EBSD analysis of a Ti-IF steel subjected to hot torsion in the ferritic region

The effect of grain size on the warm deformation behaviour of a titanium stabilized interstitial free steel was investigated using hot torsion. Tests were performed at temperatures between 765 °C and 850 °C at strain rates between 0.003 s⁻¹ and 1 s⁻¹ for samples with grain sizes of 25 μm, 75 μm and 150 μm. The structures were observed using EBSD analysis and are consistent with those expected for materials dominated by dynamic recovery. Some evidence was found for small amounts of thermally induced migration of pre-existing boundary (bulging) and for the generation of new segments of high angle boundaries by continuous dynamic recrystallization. The early onset of a steady-state flow stress in the finer grained samples is attributed to one or a combination of thermally induced boundary migration and enhanced rates of recovery near subgrain (and grain) boundaries.     

Deformation behavior and microstructural evolution during the semi-solid compression of Al–4Cu–Mg alloy

The effects of the process parameters, including deformation temperature and strain rate, on the deformation behavior and microstructure of an Al–4Cu–Mg alloy, have been investigated through isothermal compression. Experiments were conducted at deformation temperatures of 540 °C, 560 °C, and 580 °C, strain rates of 1 s⁻¹, 1×10⁻¹ s⁻¹, 1×10⁻² s⁻¹, and 1×10⁻³ s⁻¹, and height reductions of 20%, 40%, and 60%. The experimental results show that deformation temperature and strain rate have significant effect on the peak flow stress. The flow stress decreases with an increase of deformation temperature and/or a decrease of the strain rate. Above a critical value of the deformation temperature, the flow stress quickly reaches a steady value. Experimental materials A and B have equiaxed and irregular grains, respectively, prior to deformation. The microstructures vary with the process parameters in the semi-solid state. For material B, the irregular grains transform to equiaxed grains in the process of semi-solid deformation, which improves the deformation behavior.     

Compression testing of a sintered Ti6Al4V powder compact for biomedical applications

In this study, the compression deformation behavior of a Ti6Al4V powder compact, prepared by the sintering of cold compacted atomized spherical particles (100–200 μm) and containing 36–38% porosity, was investigated at quasi-static (1.6×10⁻³–1.6×10⁻¹ s⁻¹) and high strain rates (300 and 900 s⁻¹) using, respectively, conventional mechanical testing and Split Hopkinson Pressure Bar techniques. Microscopic studies of as-received powder and sintered powder compact showed that sintering at high temperature (1200 °C) and subsequent slow rate of cooling in the furnace changed the microstructure of powder from the acicular alpha () to the Widmanstätten (+β) microstructure. In compression testing, at both quasi-static and high strain rates, the compact failed via shear bands formed along the diagonal axis, 45° to the loading direction. Increasing the strain rate was found to increase both the flow stress and compressive strength of the compact but it did not affect the critical strain for shear localization. Microscopic analyses of failed samples and deformed but not failed samples of the compact further showed that fracture occurred in a ductile (dimpled) mode consisting of void initiation and growth in phase and/or at the /β interface and macrocracking by void coalescence in the interparticle bond region.     

Hot flow behavior of boron microalloyed steels

This research work studies the effect of boron contents on the hot flow behavior of boron microalloyed steels. For this purpose, uniaxial hot-compression tests were carried out in a low carbon steel microalloyed with four different amounts of boron over a wide range of temperatures (950, 1000, 1050 and 1100 °C) and constant true strain rates (10⁻³, 10⁻² and 10⁻¹ s⁻¹). Experimental results revealed that both peak stress and peak strain tend to decrease as boron content increases, which indicates that boron additions have a solid solution softening effect. Likewise, the flow curves show a delaying effect on the kinetics of dynamic recrystallization (DRX) when increasing boron content. Deformed microstructures show a finer austenitic grain size in the steel with higher boron content (grain refinement effect). Results are discussed in terms of boron segregation towards austenitic grain boundaries during plastic deformation, which increases the movement of dislocations, enhances the grain boundary cohesion and modificates the grain boundary structure.     

