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The defects due to cold-rolling deformation in the advanced reduced-activation alloy (ARAA) have been analyzed using positron annihilation lifetime spectroscopy (PALS). Four different cold-rolled conditions up to 35% reduction from 0% have been used in this study. The mean lifetime of the positron at the cold-worked ARAA samples increased over that of the as-normalized sample, which was comparative to the increase in the amount of grain boundaries in the electron backscatter diffraction images. PALS could determine microstructural change in defects of the cold-rolled ARAA.

An advanced reduced activation alloy (ARAA) has been developed as a structural material for breeding blankets in fusion reactors. A breeding blanket has a relatively complex structure, so various welding methods, such as electron beam welding and similar and dissimilar tungsten inert gas (TIG) welding have been developed for the manufacture of breeding blankets. In this article, mechanical properties of joints of dissimilar TIG welding are introduced using ARAA and SS316L. Charpy impact, tensile, and bending tests were carried out for the as-postweld heat-treated (PWHT) samples manufactured with defined welding conditions, and hardness tests were conducted for the as-welded and the PWHT samples. The average hardness values of the dissimilar welded samples were 338 HV in the heat-affected zone (HAZ) and 199 HV in the weld metal (WM). The PWHT decreased hardness to 223 HV in the HAZ but did not affect the hardness in the WM. The yield strength (YS) and ultimate tensile strength (UTS) of the dissimilar TIG welds were 335 and 617 MPa, respectively.

In order to study the mechanical properties and the anisotropy of a Zr-2.5 wt%Nb pressure tube material, tensile tests along the principal tube directions, which are axial, transverse and radial, were performed at room temperature using a pressure tube. The effect of tube length was evaluated, by using samples from the tube's back-end, middle, and front-end. Strong anisotropy among the principal directions was found, and increased strengths were obtained with increased distance from the front-end location. Microstructure analysis by X-ray diffraction, electron backscatter diffraction, and scanning electron microscopy indicated that increasing defect density along the tube was the main reason for the increasing strength. Although texture mainly determined the anisotropy of the tensile strengths, the ratio between the axial and radial strengths could be considerably altered by hydride plates. Depending on how the hydride plate was placed with regard to the deformation direction, the yield strength and strain hardening ability were reduced differently by the hydride. The deterioration effect of the hydrides did not show linear dependency on the hydrogen contents due to other microstructure variations. We had a brief discussion about which parameters might result in the non-linear change found in the deterioration along the tube.

A new second nearest-neighbor modified embedded-atom model-based PtTi binary interatomic potential was developed by improving the pure Pt unary descriptions of the pre-existing interatomic potential. Specifically, the interatomic potential was developed focusing on the shape memory-associated phenomena and the properties of equiatomic PtTi, which has potential applications as a high-temperature shape memory alloy. The simulations using the developed interatomic potential reproduced the physical properties of the equiatomic PtTi and various intermetallic compound/alloy compositions and structures. Large-scale molecular dynamic simulations of single crystalline and nanocrystalline configurations were performed to examine the temperature- and stress-induced martensitic transformations. The results show good consistency with the experiments and demonstrate the reversible phase transformation of PtTi SMA between the cubic B2 austenite and the orthorhombic B19 martensite phases. In addition, the importance of anisotropy, constraint and the orientation of grains on the transformation temperature, mechanical response, and microstructure of SMA are presented.

The hot deformation behavior of advanced reduced-activation alloy (ARAA) was investigated using hot torsion tests. The flow stress decreased as deformation temperature increased and as strain rate decreased. The flow behavior demonstrated the typical dynamic recrystallization (DRX). Based on the constitutive analysis of peak stress, the activation energy for hot deformation was found to be 330.3 kJ mol−1. Peak stress was analyzed as a function of the Zener–Hollomon parameter, and calculated and experimental values were in good agreement. A DRX kinetic model for ARAA was derived with deformation conditions based on the Avrami-type model. It was confirmed that the volume fraction of dynamically recrystallized grains increased as deformation temperature increased and as strain rate decreased. The necklace structure and grain boundary bulging were observed in the deformed microstructure of ARAA. The suggested DRX mechanism for ARAA during hot working is discontinuous DRX.

Korea has own programs toward DEMO and fusion reactors that will require further improved DEMO blanket and energy utilization systems. Primary goals of DEMO blanket development in Korea are to develop and verify the integrated blanket design tools; to develop blanket materials, cooling and tritium fuel cycle technologies; and to develop and evaluate fabrication and joining technologies. The concept of helium-cooled ceramic reflector (HCCR) blanket is adopted to be tested in ITER as Test Blanket Module (TBM). Currently, the design and R&D activities are mainly performed through the ITER TBM program in Korea. It is expected to demonstrate the major objectives of the breeding blanket: extraction of heat from burning plasma, tritium self-sufficiency and its integrity within the whole systems considering safety features. Although TBM program will provide very essential data and experience, there will be a considerable technical gap from DEMO blanket. This paper presents the technical gap in the main technical categories of the breeding blanket based on the experience of the development of HCCR TBM and breeding blanket. It is found that still about 60–70% of the DEMO blanket technology can be achieved through the HCCR TBM depending on the technology maturity level, and some ways to DEMO blanket from HCCR TBM are proposed.

