Stability of the ultrafine-grained microstructure in silver processed by ECAP and HPT

The high-temperature thermal stability of the ultrafine-grained (UFG) microstructures in low stacking fault energy silver was studied by differential scanning calorimetry (DSC). The UFG microstructures were achieved by equal-channel angular pressing (ECAP) and high-pressure torsion (HPT) at room temperature (RT). The defect structure in the as-processed samples was examined by electron microscopy and X-ray line profile analysis. The stored energy calculated from the defect densities was compared to the heat released during DSC. The sum of the energies stored in grain boundaries and dislocations in the ECAP-processed samples agreed with the heat released experimentally within the experimental error. The temperature of the DSC peak maximum decreased while the released heat increased with increasing numbers of ECAP passes. The released heat for the specimen processed by one revolution of HPT was much smaller than after 4–8 passes of ECAP despite the 2 times larger dislocation density measured by X-ray line profile analysis. This dichotomy was caused by the heterogeneous sandwich-like microstructure of the HPT-processed disk: about 175 μm wide surface layers on both sides of the disk exhibited a UFG microstructure while the internal part was recrystallized, thereby yielding a relatively small released heat.     

High temperature thermal stability of pure copper and copper–carbon nanotube composites consolidated by High Pressure Torsion

The thermal stability of ultrafine-grained (UFG) microstructures in pure copper samples and copper–carbon nanotube (CNT) composites processed by High Pressure Torsion (HPT) was compared. The UFG microstructure in the sample consolidated from pure Cu powder exhibited better stability than that developed in a casted Cu specimen. The addition of CNTs to the Cu powder further increased the stability of the UFG microstructure in the consolidated Cu matrix by hindering recrystallization, however it also yielded a growing porosity and cracking during annealing. It was shown that the former effect was stronger than the latter one, therefore the addition of CNTs to Cu has an overall benefit to the hardness in the temperature range between 300 and 1000 K. A good agreement between the released heat measured during annealing and the calculated stored energy was found for all samples.     

Stored energy and recrystallization temperature in high purity copper after equal channel angular pressing

Equal channel angular pressing (ECAP) was conducted at room temperature to impose high strain into high purity copper. Differential Scanning Calorimeter (DSC) was used to estimate the stored energy from ECAP and recrystallization temperature. It was found that the stored energy increases upon ECAP processing until a peak is reached at 12 passes of ECAP, and a slight decrease in stored energy was observed at higher ECAP passes. The recrystallization temperature decreases upon the increase of the stored energy up to ~50 J/mol, and reaches a stable valve of ~210 °C. Partial annealing of an ECAP processed (8 passes) sample by heating to ~185 °C at a heating rate of 20 °C/min released the stored energy from ~55 to ~18 J/mol, without substantial change on the recrystallization temperature of the sample. A two parameters model was used to help calculate stored energy of ultrafine-grained copper after high strain level processing.     

Service properties of ultrafine-grained Ti–6Al–4V alloy at elevated temperature

This study deals with investigation of mechanical properties and fatigue behavior of the ultra-fine grained (UFG) alloy Ti–6Al–4V at elevated temperatures. UFG samples were produced by means of combination of equal-channel angular pressing and thermomechanical treatments. Studies of the temperature dependence of mechanical properties of the UFG alloy demonstrated their thermal stability upto 175–350 °C. It was revealed that 100-hour creep rupture strength at 300 °C increased from 750 MPa in the conventional state to 890 MPa in the UFG state. The alloy demonstrates stability of the UFG structure at 300 and 370 °C in the conditions of long-term tests. The fatigue tests were conducted with axial loading applied on a sample at 175 °C, the asymmetry factor of the cycle was 0.1. The fatigue endurance limit of the UFG alloy was almost 50 % higher than that of the CG alloy.     