Microstructure evolution during dynamic recrystallization of hot deformed superalloy 718

Microstructure evolution during dynamic recrystallization (DRX) of superalloy 718 was studied by optical microscope and electron backscatter diffraction (EBSD) technique. Compression tests were performed at different strains at temperatures from 950 °C to 1120 °C with a strain rate of 10⁻¹ s⁻¹. Microstructure observations show that the recrystallized grain size as well as the fraction of new grains increases with the increasing temperature. A power exponent relationship is obtained between the dynamically recrystallized grain size and the peak stress. It is found that different nucleation mechanisms for DRX are operated in hot deformed superalloy 718, which is closely related to deformation temperatures. DRX nucleation and development are discussed in consideration of subgrain rotation or twinning taking place near the original grain boundaries. Particular attention is also paid to the role of continuous dynamic recrystallization (CDRX) at both higher and lower temperatures.     

Superplasticity and grain boundary sliding in rolled AZ91 magnesium alloy at high strain rates

The superplastic deformation characteristics and microstructure evolution of the rolled AZ91 magnesium alloys at temperatures ranging from 623 to 698 K (0.67–0.76 T_m) and at the high strain rates ranging from 10⁻³ to 1 s⁻¹ were investigated with the methods of OM, SEM and TEM. An excellent superplasticity with the maximum elongation to failure of 455% was obtained at 623 K and the strain rate of 10⁻³ s⁻¹ in the rolled AZ91 magnesium alloys and its strain rate sensitivity m is high, up to 0.64. The dominant deformation mechanism in high strain rate superplasticity is still grain boundary sliding (GBS), which was studied systematically in this study. The dislocation creep controlled by grain boundary diffusion was considered the main accommodation mechanism, which was observed in this study.     

Pressure-shear impact and the dynamic viscoplastic response of metals

Pressure-shear plate impact experiments are used to investigate the viscoplastic response of metals at shear strain rates ranging from 10⁵ s⁻¹ to 10⁷ s⁻¹. Flat specimens with thicknesses between 300 μm and 3 μm are sandwiched between two hard, parallel plates that are inclined relative to their direction of approach. Nominal stresses and strains in the specimens are determined from elastic wave profiles monitored at the rear surface of one of the hard plates. Results are reviewed for two fcc metals: commercially pure aluminum and an aluminum alloy. New results are presented for bcc high purity iron, a high strength steel alloy and vapor deposited aluminum. For commercially pure aluminum the flow stress increases strongly with strain rate as strain rate increases from 10⁴ s⁻¹ to 10⁵ s⁻¹. At strain rates above 10⁵ s⁻¹ the flow stress, based on results for thin vapor-deposited aluminum specimens, increases strongly, but less than linearly, with increasing strain rate until it saturates at strain rates between 10⁶ s⁻¹ and 10⁷ s⁻¹. Preliminary results for high purity alpha-iron indicate that the flow stress increases smoothly over eleven decades of strain rate, and faster than logarithmically for strain rates from 10² s⁻¹ to greater than 10⁶ s⁻¹. In contrast, for a high strength steel alloy the flow stress depends only weakly on the strain rate, even at strain rates at high as 10⁵ s⁻¹. Such contrasting behavior is attributed to differences in the relative importance of viscous glide and thermal activation as rate controlling mechanisms for dislocation motion in the various metals. Numerical studies indicate that experiments performed at the highest strain rates on the thinnest specimens are not adiabatic, thus requiring a full thermal-mechanical analysis in order to interpret the data.     

High strain rate superplastic behaviour of powder-metallurgy processed 7475Al+0.7Zr alloy

Superplasticity was investigated in powder-metallurgy (PM) processed 7475Al+0.7Zr alloy. Strain-rate-change (SRC) tests were carried out at various temperatures to examine the relationship between strain rate and flow stress. After the compensation by threshold stress, the superplastic flow was found to be well correlated with lattice diffusivity in aluminium, like that in the ingot-metallurgy (IM) processed 7475Al alloy having a coarser grain size. Large tensile elongations of up to 1000% could be obtained at a very high strain rate near 10⁻¹ s⁻¹ and at 515°C. Short fibre formation was observed after the superplastic deformation. This formation seemed to be related to liquid formation on the grain boundaries and similar evidences were found over a wide range of temperature, not necessarily near the incipient melting point.     