We studied effects of thermo-mechanical processing (TMP) on microstructure and mechanical properties of the Korea RAFM steel, ARAA, as a feasibility study for enhancement of creep-resistance. Austenitized ARAA plates were subjected to three different TMP schedules: i) 20% hot-rolling at 1000 °C, ii) 20% hot-rolling at 700 °C, and iii) 20% hot-rolling at 700 °C followed by 30% cold-rolling, all of which were then subjected to tempering at 750 °C. 20% hot-rolling resulted in strengthening effects on the as-tempered ARAA in both tensile and short-term creep tests, the extents being greater for that rolled at lower temperature. However, additional cold-rolling promoted recrystallization of tempered martensite structure, leading to a significant loss of strength. Enhanced tensile and creep strength of the TMPed ARAA can be attributed to the larger number of fine MX carbides and slightly higher density of dislocations. It is concluded that ausforming, which is simple and readily applicable to industry, is a method that can effectively improve the high-temperature creep strength of ferritic-martensitic steels without a loss of impact toughness.

The microstructural evolution of oxide and nitride dispersed nickel-based mechanical alloyed (MA) powders was investigated using in situ characterization methods, including high-temperature X-ray diffraction and transmission electron microscope (TEM). MA powders strengthened by Si3N4 and Er2O3 exhibited different behaviors. The added Si3N4 particles were not dissolved during the MA process, resulting in faster strain recovery and grain coarsening in the MA powders. In contrast, the Er2O3 particles were dissolved into the matrix during the MA process. Also, precipitation of Er2Ti2O7 particles with an average size of 10 nm was observed by in situ TEM. The precipitation of nanoparticles was related to strain recovery and grain growth, as both reduced the solubility of solute atoms in the matrix. The in situ TEM observation results provide direct experimental evidence for the dissolution–precipitation mechanism in nickel-based MA powders.

Nickel alloy powders with additions of Gd2O3 and Ti were mechanically alloyed (MA) using a high-energy ball mill. The formation behaviors of nanoparticles in nickel-based MA powders were investigated using in situ characterization methods, including high-temperature X-ray diffraction and in situ transmission electron microscope (TEM). The added Gd2O3 particles were dissolved into the matrix during MA process. Recrystallization of the MA powders occurred between 810 and 880 °C, followed by grain growth. Precipitation of nanoparticles was observed after grain growth using in situ TEM and was attributed to the reduced dislocations and grain boundary area. More importantly, these nanoparticles were dissolved into matrix after the specimen was reheated to a higher temperature of 1014 °C. During subsequent cooling, Gd2TiO5 particles were re-precipitated in a more uniform way, including smaller average size of 6.8 nm and reduced interparticle spacing of 26.9 nm.

As an effort to develop corrosion resistant and highly formable austenitic steels, Fe–18Mn–9Cr–2Al-(0–0.5)C steels in wt. % were designed and their microstructures and tensile properties were investigated with particular attention being paid to the effects of carbon concentration. The carbon-free steel consisted of the austenite, δ-ferrite and e-martensite phases in as heat-treated condition while those of carbon-added steels had an austenite single phase with the (Fe,Cr)23C6 carbides at grain boundaries. When tensile strained at room temperature, all the steels exhibited extensive deformation twinning, resulting in large work-hardening and excellent ductility. Deformation twinning was suppressed at 300 °C, and dynamic recrystallization took place partially at grain boundaries when strained at 600 °C. The addition of Cr and hence formation of carbides in carbon-added steels have little influence on plasticity of the conventional twinning-induced plasticity steels, suggesting the possibility of developing Cr-bearing TWIP steels with both corrosion resistance and excellent formability.

After annealing the Zr-2.5wt%Nb pressure tube material at 700 ∘ C for 24h, the largest grain aspect ratio reduced from 3.1 to 1.8, and the average minimum grain size increased from 0.2 μ m to 0.9 μ m . In addition, Kearns’ factors characterizing texture along specific directions became more anisotropic by the annealing. While the annealed microstructure resulted in the decreases of all compressive strengths along three directions—transverse, axial, and radial, the anisotropy in the yield strengths displays uneven changes. The ratio of the transverse and axial strengths remains nearly constant. On the other hand, the ratio of the radial and the axial becomes lower. In the study, the origin of the changes in the strength anisotropy was investigated by the crystal plasticity finite element method (CPFEM). To consider the alteration of grain morphology, the grain shape model is included in the material constitutive model using a well-known Hall–Petch type relation. Through iteratively comparing the calculated yield strengths to the compressive yield strengths, the best-estimated critical resolved shear stresses (CRSSs) of the deformation systems were obtained without and with the grain model. In the latter case, the dependencies of CRSSs on the grain size were found as well. Although the effect of the grain model on the yield strengths was apparent, its influence on determining the anisotropic ratio was insignificant. On the other hand, changing texture information led to a quite definite variation in the yield anisotropy. It implies that the decrement in the ratio of the radial and the axial yield strengths could be caused mainly by the texture modification after annealing. In summary, the anisotropy in the yield strengths of the Zr-2.5wt% pressure material is mainly controlled by the texture and partly affected by the grain morphology.