Tailoring structures through two-step annealing process in nanostructured aluminum produced by accumulative roll-bonding

Due to structural and textural heterogeneities and a high content of stored energy, annealing of nanostructured metals is difficult to control in order to avoid non-uniform coarsening and recrystallization. The present research demonstrates a method to homogenize the structure by annealing at low temperature before annealing at high temperature. By this two-step process, the structure is homogenized and the stored energy is reduced significantly during the first annealing step. As an example, high-purity aluminum has been deformed to a total reduction of 98.4% (equivalent strain of 4.8) by accumulative roll-bonding at room temperature. Isochronal annealing for 0.5 h of the deformed samples shows the occurrence of recrystallization at 200 °C and above. However, when introducing an annealing step for 6 h at 175 °C, no significant recrystallization is observed and relatively homogeneous structures are obtained when the samples afterwards are annealed at higher temperatures up to 300 °C. To underpin these observations, the structural evolution has been characterized by transmission electron microscopy, showing that significant annihilation of high-angle boundaries, low-angle dislocation boundaries, and dislocations characterizes the low-temperature annealing step. In a discussion, the observed annealing behavior is related to these structural changes.     

Surface hardening of biocompatible ultrafine-grained niobium zirconium alloy by two-stage oxidation treatment

The present study reports on an optimized surface hardening process for biocompatible ultrafine-grained (UFG) niobium 2.3 wt% zirconium (NbZr) alloy, a promising candidate implant material. The as-received material of conventional grain size (CG) was processed using multipass equal channel angular processing at room temperature to obtain an UFG microstructure featuring high strength and ductility. Subsequent surface hardening was performed by a heat treatment leading to internal oxidation. Using a thermogravimetric system, the influence of temperatures, time, and partial pressure of oxygen ( $ p_{{\text{O}}_2} $ ) on the oxidation kinetics were investigated. Metallographic and microscopic analyses and hardness measurements were employed to evaluate maximum hardness, penetration-depth and scale formation under various conditions. Heat treatment at 620 °C for 6 h at a $ p_{{\text{O}}_2} $ of 0.2 hPa led to sufficiently rapid oxidation kinetics yielding a relatively high depth of penetration without formation of loose Nb₂O₅ on the surface, which was observed at higher $ p_{{\text{O}}_2} $ . As compared to CG material, improved hardness profiles were reached using the same heat treatment parameters, since the UFG structure significantly changes diffusion conditions and therefore oxidation kinetics. After a second heat treatment in high vacuum the high maximum hardness of 820 HV0.01 in the UFG material was reduced effectively and a less steep hardness gradient was achieved, both contributing to a less brittle behavior under mechanical loading. High-cycle fatigue tests performed on surface-hardened UFG NbZr samples showed a substantial improvement of fatigue life in tests conducted near the endurance limit. Especially when high fatigue and wear resistance are key issues for a given application, the internal oxidation process offers an effective way to further improve the properties of UFG NbZr.     

Effect of severe plastic deformation on creep behaviour of a Ti–6Al–4V alloy

This paper examines the effect of severe plastic deformation on creep behaviour of a Ti–6Al–4V alloy. The processed material with an ultrafine-grained (UFG) structure (d ≈ 150 nm) was prepared by multiaxial forging. Uniaxial constant stress compression and constant load tensile creep tests were performed at 648–698 K and at stresses ranging between 300 and 600 MPa on the UFG processed alloy and, for comparison purposes, on its coarse-grained (CG) state. The values of the stress exponents of the minimum creep rate n and creep activation energy Q _c were determined. Creep behaviour was also investigated by nanoindentation method at room temperature under constant load. The microstructure was examined by transmission electron microscopy and scanning electron microscope equipped with an electron back scatter diffraction unit. The results of the uniaxial creep tests showed that the minimum creep rates of the UFG specimens are significantly higher in comparison with those of the CG state. However, the differences in the minimum creep rates of both states of alloy strongly decrease with increasing values of applied stress. The CG alloy exhibits better creep resistance than the UFG one over the stress range used; the minimum creep rate for the UFG alloy is about one to two orders of magnitude higher than that of the CG alloy. The indentation creep tests showed that annealing had little effect on the creep behaviour in UFG Ti alloy at room temperature.     

