important role.

岩石のサブクリティカル亀裂進展に関する既往の研究と新たな展開

A review of studies of subcritical crack growth in rock is presented. First, theoretical background of subcritical crack growth is presented. It has been considered that the main mechanism of subcritical crack growth in silicate materials is stress corrosion under low homologous temperature and pressure. Next, theoretical background of stress corrosion is presented. Especially, the theoretical relation between the crack velocity and the stress intensity factor is described. Then, progress of the study of subcritical crack growth in silicate materials is presented by reviewing previous studies for glass, quartz, and silicate rocks. It has been clarified that the reactive species of stress corrosion is water and hydroxyl ion. For anisotropic rock, the crack velocity was dependent on the propagation direction and opening direction even at the same stress intensity factor.Finally, recent results of subcritical crack growth in rock are presented. It was clarified that the relation between the crack velocity and the stress intensity factor was decided by the crack opening direction. The crack velocity was higher when the temperature and the humidity were higher. It was shown that the activation energy became smaller when the crack path was smooth from the observation of the crack path.

Characterization of Shear Banding in La-Based Bulk Metallic Glasses through Indentation

La64Al14(Cu,Ni)22, La63.1Al15.2(Cu,Ni)21.7, La57.6Al17.5(Cu,Ni)24.9, and La55Al25Cu10Ni5Co5 bulk metallic glasses (BMGs) with low glass transition temperature (Tg) were prepared by copper-mould casting method. The homologous temperature (the ratio of room temperature to Tg) of the four BMGs ranges from 0.64 to 0.73. Plastic deformation behavior of the BMGs at various loading rates was studied by nanoindentation. The results showed that the loading rate dependency of serrated flow, which is related to the nucleation and propagation of shear bands, depends strongly on the homologous temperature. The alloys with relatively high homologous temperature exhibit an increase in flow serration with increasing loading rate, whereas, the alloys with low homologous temperature exhibit prominent serrations at low rates. No distinct shear band is observed around the indents for all alloys after nanoindentation at all the studied loading rates. Alternately, shear band pattern are characterized through macro-indentation, which shows that shear band spacing decreases with the increase of the homologous temperature.

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.

Making ceramics ductile at low homologous temperatures

Polycrystalline ceramics are brittle at room temperature but may deform by dislocation slip processes at higher temperatures, where T >= 0.5 T-M (T-M, the melting temperature), leading to strain ...

Microstructural investigation of the annealing behaviour of high-pressure torsion (HPT) deformed copper

Samples of polycrystalline 99.99% Cu were deformed by high-pressure torsion at room temperature and a hydrostatic pressure of about 8 GPa to a shear strain of about 100. After annealing at different temperatures the structural-elements sizes (grain/subgrain, scattering domain) were determined by scanning electron microscopy and multiple X-ray Bragg profile analysis. The evolution of the strength of the samples was monitored by microhardness measurements and its relation to the development of the structural-elements sizes has been investigated. The strength turns out to be correlated to the grain size: it decreases when the grain size starts to increase considerably upon annealing at homologous temperatures well above 30% of the melting temperature. Compared with conventional deformation, a very high dislocation density is present in the initial state and after annealing at 0.3 T m ( T m – melting temperature) the number of dislocations is reduced considerably. Thus, in the present case, the recovery of dislocations starts at lower homologous temperatures than in conventionally deformed materials.

Plastic deformation mechanism of pure aluminum at low homologous temperatures

Contemporary theory of metallic plastic deformation mechanism at low homologous temperatures is attributed to the physical processes of dislocations passing obstacles. Physically based deformation mechanism is generally considered as thermally activated motion of dislocations past obstacles and structural evolution of the obstacles. In order to verify the thermally activated deformation theory, stress rate change experiments, which give rapid stress changes, are conducted using pure aluminum at room temperature in this study. Incremental method is employed in the numerical simulation of above deformation mechanism. Excellent agreement is observed in the comparison between experimental data and numerical calculations, especially on the very abrupt change in strain rate at the stress rate change point. Sensitivity analyses are also performed to better understand the key parameters of both flow kinetics and structural evolution law. Simple kinetic model such as the regularly distributed rectangular obstacle is adequate enough. Obstacle structure is strongly influenced by the dislocation annihilation processes. Constant obstacle structure seems impossible to maintain once the loading condition changes.