Development of the Helium Cooled Ceramic Reflector (HCCR) breeding blanket is underway according to the development strategy of core technology for K-DEMO under the fusion energy development roadmap in Korea. The main goals are to validate integrated design tools and to develop core technologies in the field of materials, manufacturing and joining technologies, helium cooling and tritium technologies, system integration, and safety technologies. A unique graphite reflector concept was adopted to save cost by reducing the amount of beryllium neutron multiplier. The main functions and design concepts of the HCCR breeding blanket are to be verified in ITER through the HCCR Test Blanket Module (TBM) program. Not only breeder modules are to be tested in ITER but also the technologies of cooling, coolant purification, tritium extraction, etc. are to be tested or proved in ITER before employed in DEMO. This paper introduces the current updates of the design and R&D activities of the HCCR TBM systems in the preliminary design stage.

Chemical compatibility between reduced activation ferritic-martensitic steel, ARAA, and lithium meta-titanate breeder was investigated under operating conditions; high temperature and helium purge gas including a low concentration of hydrogen. ARAA specimens were embedded inside lithium meta-titanate powder and compacted under a load of 200 MPa to form disk-shaped samples. The samples were heated at 550 °C for up to 1000 h in helium with up to 1% hydrogen atmosphere to simulate a fusion breeding blanket environment. The surface of the ARAA specimen was chemically reacted with the lithium meta-titanate breeder to form an oxide layer. The thickness of the oxide layer increased as the heating time increased. The hydrogen in the purge gas made the oxide layer thinner due to the reaction with oxygen in the oxide layer. The growth phenomena of the oxide layer were evaluated.

This study investigated evolution of the microstructure of oxide dispersion strengthened Alloy 617 with annealing temperature. A mixture of prealloyed Alloy 617 and Y2O3 powders was mechanically alloyed and consolidated by hot-extrusion at 1100 °C. Hot extrusion developed a submicron-sized grain structure with M23C6 carbides and finely dispersed Al2O3 and Y2Ti2O7 oxides. The fine-grained structure was stable during subsequent annealing at temperatures up to 1250 °C. Further increase of annealing temperature to 1300 °C resulted in a significantly coarsened grain structure, which was coincident with the abrupt coarsening of oxides. M23C6 carbides in the as-extruded conditions were transformed to M7C3 carbides with complex shapes when annealed at 1200 °C, and their shapes changed to very coarse hexagonal prisms at 1250 °C, which was followed by the formation of eutectic M2C carbides at grain boundaries at 1300 °C. Tensile tests of the as-extruded ODS Alloy 617 showed that the yield strength decreased steeply at a transition temperature of around 600 °C, which can be attributed to diffusional creep along the grain boundaries.

Oxide dispersion strengthened Alloy 617 was prepared by mechanical alloying and hot extrusion processes. Precipitation behavior during heat treatment was evaluated by XRD, SEM and TEM. It was revealed that the dominant precipitates in the solution-annealed sample were Cr-rich M23C6 and M6C precipitates on grain boundaries, and Al–O, Y–Al–O and Y–Ti–O nano-particles in grain interior. Different from a conventional Alloy 617, TiN and TiC precipitates were not found. Aging at 760 °C for 100 h showed that γ′ were not formed, possibly due to the high affinity of O to Al and Ti, which suppressed the formation TiN, TiC and γ′. Heat treatment at high temperatures showed that M23C6 and Y2Ti2O7 were stable up to 1300 °C, while most Al–O nano-particles evolved into Y–Al–O. The high thermal stability of fine Y2Ti2O7 particles with a high number density is expected to contribute to a superior high temperature mechanical strength than γ′ precipitates .

Oxide dispersion-strengthened (ODS) alloys possess several advantages such as excellent high-temperature creep strength and oxidation/irradiation resistance. Lanthanide oxides as well as yttria (Y2O3) have been proven to act as efficient additives in Fe-based ODS alloys; however, their potential for addition to Ni-based alloys has been limited. In this study, we fabricated four types of Ni-based ODS alloys with the combined addition of Ti and either Y2O3, cerium dioxide (CeO2), lanthanum oxide (La2O3), or gadolinium (III) oxide (Gd2O3) by mechanical alloying (MA) and hot isostatic pressing (HIP). We characterized both MA powders and HIPed samples of each alloy to compare the effect of oxide addition on the microstructure evolution. The results indicated that the addition of La2O3 had the most significant influence on refining oxide dispersoids and delaying recrystallization, resulting in a uniformly distributed ultra-fine grain structure. Based on further investigation of the contributions of various strengthening mechanisms, it was also found that the refinement of both oxide particles and grain size with the addition of La2O3 contributed to significant dispersion strengthening and grain boundary strengthening, leading to the highest Vickers hardness among the present alloys. When CeO2 and Gd2O3 were added, the mechanical properties were comparable, but better than those when Y2O3 was added. This demonstrates that all of the selected lanthanide oxides have practical potential as alternative additives to develop Ni-based ODS alloys, among which La2O3 would be the most advantageous in improving the mechanical properties.

