Unrecrystallized Regions Research Articles

Microstructure evolution and mechanical properties of the compressively deformed AgInCd alloy during annealing treatment

The microstructure evolution and mechanical properties of the compressively deformed AgInCd alloy during annealing treatment have been investigated in the present study. In the deformed AgInCd alloy, many deformation markings are formed and these deformation markings are mainly composed of nanoscale deformation twins. The AgInCd alloy exhibits a significant work hardening capability because of the formation of deformation twins. The recrystallization temperature of the deformed AgInCd alloy is between 300 °C and 350 °C. In the AgInCd alloy annealed at 300 °C, the deformation twins are eliminated and a large number of dark strips consisting of stacking faults are presented in the un-recrystallized regions, indicating that the deformation twins are transformed into stacking faults by the detwinning mechanism. The annealing twins show higher thermal stability than the deformation twins. The strength and microhardness of the deformed AgInCd alloy gradually decrease with the increase of annealing temperature.

Deformation behavior of ultra-fine-grained Mg-0.3 at% Al alloy in compression

Fine and coarse grained (5–20 μm) and ultra-fine-grained (down to 1 μm) samples of a binary Mg-Al alloy were fabricated by extrusion at various temperatures. When tested in compression, a slight stress drop was observed after yielding, followed by a plateau in flow stress. This behavior is typical of textured fine-grained samples. Examination of deformed structures within the ultra-fine-grained samples at various levels of deformation showed that the stress drop after yielding is due to an occurrence of long, thin twins across low angle boundaries and in unrecrystallized regions. In the plateau region, extensive twinning occurs inside individual grains. Then, in a rapid strain hardening region that follows the plateau region, stress concentrations were found to build up on boundaries. Such pile-ups could be responsible for a lowering of fracture strain.

Crystallography of grain boundary precipitates in a Ni-Mo-Cr-W low CTE alloy

The crystallography of grain boundary (GB) precipitates in a low coefficient of thermal expansion (CTE) Ni-Mo-Cr-W alloy has been investigated as a function of thermal history using transmission electron diffraction. A previously unreported cubic precipitate (φ) was found to form during aging at 1043 K for 16 h and remained after an additional heat treatment at 1143 K for 16 h. The space group of this phase is Pm3¯m with lattice parameter a∼6.42 Å. In addition, the topologically close packed (TCP) phase known as μ in the literature also formed on grain boundaries when the material was heat treated at 1043 K for 16 h directly after cold rolling. Significantly, the μ phase that formed on the boundaries of unrecrystallized regions contained a high density of finely spaced (0001) nanotwins, which give rise to pseudo 6/mmm symmetry instead of the 3¯m symmetry of the μ phase (R3¯m space group) that formed on the boundaries in the recrystallized regions. After additional aging at 1143 K for 16 h, these nanotwins are absent, and the μ phase develops more widely spaced {11¯02} twins.

Texture evolution and microstructural thermal stability of as-extruded AA2099 during hot deformation

Two thermal-mechanical regimes were selected to investigate the recrystallization process of an as-extruded AA2099 alloy. Specimens were deformed to strains of 0.4, 0.7 and 0.9 at 520°C and strain rates of 0.001s−1 and 0.1s−1, respectively. Electron backscatter diffraction (EBSD) and electron channeling contrast (ECC) imaging were performed for all specimens to characterize their textures and microstructures. The texture evolution is reflected by variation of deviation angle between <111> and extrusion directions, which appears to progressively approach ~35° (<110>//ED) due to lattice rotation in both regimes before ε=0.7. The reconstruction maps indicate that the nucleation of recrystallization is related to strain incompatibility near grain boundaries. The textures of specimens deformed to strain of 0.9 exhibit significant difference between the two regimes, i.e the <110>//ED fiber texture is comparatively weakened by recrystallization nuclei growth at the lower strain rate (0.001s−1). ECC images for specimen deformed at 0.001s−1 reveal significant differences in size and volume fraction of Al3Zr particles between recrystallized and unrecrystallized regions, suggesting remarkable influence of boundary migration on the thermal stability of Al3Zr particles.