Microstructure and microtexture in pure copper processed by high-pressure torsion

The evolution of microstructure and microtexture in high purity copper was examined after processing by high-pressure torsion (HPT). Copper disks were annealed for 1 h at 800 °C and later processed monotonously in HPT at ambient temperature for 1/4, 1/2, 1, and 5 turns under a pressure of 6.0 GPa. Electron backscattered diffraction (EBSD) measurements were taken for each disk at three positions: center, mid-radius, and near-edge. Results from EBSD for samples processed between 1/4 and 1 turn indicate the formation of Σ3 twin boundaries by recrystallization before complete microstructural refinement. The results show a gradual increase in the homogeneity of the microstructure with increasing numbers of turns, reaching a stabilized ultrafine-grained structure at 5 turns with a bimodal distribution of fine and coarse grains of 0.15 and 0.5 μm in diameter, respectively. The occurrence of recrystallization in the early straining stages was further supported by examining microtexture development with increasing numbers of turns, where this shows a gradual transition from a shear texture to a mixture of shear and recrystallization and later to a shear texture at high HPT strains. The promotion of recrystallization during HPT is probably related to the high purity of the copper.     

Recrystallization and grain growth of nanocomposite Ti–B–N coatings

Nanocomposite Ti–B–N coatings with different chemical composition were prepared by non-reactive co-sputtering of a segmented TiN–TiB₂ target. The coatings investigated are primarily composed of nanocrystalline TiN and TiB₂ phases. Increasing boron content results in a decreasing grain size from approximately 6 to 2 nm. During a thermal treatment of such coatings solely recovery and recrystallization with subsequent grain growth would appear, since the two phases are in thermodynamic equilibrium. Differential scanning calorimetry (DSC) and X-ray diffraction analysis were used to investigate the recrystallization behavior and subsequent grain growth of the nanocomposite Ti–B–N coatings. On heating the coating samples, which were removed chemically from their low alloyed steel substrates, an exothermal peak appeared during the DSC measurements indicating grain growth. From the onset temperature of this peak the recrystallization temperature was found which increases with increasing boron content from 1032 to 1070 °C. Activation energies for grain growth are obtained from Kissinger plots and yield values decreasing from 7.9 to 4.4 eV with increasing boron content. After heat treatment up to 1400 °C during the DSC measurements the coatings showed grain sizes within the range of 15–30 nm. It was found that the highest recrystallization temperature does not imply the highest activation energy for grain growth.     

Assessment of Degree of Disorder (Amorphicity) of Lyophilized Formulations of Growth Hormone Using Isothermal Microcalorimetry

When determining the degree of disorder of a lyophilized cake of a protein, it is important to use an appropriate analytical technique. Differential scanning calorimetry (DSC) and X‐ray powder diffraction (XRPD) are the most commonly used thermo‐analytical techniques for characterizing freeze‐dried protein formulations. Unfortunately, these methods are unable to detect solid‐state disorder at levels < 10%. Also, interpretation of DSC results for freeze‐dried protein formulations can be difficult, as a result of the more complex thermal events occurring with this technique. For example, proteins can inhibit the thermally induced recrystallization of the lyophilized cake, resulting in potential misinterpretation of DSC degree of disorder results. The aim of this investigation was to study the use of isothermal microcalorimetry (IMC) in the assessment of degree of solid‐state disorder (amorphicity) of lyophilized formulations of proteins. For this purpose, two formulations of growth hormone were prepared by lyophilization. These formulations consisted of the same amounts of protein, mannitol, glycine, and phosphate buffer, but differed in the freeze‐drying procedure. After lyophilization, the recrystallization of the samples was studied using IMC at 25°C under different relative humidities (58–75%). The effect of available surface area was studied by determining the heat of recrystallization (Q) of the samples before and after disintegration of the cakes. The results showed that, in contrast to DSC, IMC allowed detection of the recrystallization event in the formulations. Although both formulations were completely disordered and indistinguishable according to XRPD method, IMC revealed that formulation B had a different solid‐sate structure than formulation A. This difference was the result of differences in the freeze‐drying parameters, demonstrating the importance of choosing appropriate analytical methodology.     