Korea has developed an advanced reduced-activation alloy (ARAA) as a structural material for helium-cooled ceramic reflector test blanket module (HCCR-TBM) applications. The present paper describes the history of alloy development and the properties of ARAA, which has been produced at a 5 t scale using vacuum induction melting and electro-slag re-melting methods. ARAA is a 9Cr–1.2W based ferritic–martensitic steel with 0.01 wt.% Zr. The mechanical properties, thermal properties and physical and magnetic properties of ARAA show similar temperature dependencies to those observed for Eurofer 97. However, ARAA exhibits a much longer creep–rupture time than conventional RAFM steel, which suggests a positive effect on Zr addition. The enhanced creep strength of ARAA by the addition of Zr is attributed to the reduced temperature-dependence of the yield strength.

The influence of high-temperature annealing on the microstructure of zirconium carbide (ZrC) was investigated in relation to its application as a coating layer of a nuclear fuel in a very high temperature gas cooled reactor. ZrC was deposited as a constituent coating layer of TRISO coated particles by a fluidized bed chemical vapor deposition method using a ZrCl 4 -CH 4 -Ar-H 2 system. The grain growth of ZrC during high-temperature annealing was strongly influenced by the co-deposition of free carbon. Sub-stoichiometric ZrC coatings have experienced a significant grain growth during high-temperature annealing at 1800 °C and 1900 °C for 1 h. On the other hand, a dual phase of stoichiometric ZrC and free carbon experienced little grain growth. It was revealed that the free carbon of the as-deposited ZrC was primarily distributed within the ZrC grains but was redistributed to the grain boundaries after annealing. Consequently, carbon at the grain boundary retarded the grain growth of ZrC. Electron backscatter diffraction (EBSD) results showed that as-deposited ZrC had (001) a preferred orientation that kept its favored direction after significant grain growth during annealing. The hardness slightly decreased as the grain growth progressed.

The cyclic softening behavior of reduced activation ferritic-martensitic (RAFM) steel was investigated by low-cycle fatigue tests with and without dwell times at 550 °C, and a mechanistic understanding of the softening was achieved by analyzing the grain boundaries and crystal orientation in the grains using the electron backscatter diffraction (EBSD) technique. The cyclic stress response of the steel was divided into two distinct rate stages with the number of cycles, namely, the initial rapid stage and the uniform softening stage. The former stage was dependent on and the latter was independent of the input strain amplitudes and fatigue modes. The softening in the two stages was led by both the disappearance of lath and packet boundaries and the release of lattice distortion by dislocations in the initial RAFM steel; their relative contributions in each stage were different depending on the input strain ranges and fatigue modes. A new equation was proposed as a measure of the lattice distortion in grains, which showed a strong correlation with the amount of cyclic softening under different fatigue conditions.

The influences of Zr addition on the mechanical properties of reduced-activation ferritic–martensitic steel were investigated. 9Cr-based steels with Zr contents varying 0–0.01 wt% were designed and produced by vacuum induction melting, and then formed into a plate by hot rolling, which was followed by normalizing and tempering. The addition of 0.005 wt% Zr improves the impact resistance with a slight sacrifice of yield strength for a given tempering condition. A short-term creep test shows that the addition of 0.01 wt% Zr significantly increases the time to rupture. Such improvement in impact and creep resistances by the addition of small amounts of Zr is attributed to the solution softening effect of Zr, which is associated with a reduced temperature-dependence of the yield strength. It is concluded that Zr has great potential as an alloying element in reduced-activation ferritic–martensitic steel for improving the mechanical properties.

In this study, Fe3+ self-ion irradiation is used as means of introducing irradiation damage in ferritic/martensitic (F/M) steel and oxide dispersion strengthened (ODS) steel. The ion accelerator named DuET (at Kyoto University, Japan) was used for irradiation with 6.4 MeV Fe3+ ions at 300 °C. The total number of accelerated ions was 2.5 × 1020 ions/m2, and the maximum damage rates in the F/M and the ODS steels were estimated to be roughly 6 dpa. The irradiation-induced hardness change in the damaged layer was evaluated by using nano-indentation. The F/M steel and the ODS steel commonly exhibited irradiation hardening; however, the irradiation hardening was more active in the F/M steel than in the ODS steel. The microstructure evolutions after the irradiation were investigated; point or line defects were dominantly observed in the F/M steel, while small circular cavities were typically observed in ODS steel.