Microstructure evolution and texture development during high-temperature uniaxial compression of magnesium alloy AZ31

Texture development in magnesium alloy AZ31 was studied by uniaxial compression tests at temperatures, strain rates and final strains ranging from 573 to 773 K, 1.0 × 10⁻³ to 5.0 × 10⁻⁵ s⁻¹ and −0.2 to −1.5, respectively. Fiber texture was formed in all of the deformation conditions. The main component of the texture varied depending on deformation conditions; it appeared about 33–38° away from the basal pole after the deformation at higher temperatures and lower strain rates. This can be attributed to the increased activity of the secondary pyramidal slip system. With a decrease in temperatures and an increase in strain rate, the tilting angle of the main component (compression plane) from the basal pole decreased down to about 20°. Construction of a basal fiber texture was detected after deformations at the lowest temperature and high strain rates.     

Effect of dynamic strain ageing on the deformation behavior of Incoloy alloy MA956

Incoloy alloy MA956 is an oxide dispersion hardened ferritic stainless steel produced by powder metallurgy. It is used as a candidate material for the high temperature components of gas turbines. This material underwent dynamic strain ageing at 300–400°C and strain rate of 1.2 × 10⁻³ s⁻¹. The following features of dynamic strain aging were observed: serrated flow at 300 and 400°C, a peak in the ultimate tensile strength normalized by the elastic modules versus temperature curve at 400°C, a plateau in the 0.2% offset yield stress-temperature curve at 300–400°C, a peak in the deformation rate-temperature curve at 300°C and the elongation-temperature plot showed a minimum at 400°C associated with shear fracture and with a minimum in the reduction in area-temperature plot. These features of dynamic strain ageing were discussed in the view of the recent models of dynamic strain ageing. The effect of dynamic strain ageing on the deformation and fracture behavior of this material was discussed.     

Molecular dynamics study of the mechanical behavior of nickel nanowire: Strain rate effects

We present the analysis of uniaxial deformation of nickel nanowires using molecular dynamics simulations, and address the strain rate effects on mechanical responses and deformation behavior. The applied strain rate is ranging from 1 × 10⁸ s⁻¹ to 1.4 × 10¹¹ s⁻¹. The results show that two critical strain rates, i.e., 5 × 10⁹ s⁻¹ and 8 × 10¹⁰ s⁻¹, are observed to play a pivotal role in switching between plastic deformation modes. At strain rate below 5 × 10⁹ s⁻¹, Ni nanowire maintains its crystalline structure with neck occurring at the end of loading, and the plastic deformation is characterized by {1 1 1} slippages associated with Shockley partial dislocations and rearrangements of atoms close to necking region. At strain rate above 8 × 10¹⁰ s⁻¹, Ni nanowire transforms from a fcc crystal into a completely amorphous state once beyond the yield point, and hereafter it deforms uniformly without obvious necking until the end of simulation. For strain rate between 5 × 10⁹ s⁻¹ and 8 × 10¹⁰ s⁻¹, only part of the nanowire exhibits amorphous state after yielding while the other part remains crystalline state. Both the {1 1 1} slippages in ordered region and homogenous deformation in amorphous region contribute to the plastic deformation.     

Precipitation of Cu in Fe–Cu alloys by high-speed deformation

The differences between defect structures in Fe–Cu alloys deformed at the high (4.3×10⁵ s⁻¹) and the low strain rate (67 s⁻¹) were studied. Positron lifetime and coincidence Doppler broadening (CDB) measurements were carried out to investigate the formation of vacancy clusters and Cu precipitates. Both the size of vacancy clusters and the total amount of vacancy-type defects were larger after high-speed deformation at room temperature. Cu precipitation in the specimen deformed at the high-speed stopped for 10 h after annealing at 400 °C, while that in the specimen deformed at the low-speed continued for 100 h. Transmission electron microscopy (TEM) observations showed a heterogeneous distribution of dislocations in the case of low-speed deformation but a homogeneous distribution in the case of high-speed deformation. These results suggested that the sink efficiency for defects was higher in the specimen deformed at the high-speed.     