Effect of grain size from mm to nm on the flow stress and plastic deformation kinetics of Au at low homologous temperatures

Three grain size regimes were identified: Regime I ( d > ∼0.5 μm), Regime II ( d ≈ 10–500 nm) and Regime III ( d < ∼10 nm). Grain size hardening in accord with the Hall–Petch (H–P) equation occurred in Regimes I and II, grain size softening with the flow stress proportional to d −1 occurred in III. The rate-controlling mechanism in Regime I was concluded to be the intersection of dislocations, that in II grain boundary shear promoted by the pile-up of dislocations and that in III grain boundary shear without pile-ups. The transition from I to II occurred when the dislocation cell size became larger than the grain size, that from II to III when the dislocation elastic interaction spacing became larger than the grain size. The H–P behavior in Regime I is in accord with the dislocation density model, that in II with the dislocation pile-up model. The general behavior of Au is similar to that reported previously for Cu and Ag.

Plastic deformation mechanism of pure copper at low homologous temperatures

Plastic deformation mechanism of pure metals at low homologous temperatures is attributed to the motion of dislocations and their interactions with each other or other kinds of obstacles. Physically based modeling of deformation mechanism is generally considered as thermally activated motion of dislocations past obstacles and structural evolution of the obstacles. In this study, stress rate change experiments, which give rapid stress changes, are conducted using pure copper at room temperature. Incremental method is employed in the numerical simulation of above modeling. Excellent agreement is observed in the comparison between experimental data and numerical calculations, especially on the very abrupt change in strain rate at the stress rate change point. Sensitivity analyses are also performed to better understand both flow kinetics and structural evolution law. Simple kinetic model such as the regular distributed rectangular obstacle is adequate enough. Obstacle structure is strongly influenced by the dislocation annihilation processes.

A hybrid model for computationally efficient fatigue fracture simulations at microelectronic assembly interfaces

Modern microelectronic assemblies are heterogeneous, layered structures joined by interconnects made of solder alloys with low homologous temperatures. The solder interconnections join devices to circuit boards and they fail by thermal fatigue fracture at their interfaces either to the device or to the circuit board. Predicting the fatigue fracture of the solder interconnections is a challenge due to the fact that they undergo large inelastic deformations during temperature cycling tests. In this paper we develop a hybrid approach inspired by cohesive zone fracture mechanics and the Disturbed State Concept to predict the crack trajectory and fatigue life of a solder interconnection subjected to both isothermal temperature cycling and anisothermal temperature cycling conditions (representing the two common accelerated test conditions for microelectronic products). A hybrid computational approach is used in which a first order approximation of the disturbance is used to estimate incremental cycles to criticality and thereby propagate the crack. The modeled crack fronts and the fatigue lives are validated through a comparison to results from the two types of accelerated tests. Overall, the model is shown to predict the fatigue life of the critical interconnection in the assembly to within 20% of the experimentally determined life. More importantly, the predicted crack trajectory is demonstrated to agree very well with the experimentally observed trajectory. Strikingly, the microscopically observed microstructural changes during crack propagation from that corresponding to creep fatigue to that of shear overload were found to be excellently correlated with the rate of change of the disturbance calculated in the model.

Effect of grain size from millimeters to nanometers on the flow stress and deformation kinetics of Ag

Abstract Data on the effects of grain size, d = 9 nm to 0.26 mm on the flow stress and plastic deformation kinetics of Ag at low homologous temperatures were evaluated. Two regimes were identified for this grain-size range: Regime I ( d > 500 nm) and Regime II ( d ≈ 10–500 nm). The effect of grain size on the flow stress in both regimes could be described by a Hall–Petch equation. K H–P was however significantly smaller and σ i larger for Regime II compared to I. Dislocation activity occurred in both regimes; dislocation cells were observed in Regime I, but not reported for II. The available data suggest that the dislocation density model governs the grain-size dependence of the flow stress in Regime I and that dislocation pile-up applies in Regime II. The rate-controlling mechanism(s) in each of the two regimes are discussed.

A hypoelasto-plastic finite strain simulation of powder compaction processes with density-dependent endochronic model

In this paper, a new approach is developed based on an endochronic density-dependent plasticity model for describing the isothermal deformation behavior of metal powder at low homologous temperature. As large deformations are observed in powder compaction processes, the endochronic constitutive model is presented based on large strain plasticity and an integration scheme is established for the rate constitutive equations. Endochronic constitutive equations are established based on coupling between deviatoric and hydrostatic behavior. The elastic response is stated in term of hypoelastic model and endochronic constitutive equations are stated in unrotated frame of reference. Finally, the algorithmic modulus consistent with numerical integration algorithm of constitutive equations is extracted. Although the concept of yield surface has not been explicitly assumed in endochronic theory, it is demonstrated that the cone-cap yield surface can be derived as a special case of the proposed endochronic model. The material parameters in endochronic model are calibrated for two samples of metal powder by fitting the model to reproduce data from true-triaxial compression experiments. The numerical schemes are examined for efficiency and accuracy in the modeling of three powder compaction components.