Processing and microstructure–property relations of high-strength low-alloy (HSLA) Mg–Zn–Ca alloys

Deformation dilatometry and semi-industrial extrusion were used to investigate the effect of different thermomechanical processing routes on the microstructure and mechanical properties of the low-alloy Mg alloys ZX10 (Mg–1Zn–0.3Ca) and ZX00 (Mg–0.5Zn–0.15Ca). It is shown that the deliberately adjusted formation of intermetallic particles beneficially influences dynamic recrystallization and grain growth, with the result of a fine-grained microstructure (grain size<2μm). The presence of unrecrystallized regions with its unfavorable influence on ductility and mechanical anisotropy can be controlled by the selection of an indirect extrusion mode. Meta-dynamic recrystallization generates almost fully recrystallized microstructures and hence the desired properties, which are characterized by high strength (yield strength≈240MPa), simultaneously high ductility (elongation to fracture≈30%), and low structural and mechanical anisotropy. These properties are of great interest for light-weight applications and for deployment as biodegradable implants in medical technology.

Microstructure evolution at the onset of discontinuous dynamic recrystallization: A physics-based model of subgrain critical size

• Evolution of subgrains to recrystallization nuclei is possible at very low strains. • Dynamic recrystallization nuclei increases on a sigmoidal curve with strain. • Nucleation rate increases up to a maximum and decreases to zero at peak strain. • Growth of recrystallized nuclei diminishes to zero at or over the peak. • Site saturation is the mechanism of dynamic recrystallization up to the peak. A physical model based on the evolution of subgrains size is proposed to describe the nucleation and growth processes during discontinuous dynamic recrystallization. The evolution of subgrains to viable recrystallization nuclei was found possible at very low strains. Afterwards, the number of stable nuclei considerably increased on a sigmoidal trend with strain and reached a saturated state at about 0.6 times the peak strain. The dependence of nucleation rate on strain was modeled using an Avrami-type equation and the driving force for the growth of recrystallized nuclei was similarly modeled in terms of strain. It is also shown that “site saturation” is the governing mechanism for the initiation of the discontinues dynamic recrystallization at the grain boundaries. The flow stress of the material was calculated using the law of mixture of recrystallized and unrecrystallized regions with fractional softening as the stress-partitioning factor. Satisfactory agreement between predicted and experimental results was obtained, thereby confirming the validity of the proposed model.

Extruded Mg–Zn–Ca–Mn alloys with low yield anisotropy

Rare-earth-free Mg–5.99Zn–1.76Ca–0.35Mn (wt%) magnesium (Mg) alloys with low yield anisotropy and moderate performances were developed by utilizing hot extrusion. The sample extruded at 350°C exhibited a high compression/tension yield ratio close to 1 along the extrusion direction due to its high volume fraction of the dynamically recrystallized (DRXed) grains (approximately 98%), its fine DRXed grains (average of 2.7μm) and its extremely weak basal texture (nearly the same as that of the as-homogenized sample). The sample extruded at 300°C also exhibited a compression/tension yield ratio of 0.89 with a tensile elongation to failure of 16% and a tensile 0.2% proof stress of 289MPa. The high compression/tension yield ratio and the medium performances of the sample extruded at 300°C are attributable to its refined microstructure consisting of the fine DRXed grains (average of 1.4μm) and the elongated unrecrystallized regions of approximately 12μm in width with high number density of fine Mg6Zn3Ca2 and a-Mn precipitates. During compression test, nearly no {10–12} extension twinning was observed in the fine DRXed grains region and the formation of {10–12} extension twinning was also effectively restrained in the unrecrystallized regions due to their refined size in width, their high density of precipitates and their relatively weaker basal texture than other basal-textured Mg alloys.