Strain rate sensitivity of the commercial aluminum alloy AA5182-O

The mechanical behavior of the commercial aluminum alloy AA5182-O is investigated at temperatures ranging from −120 to 150 °C and strain rates from 10⁻⁶ to 10⁻¹ s⁻¹. The strain rate sensitivity parameter is determined as a function of temperature and plastic strain, and the strain rate and temperature range in which dynamic strain aging leads to negative strain rate sensitivity is mapped. The effect of dynamic strain aging on ductility and strain hardening is investigated. The sensitivity of the measured quantities to the experimental method employed and their dependence on grain shape are discussed. The experimental data are compared with the predictions of a model constructed based on a recently proposed mechanism for dynamic strain ageing. The mechanism is based on the effect solute clustering at forest dislocations has on the strength of dislocation junctions. The model is shown to reproduce qualitatively the experimental trends.     

Mechanical stability of Ca65Mg15Zn20 bulk metallic glass during deformation in the supercooled liquid region

Ca₆₅Mg₁₅Zn₂₀ bulk metallic glass (BMG) samples of dimensions 3.2 mm × 7 mm × 125 mm were prepared using a low-pressure die casting technique. These samples were ground to produce tensile test pieces in compliance with ASTM E8-04. This work is the first reported study of the tensile behaviour of Ca₆₅Mg₁₅Zn₂₀ BMG in the supercooled liquid region (105–120 °C). Two deformation conditions were used for testing: (i) constant strain rate testing from 10⁻³ to 10⁻⁴ s⁻¹ and (ii) constant load testing using loads of 20–50 N applied to a tensile sample during heating at a constant rate of 5 °C s⁻¹. The maximum elongation to failure in the BMG was in excess of 850% for constant load testing although, under isothermal testing conditions, most samples failed after 200% elongation. It is concluded that large superplastic elongations (>500%) during isothermal tensile straining is difficult in this alloy due to the onset of crystallization.     

The characteristics of serrated flow in superalloy IN738LC

Serrated flow was investigated in superalloy IN738LC, a nickel-base γ′ age-hardened alloy. In this material serrated flow appeared between 350 and 450 °C and strain rate of (8.77 × 10⁻⁵ to 8.77 × 10⁻³) s⁻¹. Activation energy for this process was calculated to be 0.69–0.86 eV which is in good agreement with the values reported for similar alloys. Results show that the diffusion rate of substitutional solute atoms at this temperature range is too low to cause this effect. This suggests that the interaction of solute atoms and moving dislocation is responsible for the observed serrated flow in this alloy.     

Effect of strain rate on high-temperature low-cycle fatigue of 17-4 PH stainless steels

The effect of strain rate (10⁻², 10⁻³ and 10⁻⁴ s⁻¹) on the low-cycle fatigue (LCF) behavior was investigated for 17-4 PH stainless steels in three different conditions at temperatures of 300–500 °C. The cyclic stress response (CSR) for Condition A tested at 300 and 400 °C showed cyclic hardening due to an influence of dynamic strain aging (DSA). An in situ precipitation-hardening effect was found to be partially responsible for the cyclic hardening in Condition A at 400 °C. For H900 and H1150 conditions tested at 300 and 400 °C, the CSR exhibited a stable stress level before a fast drop in load indicating no cyclic hardening or softening. At 500 °C, cyclic softening was observed for all given material conditions because of a thermal dislocation recovery mechanism. The cyclic softening behavior in Conditions A and H900 tested at 500 °C is attributed partially to coarsening of the Cu-rich precipitates. The LCF life for each material condition, tested at a given temperature, decreased with decreasing strain rate as a result of an enhanced DSA effect. At all given testing conditions, transgranular cracking was the common fatigue fracture mode.     