As part of an alloy development program for Korean reduced-activation ferritic–martensitic (RAFM) steel, a total of 37 program alloys were designed and their mechanical properties were evaluated with special attention being paid to the effects of alloying elements and heat treatments. A reduction of the normalizing temperature from 1050 °C to 980 °C was found to have a positive effect on the impact resistance, resulting in a decrease in ductile–brittle transition-temperature (DBTT) of the program alloys by an average of 30 °C. The yield strength and creep rupture time are affected strongly by the tempering time at 760 °C but at the expense of ductility. Regarding the effects of the alloying elements, the addition of trace amounts of Zr enhances both the creep and impact resistance: the lowest DBTT was observed for the alloys containing 0.005 wt.% Zr, whereas the addition of 0.01 wt.% Zr extends the creep rupture-time under an accelerated condition. The enhanced impact resistance owing to the normalizing at lower temperature is attributed to a more refined grain structure, which provides more barriers to the propagation of cleavage cracks. Solution softening by Zr addition is suggested as a possible mechanism for enhanced resistance to both impact and creep of the program alloys.

Korea plans to test a Helium Cooled Ceramic Reflector (HCCR) Test Blanket Module (TBM) in ITER. The HCCR TBM adopts a four sub-module concept considering the fabricability and the transfer of irradiated TBM for post irradiation examination. Each sub-module has seven-layer breeding zone, including three neutron multiplier layers packed with beryllium pebbles, three lithium ceramic pebble bed packed tritium breeder layers, and a reflector layer packed with graphite pebbles. Based on this configuration, neutronic and electromagnetic calculations were performed and their results were applied for the conceptual design of HCCR TBM that considers manufacturing feasibility. Also, a design and safety analysis of HCCR Test Blanket System (TBS) was performed using integrated design tools modifying nuclear system codes for helium coolant and tritium behavior evaluation. The Advanced Reduced Activation Alloy (ARAA) is being developed as a structural material. A total of 73 candidate ARAA alloys were designed and their out-of-pile performance was evaluated. The graphite pebbles as the neutron reflector were fabricated by using mechanical machining and grounding method with the surface coated with SiC. The hydrogen permeation characteristics of structural materials were evaluated using the Hydrogen PERmeation (HYPER) facility. The recent design and R&D progress on these areas are addressed in this paper.

Korea has put in a huge effort to develop the future nuclear systems such as a sodium-cooled fast reactor (SFR), a very high temperature reactor (VHTR) and fusion reactor. As the realization of such nuclear systems relies heavily on performance of materials, Korea has made steady and persistent effort to develop structural materials that can withstand the extremely harsh operating conditions. The present paper summarizes the current status of the Korean R&D activities on advanced nuclear materials including oxide dispersion strengthened (ODS) steels, Ni- base alloys and their ODS variations, reduced-activation ferritic-martensitic (RAFM) steels, and SiC composites.

We investigate the effects of texture on shear banding in highly textured Mg–3Al–1Zn alloy. The plates with different textures were rolled to 10% reduction in thickness at various temperatures, and the microstructure, texture and misorientation distributions within the regions undergoing shear banding were analyzed with optical microscopy and X-ray diffraction and electron backscattering diffraction techniques. It is found that texture prior to rolling significantly affects the various features of shear bands such as the formation frequency of shear bands, the inclined angle of shear bands to the rolling plane, and orientations of grains in shear bands. We infer that shear bands in AZ31 rolled to a relatively low strain occur as a result of geometric (texture) softening. Shear banding is most pronounced in the plate with conventional strong basal texture, whereas the plate oriented favorably for the prism 〈a〉 slip shows no sign of shear banding. The inclined angle of shear bands in the plate with basal texture is around 30°, while those in the plates oriented for basal slip or tension twinning is around 55°. The regions of shear bands receive more strain than the region outside the bands, which results in significant lattice rotation of the grains in the bands, leading to off-basal texture.

The magnesium alloy AZ91 was hot rolled and annealed to produce a plate with a strong basal texture. The resultant plate was mechanically tested in five different test orientations in the solution-treated, underaged, peak-aged and overaged conditions. The critical resolved shear stresses for activation of basal slip, prismatic slip and twinning were then estimated for each of the ageing conditions. We found that the age hardening of the slip systems is consistent with Orowan hardening, but that this theory significantly underestimates the hardening of the { 1 0 1 ¯ 2 } twin system. It is proposed that back stresses and hardening of the slip systems that accommodate twinning also contribute to precipitate hardening of the twin system.

Based on analysis of texture and in-grain misorientation axes (IGMA) distribution, we investigate the effects of initial orientation and deformation temperature on the rollability of magnesium alloy AZ31 and the associated deformation mechanisms. Plate samples oriented favorably for basal 〈a〉 slip exhibited the best rollability at room temperature, whereas under the warm-rolling condition, surprisingly, the plate oriented for prism 〈a〉 slip exhibited the best rollability. The enhanced rollability of the latter plate is attributed to increased activity of prism 〈a〉 slip, which exhibits a lower texture hardening rate than basal 〈a〉 slip. The increased activity of prism 〈a〉 slip is shown experimentally by the development of the \( \langle 10\bar{1}0 \rangle \)//RD texture and 〈0001〉-type IGMA distribution. Asymmetric texture is also suggested to impair the rollability of plate oriented for basal 〈a〉 slip.