Reverse transformation of deformation-induced martensite in austenitic stainless steel studied by positron annihilation

The reversion of deformation-induced α′-martensite in tension-deformed 1.4301 (EN) stainless steel was investigated using positron annihilation spectroscopy. The Doppler broadening of the annihilation line and positron lifetime spectroscopy were applied to study defect structure and its annealing behavior in samples with a similar deformation level but varying in α′-martensite amount. The difference in α′-martensite was obtained by applying different deformation temperatures, i.e., liquid nitrogen temperature, room temperature, and 200 °C. The cumulative annealing of the tension-deformed samples and measurement of the positron annihilation characteristics show the gradual annealing of defects in the temperature range between 200 and 400 °C due to the recovery and recrystallization. However, in the temperature range between 450 and 650 °C, the generation of vacancy clusters which trap positrons is revealed. This temperature range coincides with the temperature range of α′-martensite reversion which is confirmed by microhardness and magnetization measurements. The detected large vacancy clusters consisting of 6–9 vacancies can occur at the interface between austenite and α′-martensite phases due to the volume contraction accompanying bcc/fcc change.     

基于不同原始组织预设变形第四代单晶高温合金的再结晶行为

第四代单晶高温合金标准热处理试样和铸态试样压痕后分别在1100,1150,1200,1250,1300℃和1340℃退火处理,采用光学显微镜、扫描电镜、电子背散射仪研究不同条件的再结晶组织。结果表明:1100,1150,1200℃退火处理后,标准热处理试样和铸态试样都出现胞状再结晶。1250℃退火处理后,标准热处理试样和铸态试样都为混合再结晶。1300℃退火处理后,标准热处理试样再结晶组织全部为等轴再结晶,而铸态试样仍为混合再结晶。1340℃退火处理后,标准热处理试样和铸态试样都形成了等轴再结晶。随着退火温度升高,标准热处理试样和铸态试样的再结晶层深度明显增加,标准热处理试样再结晶深度明显大于铸态试样,相同条件下标准热处理试样的再结晶晶粒更容易长大。再结晶与基体的界面为小角度晶界、大角度晶界,而再结晶晶粒之间为小角度晶界、大角度晶界和孪晶界。孪晶在单晶高温合金再结晶的过程中发挥了重要作用。     

Magnetic and interface microstructure contribution to bulk specific heat of nanocrystalline Ni–P alloy

The bulk specific heat of fully dense nanocrystalline Ni–P electrodeposits with essentially constant P content (about 4 at%) and varying average grain sizes from 6.9 to 28.9 nm was investigated using modulated differential scanning calorimetry. In the lower temperature range from room temperature to 120 °C, at which the as-deposited sample microstructure was thermally stable, the bulk specific heat varied only within ~2 % despite the substantial variation of interface volume fractions from 0.11 to 0.39 for this series of samples. Moreover, the measured bulk specific heat values of the Ni–P samples were all located within the reported specific heat value range for conventional polycrystalline Ni. Evidently, the contribution due to grain size-related interface excess free volume is negligible and the bulk specific heat for the materials can be characterized as a structure-insensitive property. In the elevated temperature range from 150 °C to the Curie temperature of 357 °C, the magnetic contribution to the specific heat was significantly influenced by the chemical environment of P in the Ni–P samples. When P atoms were in the form of supersaturated solution in the nickel matrix, a complete suppression of the characteristic λ peak of the magnetic contribution in the specific heat curves was observed for all materials. The λ peak re-appeared in the specific heat curve after the Ni–P sample underwent a transformation to a two-phase microstructure consisting of Ni and Ni₃P grains. It can be concluded that at a given P content, paramagnetic phosphorus atoms in the form of solutes are more effective in reducing the magnetic contributions to the specific heat than the form of paramagnetic Ni₃P second-phase particles for the nanocrystalline Ni–P alloy.     