Exceptional superplasticity in an AZ61 magnesium alloy processed by extrusion and ECAP

Experiments were conducted on a commercial AZ61 alloy to evaluate the potential for achieving an ultrafine grain size and superplastic ductilities through the use of the EX-ECAP two-step processing procedure of extrusion plus equal-channel angular pressing. The results show that EX-ECAP gives excellent grain refinement with grain sizes of 0.6 and 1.3 μm after pressing at 473 and 523 K, respectively. The alloy processed by EX-ECAP exhibits exceptional superplastic properties including a maximum elongation of 1320% after pressing through four passes when testing at 473 K with an initial strain rate of 3.3 × 10⁻⁴ s⁻¹. This result compares with an elongation of 70% achieved in the extruded condition without ECAP under similar testing conditions.     

Flow pattern and heat transfer characteristics during flow boiling of CO2 in a horizontal micro fin tube and comparison with smooth tube data

Flow pattern observations and measurements of the heat transfer in a helical grooved micro fin tube are presented and compared with results for a smooth tube. The micro fin tube used (OD of 9.52 mm) was a Wieland Cuprofin EDX tube with 60 fins (height 0.25 mm) and a helix angle of 18°. The flow pattern observations at 39.7 bar (T_s=+5 °C, p_r=0.54) and 26.4 bar (T_s=−10 °C, p_r=0.36) show a wide range of the annular flow region. The transition from slug to annular flow does not occur, as expected, at constant vapour quality for all mass fluxes but there is an interdependence between transition vapour quality and mass flux. For the heat transfer in the micro fin tube, measurements at 39.7 bar are presented for heat fluxes up to 120 kW m⁻², mass fluxes between 75 and 250 kg m⁻² s⁻¹ and vapour qualities between 0.1 and 0.9.     

Sintering kinetics of 8Y–cubic zirconia: Cation diffusion coefficient

Zirconia ceramics, mainly of cubic phase, are used in different applications because of their particular electrical and structural properties.

After the forming stage, sintering leads to a material with suitable microstructural characteristics. The sintering process mainly depends on thermal cycle and on starting particle size and its distribution; it also depends on density and the microstructure of green material. Cubic zirconia has a high (2680 °C) melting temperature; however, effective sintering could be observed for temperatures higher than 900 °C (nanoparticles), and it may reach a final density of 96–98% the theoretical value at relative low temperatures.

The objective of this paper is to study the sintering kinetics of stabilized zirconia in its cubic phase with 8% molar of Y₂O₃ under fast firing rates up to nearly isothermal conditions. Samples were shaped from suspensions dispersed with ammonium polyacrylate by slip casting. Sintering was performed in the temperature range between 1200 °C and 1400 °C. The sintering kinetic process was followed by measuring density as a function of time. A sintering model was applied to fit the experimental data of the first steps of densification. It was observed that sintering obeys the same mechanism in the temperature and time ranges under study, which results in an activation energy of 170 kJ mol⁻¹. Sintering is controlled by Zr cation diffusion, for which a lattice diffusion coefficient of D_l = 8 × 10⁻¹² cm² s⁻¹ at 1400 °C was found, and the activation energy of the diffusion process was 223 kJ mol⁻¹.     

Deformation of thin foil of fcc and bcc metals containing pre-introduced He bubbles

Thin foil of fcc and bcc metals subjected to tensile deformation has been found to exhibit an anomalously high density of small vacancy clusters, probably in the absence of dislocations. Deformation of fcc Au and bcc Fe containing pre-introduced He bubbles is carried out, at strain rates ranging from 10⁻³ to 10⁵ s⁻¹ to a 10²% strain at −180 and 25 °C. Microstructures in the deformed regions are examined by transmission electron microscopy. Rows of bubbles are formed due to extreme elongation of bubbles under stress and its subsequent division into smaller pieces in response to vacancy diffusion around the bubble surfaces. The bubble rows are parallel to the low-index crystallographic directions, 001, 011, and 012 for Au and 011 and 001 for Fe, which can be resolved into ‘slip directions’. The results indicate that displacement of atoms in these thin-foil specimens during tensile deformation progresses while conforming to the nature of the crystal, even in the absence of dislocations.