Commercial purity Ti was subjected to channel die compression in liquid nitrogen for the purpose of studying its deformation characteristics from the viewpoint of the grain refinement induced by severe plastic deformation. Deformed specimens showed microstructural heterogeneity in that the initial blocky grains of about 50 μm in size turned into a mixture of the easily deforming soft grains and the hard grains revealing local concentration of dislocations and deformation twins. Using electron back scattered diffraction analysis the in-grain misorientation-axis distribution was studied, from which it was found that the deformation heterogeneity resulted from the anisotropy of individual grains with respect to the compression die: while the soft grains were those oriented favorably for the prism slip, the hard ones were oriented for non-prism slips and deformation twinning. The difference in the response of individual grains, therefore, led to a remarkable effect of the crystal texture of specimens on the maximum strain that can be imposed on specimens without cracking. Based on the present results, a way to achieve expedient grain refinement through the cryogenic plane-strain compression was suggested.

This paper reports the effects of the texture and strain on the strain rate sensitivity of wrought magnesium alloy AZ31. Specimens were prepared from strongly textured AZ31 plate such that the uniaxial loading direction was either parallel or perpendicular to the basal planes of the majority of grains. The strain rate sensitivity of the specimens was determined from strain rate jump tests in tension and in compression, and the microstructure evolution during the tests was tracked by electron backscatter diffraction microscopy of ex situ samples. Specimens oriented unfavorably for { 0 1 1 ¯ 2 } tension twinning show a decreasing strain rate sensitivity with increasing strain. In contrast specimens oriented favorably for tension twinning exhibited negative strain rate sensitivity at low strains followed at higher strains by positive strain rate sensitivity. The flow stress and consequently the strain rate sensitivity was modeled using an analytical constitutive equation. This model was able to account for not only the negative strain rate sensitivity of the samples undergoing tension twinning at low strains but also the observed trend of strain rate sensitivity with strain for all samples. From this we deduce that an increase in twinning activity with strain rate is responsible for the strain rate sensitivity.

We show that any highly textured metal that deforms predominantly by deformation twinning can exhibit a negative lateral strain ratio under uni-axial loading. Theoretical calculations of lateral strains caused by tension twinning on the { 1 0 1 ¯ 2 } plane in a magnesium single crystal predicts this behavior, and we verify this by direct measurements of lateral strains in highly textured commercial magnesium alloy AZ31 deformed in uniaxial compression. The R values of AZ31 plate compressed in the rolling direction at −100 °C and 25 °C show negative values at compressive strain up to ∼0.03, and increase with strain and temperature. These evolution trends of R value are found be closely related to activity of { 1 0 1 ¯ 2 } twinning. This approach is extended to compression twinning in zinc, for which theoretical calculations show the same effect on lateral strain ratio.

The evolution of stress and lateral strains with applied strain was determined by uniaxial loading in the principal directions of a highly textured Mg–3Al–1Zn plate. The material exhibits negative lateral strain ratios when twinning dominates deformation. This leads to a yield surface with a distinct shape, with a high curvature under a balanced biaxial stress state and a shift in plane-strain compression conditions from the third to the second and fourth quadrants of the yield surface.

Uniaxial tension tests and elastoplastic self-consistent (EPSC) simulations were conducted to study the effect of texture on the microyielding of highly textured AZ31 plate. The onset of microyielding is always associated with the activation of basal 〈a〉 slip irrespective of starting texture, while the value of the microyield stress does depend on the texture. The macroscopic yield stress is largely affected by the critical resolved shear stress of the deformation mode with the highest Schmid factor. An inverse relation is found between the microyield strength normalized by the macroscopic yield strength and the average Schmid factor of prism 〈a〉 slip, which is useful for a rough estimate of the microyield strength.

A Mg–6Gd–1Zn–0.6Zr alloy has been mechanically tested in the aged and solution treated conditions. The plate showed negligible yield asymmetry in either the aged condition or the particle free condition. This has been attributed to the weak texture that develops during recrystallisation. It is proposed that a negligible yield asymmetry can be achieved with textures weaker than ∼ 5 times random intensity, and that it is not necessary to have a fully random texture, but simply a texture weak enough for basal slip to dominate the deformation in both tension and compression. The ageing treatment provided little strengthening in tension or compression, and this is because the precipitates do not possess the strongest hardening characteristic of coherent shearable particles, which is internal order.