Impurity Effect on Wear Resistance of Ultrafine-Grained Copper

The wear properties of ultrafine-grained(UFG) Cu samples of different purities were investigated in comparison with the coarse-grained(CG) Cu.The UFG Cu samples,prepared by means of plastic deformation via quasistatic compression,exhibit an enhanced wear resistance relative to the CG Cu samples.For both the UFG and the CG Cu samples,wear volumes increase at higher purities.A steady state worn subsurface structure was formed in each sample after sliding for 60 min,consisting of a heavily deformed nanostructured mixing layer(NML) on top of a continuous dynamic recrystallization(DRX) layer.A pronounced correlation is identified that wear volume increases monotonically with an increasing grain size of the DRX layer.The impurity level of the Cu samples has an obvious influence on the DRX grain sizes,which in turn determines the wear resistance of the Cu samples.     

Effect of Heat Treatments on Austempered Ductile Iron

This work presents the influence of austempering heat treatment carried out in one-step and two-step processes on the microstructures and mechanical properties of ductile cast iron. The samples were extracted from as-cast pieces and heat treated by austempering. For the one-step process the samples were heated at 910°C for 90 min for austenitization and cooled in salt bath at a temperature of 300°C for 30 min. For the two-step process the samples were cooled from 910°C to 245°C, kept at this temperature for 5 min in salt bath, then heated in another salt bath at a temperature of 300°C for 30 min. The samples were analyzed by optical microscopy and mechanical tests. After the one-step austempering, microscopic analysis of the samples showed ausferrite microstructure matrix and graphite in nodules surrounded by fine pearlite. For the two-step austempering, the presence of ausferrite matrix with graphite in nodules and retained austenite was observed. As to mechanical properties, the results showed that, with the two-step process there was gain (4.7%) in the average hardness and loss (3.5%) in the impact resistance. The microhardness of the ausferrite was 6.2% higher in the one-step austempering when compared to the two-step process.     

Mechanical properties of ultrafine grained ferritic steel sheets fabricated by rolling and annealing of duplex microstructure

A new route to fabricate ultrafine grained (UFG) ferritic steel sheets without severe plastic deformation is proposed in this article. A low-carbon steel sheet with a duplex microstructure composed of ferrite and martensite was cold-rolled to a reduction of 91% in thickness, and then annealed at 620–700 °C. The microstructure obtained through the process with annealing temperatures below 700 °C was the UFG ferrite including fine cementite particles homogenously dispersed. The grain size of ferrite matrix changed from 0.49 to 1.0 μm depending on the annealing temperature. Dynamic tensile properties of the produced UFG steels were investigated. The obtained UFG ferrite–cementite steels without martensite phase showed high strain rate sensitivity in flow stress. The UFG ferritic steels are expected to have high potential to absorb crash energy when applied to automobile body.     

Characterization of the evolution of recrystallization by fluctuation and fractal analyses of the magnetic hysteresis loop in a cold rolled non-oriented electric steel

Silicon steels with non-oriented grains are widely used in the fabrication of electrical motor nucleus where a low magnetic loss is an important point. The performance of these motors is affected by the level of recrystallization of these steels which can come from the steel plant in a semi-processed condition. In this condition, they have a partially deformed structure and are submitted to an adequate annealing heat treatment, after reaching the end shape, to get an appropriate magnetic property. In this study, samples of an electric steel, cold rolled 50% in thickness, were withdrawn during the industrial heat treatment at temperatures of 575, 580, 600, 620 and 730 °C with the objective of evaluating the evolution of recrystallization with temperature. Magnetic properties were measured at room temperature in a vibrating sample magnetometer and the changes in magnetic hysteresis loop with temperature have been identified by using two pattern classification techniques, principal-component analysis and Karhunen-Loève (KL) expansion, associated with statistical fluctuations and fractal analyses. The fluctuation and fractal analyses were used as preprocessing tools of the series which are built from each hysteresis loop, properly renormalized, whose values correspond to the amplitudes of the loop at given equally spaced values of the renormalized field interpolated between the experimental data. The samples have been classified in four sets corresponding to different temperatures, and to samples without annealing heat treatment and recrystallized ones. The classification of the different microstructures have been obtained by applying the two pattern classification techniques to the vectors obtained from the preprocessing, and in particular a 100% success rate has been reached by using KL expansion.     