Room and elevated temperature mechanical properties in the as-extruded Mg–Al–Ca–Mn alloys

Fine DRXed grains are formed in conjunction with coarse unrecrystallized regions in the as-extruded Mg–4Al–3Ca–0.3Mn (mass%) alloy produced by gravity casting technology. The unrecrystallized region exhibits strong basal texture whose basal plane is parallel to the extrusion direction. Yield anisotropy is appeared in the compressive test sample measured at ambient temperature due to formation of tensile twinning in the unrecrystallized region. The obtained 0.2% proof stresses in tension and compression are 331 and 285 MPa, respectively. The yield anisotropy is improved when the test temperature increases, leading to remarkable elevated temperature mechanical properties. The room and elevated temperature mechanical properties are comparable to those in WE43A Mg and T6-treated A4032 Al alloys.

High strength magnesium alloy with α-Mg and W-phase processed by hot extrusion

Fine-grained Mg-6Zn-4Y alloy was prepared by an ingot metallurgy process with hot extrusion at 300 °C. The microstructure was studied by XRD, OM, SEM and TEM, and the tensile properties were tested at room temperature. The results show that the alloy is composed of α-Mg and W-phase. The microstructure of the as-extruded alloy has a bimodal grain size distribution. The fine grains with the mean size of 1.2 μm are formed by dynamic recrystallization. The coarse grains (about 23% in area fraction) are unrecrystallized regions which are elongated along extrusion direction. The engineering stress—strain curve shows a pronounced yield point. The ultimate tensile strength, yield strength, and elongation are (371±10) MPa, (350±5) MPa and (7±2)%, respectively. The high strengths are attributed to the fine-grained matrix structure enhanced by W-phase particles, nano-scaled precipitates, and strong basal plane texture.

Effect of Zr addition on the mechanical properties of as-extruded Mg–Zn–Ca–Zr alloys

Extrudable Mg–6Zn–0.2Ca(–0.8Zr) alloy (mass%) has been developed. The Mg–Zn–Ca–Zr alloy shows excellent mechanical properties in the as-extruded state with the ultimate tensile strength of 357 MPa and elongation to failure of 18%. These properties are related to dynamic recrystallization, texture and precipitation. Dynamically recrystallized grains are pinned by fine precipitates, resulting in very fine grain size. The extruded sample also exhibits a higher average {0 0 0 2} 〈 1 1 2 ¯ 0 〉 Schmid factor due to the presence of unrecrystallized regions, leading to texture strengthening. Zr may work as a microalloying element, resulting in fine and dense distribution of a MgZn 2 based phase involving Ca and Zr.

Structure of titanium after dynamic channel angular pressing at elevated temperatures

Dynamic channel-angular pressing of titanium at a temperature of 500°C has been performed. An increase in the temperature prevented the formation of cracks and adiabatic-shear bands that usually occur upon pressing at room temperature. As a result of dynamic pressing of titanium at 500°C, there is obtained a structure which represents a dispersed mixture of fine recrystallized grains and unrecrystallized regions (duplex structure). The formation of recrystallized grains is caused by a local increase in the temperature in the sites of deformation localization and is a mechanism of relaxation of accumulated stresses. The recrystallized grains are grouped into inclined extended shear bands (large-scale relaxation) and into short bent chain of clusters, which are located between inclined bands (small-scale stress relaxation). The unrecrystallized regions consist predominantly of elongated subgrains formed as a result of deformation and dynamic polygonization. The microhardness of titanium in the recrystallized regions is equal to 1980 MPa, that in unrecrystallized regions, to 2150 MPa.

The role of Zr-rich cores in strength differential effect in an extruded Mg–Zn–Zr alloy

This work examines the effects of extrusion parameters namely ratio and temperature on recrystallization behavior of a Mg–Zn–Zr alloy, and their consequent effects on anisotropy in the mechanical properties. Upon extrusion, the characteristic Zr-rich cores that do not recrystallize form the so-called “soft stringers” which are deformed bands elongated in the extrusion direction and curled around the extrusion axis. At higher extrusion ratio, there is more twinning contribution and the DRX response is improved, making the recryatallized grains finer and increasing the proportion of recrystallized Zr-rich cores. A basal texture after extrusion and the directional activation of tensile twinning cause anisotropy in the mechanical properties. In addition, the microstructural features such as large unrecrystallized regions and coarse crystallized grains also contribute in the strength differential effect. Further slip in the strain-hardened unrecrystallized grains is inhibited while twin activation under favorable orientation becomes easier in the coarse recrystallized grains. A higher proportion of large unrecrystallized and coarse crystallized gains in the case of lower extrusion ratio result in a much higher strength differential effect (∼100 MPa) in comparison to the one caused by the crystallographic texture only (∼25 MPa).