Commercial purity Ti was grain-refined with a channel die compression method at −190 °C. Repeated cryogenic compression cycles with brief intermittent annealing treatments at 620 °C resulted in the reduction of the grain size from 50 μm to 100 nm. The microstructure in the final state consisted of narrow lamellar structure intermixed with equiaxed grains surrounded by high angle grain boundaries. The effective microstructural refinement was attributed to the suppression of the dynamic recovery and maximization of the stored energy. Due to the fine grain size and the residual dislocation density, the room temperature yield strength and the ultimate tensile strength increased by 80% over the starting values. Using the electron back-scattered diffraction analysis, the deformation mechanism of individual grains was studied in terms of the Taylor axis distribution. While the grains without mechanical twin showed the predominant concentration of the individual grain misorientation axes along [0 0 0 1] the ones containing many twins revealed random distribution of the axes. Based on this observation, a future study was suggested as to the control of the initial texture to enhance the cryogenic deformability of Ti and subsequently a finer microstructure.

The distribution characteristics of in-grain misorientation axes (IGMA) in cold-rolled pure titanium were investigated using electron backscatter diffraction (EBSD). Depending on the orientation of individual grains, two distinct IGMA distribution patterns were observed: one with strong intensities of IGMA around ⟨0001⟩ and the other with those around ⟨uvt0⟩. Analyses based on the Taylor axes and Schmid factors of possible slip modes suggested that the former pattern arises from predominant activation of prism ⟨a⟩ slip, while activation of $$ \{ 11\bar{2}2\} \langle \bar{1}\bar{1}23 \rangle $$ slip under the suppression of prism ⟨a⟩ slip results in the latter pattern. It was also found that prism ⟨a⟩ slip becomes more active with increasing strain, playing a critical role in the plasticity of pure titanium. The present work demonstrates that IGMA analysis of EBSD data may be used to explore the active slip mode in polycrystalline hexagonal-close-packed (hcp) metals deformed to moderate to large strains.

The recrystallization behavior of cold-rolled, commercial-purity titanium was studied experimentally and with Monte-Carlo (MC) modeling. Utilization of EBSD-OIM as input for MC modeling resulted in realistic predictions of recrystallization kinetics, microstructure and texture, which were in good agreement with experimental results. MC modeling of recrystallization kinetics predicted that the non-uniform stored energy distribution, heterogeneous nucleation of recrystallization and recovery in combination leads to a negative deviation from linear JMAK kinetics. It was found that concurrent recovery that takes place during recrystallization is an important process that controls both the overall recrystallization kinetics and the deviation of linear JMAK kinetics. On the other hand, the non-uniformly distributed stored energy itself has little effect on the negative deviation from JMAK kinetics but intensifies the deviation when heterogeneous nucleation is combined. Modeling results also revealed that heterogeneous nucleation of recrystallized grains and their early impingement in local areas of high deformation are essential for producing a log-normal distribution of grain size and a typical recrystallization texture of rolled titanium.

Measurements of strain rate sensitivity (SRS) provide a key link between dislocation-based interpretations of plastic deformation and macroscopic measurements made in mechanical tests. It is well known that plastic deformation of hexagonal close-packed (hcp) metals is achieved not only by dislocation glide but also by twinning and that the atomic rearrangement underlying the latter mode is different from that of slip. This leads to an expectation that co-activation of twinning may affect SRS of hcp metals. This assumption was tested in the present work where strain rate jump tests in both tension and compression were conducted on highly textured AZ31 plate. It was found that the SRS of the alloy in tension decreased with strain whereas that in compression increased with strain, exhibiting negative values at low strain and positive values at higher strain. Microstructure analyses revealed that the strain regimes where negative SRS or decreasing trend in SRS with strain was observed correspond to extensive twinning, implying a negative SRS of twinning. It is concluded that dislocation model alone cannot explain the strain rate dependence of flow stress in metals whose deformation is assisted by twinning.

Recent advances in the severe plastic deformation technique have shown that effective refinement of the microstructure can be achieved in pure metals as well as in alloys. Among the various methods of severe plastic deformation, equal channel angular pressing has been the subject of numerous research works. Since the grain refining effect of this technique appears to reach a peak at a level of approximately 200 nm further microstructural changes are sought—deformation at a cryogenic temperature being one of the candidate routes. In the present study, we opted to combine equal channel angular pressing and low temperature plastic deformation to refine the microstructure of commercially pure V. The starting microstructure consisted of equiaxed grains with an average size of 100 micrometers. This microstructure was refined to a 200 nm thick lamellar microstructure by 8 passes of equal channel angular pressing at 350°C. The lamellar thickness was further reduced to 140 nm upon subsequent cryogenic rolling, which resulted in room temperature yield strength of 768 MPa. In the specimens, recrystallization annealed at 850°C, the grain size reached 1000 nm or larger, and the yield strength obeyed the Hall-Petch relationship with that grain size. The tensile elongation value, which was low and insensitive to the grain size in the as-deformed state, increased significantly up to 43% with the recrystallization annealing.