Study of welding peak temperatures on microstructures and hardness of heat affected zone in 2024-T3 aluminium alloy

Abstract

The influence of welding thermal cycle peak temperatures and post-weld heat treatments on the microstructures and mechanical properties of the heat affected zone (HAZ) for 2024-T3 aluminium alloy have been investigated by Gleeble HAZ simulation. Differential scanning calorimetry (DSC) in conjunction with transmission electron microscopy (TEM) is used to characterise the HAZ microstructures. The welded HAZ in the region of peak temperature 414°C has the lowest hardness after natural aged temper, which is primarily due to the precipitation and coarsening of stable S phases. When the peak temperature of welded HAZ is larger than 414°C, the hardness of HAZ increasing with an increasing peak temperature can be seen, which is due to higher peak temperature thermal cycles treatment inducing the dissolution of precipitations in the matrix, and, after the natural aging treatment, Guinier-Preston (GPB) and GPB2 zones precipitating out in the matrix again can be seen. Post-weld T81 artificial aging (PWAA-T81) heat treatment has no effect on improving the HAZ hardness; the HAZ hardness of the 2024-T3 alloy obtained by PWAA-T81 is less than that obtained by natural aging, and its lowest hardness is shifted to the region of peak temperature, which is 452°C, because overaging induces coarse and sparse amounts of stable S phase.     

Hot working behavior of AZ31 and ME21 magnesium alloys

The plastic deformation and recrystallization behavior of the commercial magnesium alloys AZ31 and ME21 were analyzed in a wide temperature range. Using the conventional hyperbolic sine equation the flow stress dependence on temperature and strain rate was modeled. The activation energy for plastic deformation significantly increased with increasing temperature and delivered values above 180 kJmol^?1 for both alloys in the very high-temperature regime (400–550 °C). At lower temperatures (250–400 °C) the activation energy of the AZ31 alloy was approximately 108 kJmol^?1 considering the peak stress as well as 120 kJmol^?1 considering the flow stress at a strain of 0.5. The stress exponent varied in a range between 4.5 and 6.5. During the high-temperature compression tests a partial recrystallized microstructure was formed, which was distinctly different in AZ31 compared to ME21 due to the different onset of dynamic recrystallization (DRX) mechanisms.     

Assessment of degree of disorder (amorphicity) of lyophilized formulations of growth hormone using isothermal microcalorimetry

When determining the degree of disorder of a lyophilized cake of a protein, it is important to use an appropriate analytical technique. Differential scanning calorimetry (DSC) and X-ray powder diffraction (XRPD) are the most commonly used thermoanalytical techniques for characterizing freeze-dried protein formulations. Unfortunately, these methods are unable to detect solid-state disorder at levels < 10%. Also, interpretation of DSC results for freeze-dried protein formulations can be difficult, as a result of the more complex thermal events occurring with this technique. For example, proteins can inhibit the thermally induced recrystallization of the lyophilized cake, resulting in potential misinterpretation of DSC degree of disorder results. The aim of this investigation was to study the use of isothermal microcalorimetry (IMC) in the assessment of degree of solid-state disorder (amorphicity) of lyophilized formulations of proteins. For this purpose, two formulations of growth hormone were prepared by lyophilization. These formulations consisted of the same amounts of protein, mannitol, glycine, and phosphate buffer, but differed in the freeze-drying procedure. After lyophilization, the recrystallization of the samples was studied using IMC at 25 degrees C under different relative humidities (58-75%). The effect of available surface area was studied by determining the heat of recrystallization (Q) of the samples before and after disintegration of the cakes. The results showed that, in contrast to DSC, IMC allowed detection of the recrystallization event in the formulations. Although both formulations were completely disordered and indistinguishable according to XRPD method, IMC revealed that formulation B had a different solid-sate structure than formulation A. This difference was the result of differences in the freeze-drying parameters, demonstrating the importance of choosing appropriate analytical methodology.