The Avrami kinetics of dynamic recrystallization

Twenty-six flow curves determined under dynamic recrystallization (DRX) conditions on 11 different steels were analyzed. The double-differentiation method was used to define the critical strain ε c for initiation of DRX. The “athermal” hardening parameter h and the dynamic recovery parameter r were evaluated from the work-hardening behavior prior to ε c. The saturation stresses σ sat pertaining to the unrecrystallized regions as well as the associated work-hardening curves, σ recov, were derived in this way. The net softening attributable to DRX was then defined as the difference between the σ recov and experimental curves. Avrami plots were prepared from all 26 sets of data. The time exponents were all in the neighborhood of 3.0; however, the strain rate and temperature dependences differed considerably from their static values. The dependence of the time of half-softening, t 50, on strain rate and temperature is used to predict the DRX curves expected under industrial conditions.

Grain Refinement and Superplasticity Induced by Hot Compression of Continuously-Casted Mg-9Al-1Zn-1Ca and Mg-9Al-1Zn Alloys

Compression tests were conducted at the temperature of 573 K with the true strain rates of 0.1 s � 1 on Mg-9Al-1Zn-1Ca and Mg-9Al-1Zn (in mass%) alloys, and the dynamic recrystallization behaviors were examined. Superplastic deformation of the alloys at 573 K after the hot compression was also investigated. When the homogenized Mg-9Al-1Zn-1Ca alloy specimens were deformed to the true compressive strain of 1.6, a fully recrystallized microstructure with the grain size of approximately 5 mm was obtained. However, unrecrystallized regions were observed locally in the hot-compressed as-cast Mg-9Al-1Zn-1Ca and homogenized Mg-9Al-1Zn alloys, which is induced by initial microstructural heterogeneity. The superplastic deformation at 573 K of hot-compressed Mg-9Al-1Zn-1Ca was negligibly impaired by the second phase particles below the strain rates of 10 � 2 s � 1 because the stress concentration due to the particles can be relaxed by diffusion. Microstructural observations of the hot-compressed Mg-9Al-1Zn alloy deformed to fracture suggested the detrimental effect of the unrecrystallized regions on the superplasticity. [doi:10.2320/matertrans.MBW200819]

Resting and ambulatory EKG changes in patients receiving cell therapy for severe coronary artery disease

The landing gear of an aircraft failed during its landing procedure due to the fracture of the outer cylinder, which is manufactured from an Al–5.7Zn–2.7Mg–0.5Mn–0.5Cu alloy. The material presented an oriented partially recrystallized microstructure, with the unrecrystallized areas showing intense intragranular precipitation, while the recrystallized areas presented fine intergranular precipitation. Fractographic examination revealed that the unstable crack propagation was originated in the fatigue cracks, which were approximately 0.6 mm deep, and occurred mainly by a mechanism of ductile intergranular fracture. Both stable and unstable crack propagation occurred preferentially parallel to the orientation of the microstructure. Fracture toughness KIC tests confirmed that LT orientation presented KIC values 60% higher than TL, although it presented comparatively a higher proportion of intergranular fracture. Fractographic observation of KIC test pieces showed that unstable crack propagation occurred preferentially along the boundaries of recrystallized grains. TL orientation showed additional propagation along the boundaries between recrystallized and unrecrystallized regions. The elongated morphology of primary and secondary precipitates aligned perpendicular to the loading direction is thought to be responsible for the lower fracture toughness observed for TL orientation. The fatigue crack observed in the landing gear was nucleated on the intersection of the aligned microstructure with the free surface, condition that should be avoided by efficient design of structural components. The abnormal loads absorbed by the outer cylinder during the landing procedure, associated with the presence of fatigue pre-cracks, may be concluded as the cause for this failure.