A three-dimensional Monte-Carlo method of simulating the microstructure evolution was developed and applied to studying recrystallization of Zr702, a commercial purity Zr. In this modeling, nuclei of recrystallization, extracted from severely deformed regions of real specimen by electron back scattered diffraction, were distributed at the boundaries of hypothetically elongated grains. Within each grain the stored energy was allowed to have a gradient, in a functional form with the highest energy level at the grain boundary and the level diminishing toward the grain interior. It was found that the recrystallization kinetics under the condition of no stored energy gradient showed ideal characteristic with the JMAK exponent approaching 3. In this condition, however, the kinetics was independent to the initial deformation. Under the condition of the stored energy gradient, however, the kinetics deviated negatively from the ideal case and also increasing the amount of initial deformation enhanced the recrystallization kinetics. Based on the recrystallization kinetics and grain size distribution characteristics, it was proposed that an exponential type of the stored energy gradient existed in as-deformed specimen.

Commercial vanadium of 99.8% purity was extensively grain-refined by application of equal channel angular pressing at 350 °C. A total of eight passes via route B C reduced the grain size from 100 μm to 200 nm. In the as-deformed state, the microstructure consisted mainly of lamellar grains and a smaller fraction of equiaxed grains. A strong fiber texture consisting of θ and {1 1 0} θ components was also produced by severe plastic deformation. More than a threefold increase of the yield strength was realized by the processing, and a balanced combination of strength and tensile elongation, 406 MPa and 32%, respectively, was obtainable by subsequent recrystallization heat treatment at 700 °C. Dynamic recovery and continuous recrystallization are thought to be the key mechanisms with regard to characterizing the microstructure evolution during deformation and recrystallization, respectively.

The recrystallization characteristics of commercially pure titanium (CP-Ti) rolled at various temperatures between 25 and 600 °C was investigated. Increasing the rolling temperature up to 450 °C enhanced the progress of recrystallization and refined the grain structure in the as-recrystallized state, although it reduced the overall stored energy in the as-rolled state. This peculiar phenomenon was attributed to characteristic features of the microstructure that developed during warm rolling, such as deformation-induced high-angle grain boundaries and steep in-grain orientation gradients. These microstructural inhomogeneities, which originate from diverse slip systems – pyramidal 〈 c + a 〉 in addition to the main prism 〈 a 〉 – activated at the working temperature, are expected to promote the subsequent nucleation of recrystallization. It is proposed, therefore, that the rate of static recrystallization will accelerate when the factor of the microstructural inhomogeneity outbalances that of the stored energy that is reduced by dynamic recovery.

Commercial purity zirconium (Zr702) was deformed by equal channel angular (ECA) pressing up to eight passes, and the resulting microstructure and texture were studied by electron backscattered diffraction, transmission electron microscopy (TEM), and X-ray diffraction. The most prominent feature of the substructure was the dislocation cell blocks (CBs), indicating that the dislocation slip rather than twinning was the main mechanism of deformation. With two passes of ECA pressing, pancake-shape grains of 0.25 μm in thickness were obtained. The grain refinement achieved by the ECA pressing was attributed to the evolution of low-angle geometrically necessary boundaries (GNBs) into high-angle grain boundaries (HAGBs) during accumulation of strain by repeated pressing. The texture characteristics were such that a shear texture was predominant in the single-pass specimen, whereas a high-strain rolling texture became apparent in the specimens repeatedly pressed.

Effect of deformation temperature on subsequent recrystallization characteristics of commercial purity titanium (CP-Ti) was investigated using EBSD and transmission electron microscopy. CP-Ti plates were rolled to at various temperatures between 25°C and 600°C to achieve 60% reduction, and were subsequently annealed at 600°C for various time periods. Increase in rolling temperature resulted in more refined microstructures, which could be attributed to coactivation of secondary slip systems such as basal slip or pyramidal slip in addition to prism slip. During subsequent annealing, a large number of recrystallization nuclei formed in the warm-worked (below 450°C) CP-Ti while nucleation of recrystallization was slower for coldrolled one. This led to the faster recrystallization kinetics and a finer microstructure in asrecrystallized state for warm-worked CP-Ti. Meanwhile, the rate of recrystallization in CP-Ti rolled at 600°C was significantly retarded because a good portion of strain energy stored in deformed matrix was released by dynamic recovery. In the present work, we show that the widely accepted explanation for the influence of the deformation temperature on the recrystallization characteristics, i.e., increase of the deformation temperature retards the recrystallization process, is not always true for the hcp metals since deformation at elevated temperature activates secondary slip systems, providing more sites for nucleation of recrystallization.

Behaviors of random high-angle grain boundaries of a Pb-base alloy during thermo-mechanical processing and heat treatment were studied with the electron back-scattered diffraction method. Repetitive thermo-mechanical processing was found to be effective in increasing the fraction of special boundaries, and this was attributed to the ∑3 boundaries associated with annealing twins. Evidence was found that the dissociation of fast moving random high-angle boundaries to ∑3 boundaries correlates with other random boundaries. Also formed during intermittent thermal exposure were ∑3n networks, which evolved mainly into ∑3 boundaries of lower energy. The present work thus supports the ‘growth accident’ model of annealing twin formation. Viable thermo-mechanical treatments for promoting the special boundaries are also proposed.