The Effect of Nb on the Recrystallization and Grain Growth of Ultra-High-Purity α-Fe: A Combinatorial Approach

A combinatorial approach to the problem of the effect of Nb in solid solution on the conditions for recrystallization and grain growth in ultra-high-purity (UHP) α-Fe is presented. It is shown to be an efficient means for estimating parameters such as the binding energy of solute to grain boundaries, the transboundary solute diffusivity, and the intrinsic mobility of grain boundaries, crucial for practical applications of existing solute drag theories to questions related to alloy design. Using a Fe-Nb diffusion couple (Nb contents ranging from 0 to 0.095 wt pct) transformed within a temperature gradient (RT-850 °C), the boundary between the recrystallized and unrecrystallized regions and the grain growth behavior as a function of Nb content and temperature could be quantitatively rationalized using the slow branch of Cahn’s solution to the solute drag problem.

Phase field simulation of stored energy driven interface migration at a recrystallization front

Phase-field simulation of the migration of a recrystallization front was performed based on the unified subgrain growth theory. The effects of the subgrain structure developed in the deformed state on a recrystallization front were investigated with the focus on the growing of the recrystallizing grain. Especially, the effects of the mean misorientation, 〈 θ CC〉, and the initial mean subgrain radius, 〈 R C〉(0), in unrecrystallized region were evaluated. The critical misorientation angle, which gives the maximum migration distance, was highly dependent on the relations between misorientation and boundary properties (i.e. energy and mobility). And the value of 〈 R C〉(0) had significant effects on the velocity of the recrystallization front if the value of 〈 θ CC〉 was favorable for discontinuous recrystallization.

Tensile properties in W-containing high Co–Ni steels

The effect of the addition of Cr on tensile properties in high Co–Ni secondary hardening steels containing W has been seen in as-quenched and aged conditions. Two kinds of Co–Ni steels, containing only W (W steel), and W and Cr (WCr steel), were severely multipass-rolled in the unrecrystallized region, followed by direct quenching. Both steels showed secondary hardening peaks with precipitation of fine M 2C carbides during aging. The secondary hardening reaction was promoted by the Cr addition although there was some decrease in tensile strength. In terms of tensile behavior, it is pointed out that both yield strength and yield ratio (YS/UTS) were considerably decreased by the Cr addition. In addition, the tensile strength to hardness ratio is larger in the as-quenched condition compared with the aged condition.

Dynamic recrystallization in Al-single crystals revealed by synchrotron radiation Laue technique

An experimental study on the microstructure development and stress–strain behavior during high temperature deformation of aluminum (Al) single crystals was made by using the synchrotron radiation Laue technique and the electron backscatter pattern technique. The main purpose was to clarify the process of dynamic recrystallization (DRX). The measured stress–strain curves with large stress peaks and the in situ observed new Laue spots without streaks at around the stress peaks confirmed the occurrence of DRX. Crystallographic analysis shows that the common axis between the DRX grain and the matrix is close to 〈1 1 2〉. The unrecrystallized region near the DRX grain consists of subgrains adjoined each other with 〈1 1 2〉 tilt boundaries and the size of subgrains becomes smaller and the misfit at subgrain boundaries becomes larger as the DRX grain boundaries are approached. These experimental results suggest that DRX grains are nucleated through the development of subgrains.

Recrystallization and Texture Development in Hot Rolled 1050 Aluminum

The evolution of texture as a function of recrystallization has been characterized for hotrolled AA1050. Samples prepared from hot rolled sheet were annealed isothermally for sufficient time to allow complete recrystallization. The microstructural variation and texture evolution in the samples was observed by automatic indexing of Electron Back Scatter Diffraction (EBSD) patterns in a Scanning Electron Microscope (SEM). The spatial orientation variation within the deformed microstructure of nucleation, growth and orientations of recrystallized grains was examined. The orientation spread within grains was found to be a useful quantity for partitioning recrystallized and unrecrystallized regions. Also the effect of deformation texture on the evolution and growth of various recrystallization texture components was analyzed. The analysis is aimed at obtaining a correlation between the deformation microstructure, texture development and recrystallization kinetics in the hot-rolled condition. Preliminary results suggest only a weak correlation between the rate of recrystallization and the deformation texture component.