Fraction Of Low-angle Boundaries Research Articles

Post-hot-deformation microstructure development in austenitic stainless steel

In the present work, austenitic stainless steel has been thermo-mechanically processed to study the post-deformation microstructure evolution at elevated temperatures. The samples were axially compressed (at a strain rate 0.1 s−1) and isothermally held at temperatures 900, 1000 and 1100 °C, for different time spans (2–1000 s). The electron back-scattered diffraction technique has been used to study the microstructure of the thermo-mechanically processed samples. The result shows for temperatures of 900 °C and 1000 °C, the resultant microstructures are comprised of deformed grains for samples held for 2 s. However, at 1100 °C sample isothermally held for 2 s shows a nearly complete recrystallization. The grain average misorientation approach has been used to estimate the softening fraction caused by recrystallization. The average grain size of the thermo-mechanically processed samples reveals grain refinement for the samples processed at 900 °C and 1000 °C. In addition to the grain refinement, the samples processed at 1000 °C shows a more consistent grain size. At all three temperatures, the increase in the holding time leads to a gradual decrease in the fraction of low angle boundaries and a simultaneous increase in high angle grain boundaries (HAGBs). The HAGBs are mainly contributed by Σ3 coincident site lattice boundaries. At the end, a discussion on post-deformation softening, considering strain-induced boundary migration, has been presented.

Synergistic enhancement in strength and ductility of nitrogen-alloyed CoCrFeMnNi high-entropy alloys via multi-pass friction stir processing

The dynamic microstructural evolution and tensile performances of CoCrFeMnNi high-entropy alloys (HEAs) with and without nitrogen alloying subjected to multi-pass friction stir processing (FSP) were comparatively investigated. Results showed that the strength and ductility of nitrogen-alloyed CoCrFeMnNi HEAs were synergistically enhanced by multi-pass FSP. Compared to free-nitrogen CoCrFeMnNi HEAs, the gains in the processed zone of the nitrogen-alloyed CoCrFeMnNi HEAs exhibited not only more refinement，but also a monotonic decrease in average gain size (AGS) with the increase in FSP pass. In addition, nitrogen-alloyed CoCrFeMnNi HEAs had higher fraction of low angle boundaries (LABs) than free-nitrogen CoCrFeMnNi HEAs after multi-pass FSP. The interaction between nitrogen atoms and LABs during multi-pass FSP was indirectly proved by the comparison of microstructural thermal-stability between two groups FSP HEAs. Meanwhile, the mechanism of synergistical enhancement in strength and ductility of nitrogen-alloyed CoCrFeMnNi HEAs was discussed.

Superplastic behavior of friction-stir welded Al–Mg–Sc–Zr alloy in ultrafine-grained condition

The present work was undertaken to improve superplastic ductility of friction-stir welded joints of ultrafine-grained (UFG) Al–Mg–Sc–Zr alloy. In order to suppress the undesirable abnormal grain growth, which typically occurs in the heavily deformed base material, the UFG material was produced at elevated temperature. It was suggested that the new processing route could reduce dislocation density in the UFG structure and thus enhance its thermal stability. It was found, however, that the new approach resulted in a relatively high fraction of low-angle boundaries which, in turn, retarded grain-boundary sliding during subsequent superplastic tests. Therefore, despite the successful inhibition of the abnormal grain growth in the base-material zone, the superplastic deformation was still preferentially concentrated in the fully-recrystallized stir zone of the material. As a result, the maximal elongation-to-failure did not exceed 700%.

Stress corrosion cracking resistant nanostructured Al-Mg alloy with low angle grain boundaries

For decades, high susceptibility to intergranular corrosion and stress corrosion cracking is the Achilles's heel for the 5xxx series Al alloys with high Mg (> 3 wt%) owing to precipitation of electrochemically active Mg-rich β phase at grain boundaries (GBs) at moderately elevated temperatures. Herein, nanostructured Al-5Mg alloy with a high proportion of low angle grain boundary equiaxed grains were generated by dynamic plastic deformation and appropriate annealing treatment. Electron back-scattered diffraction discloses that the fraction of low angle boundaries reaches 70%, which has been demonstrated to greatly suppress the precipitation of β phase at GBs. The nanostructured Al-5Mg alloy exhibits excellent intergranular corrosion resistance and stress corrosion cracking resistance after being sensitized at 150 °C for 100 h. Moreover, its strength and ductility are much higher than the corresponding coarse-grained alloys, thus solving the classic problem that has existed for decades. The introduction of a high fraction of low angle grain boundaries by dynamic plastic deformation and appropriate annealing provides a novel strategy for the development of advanced nanostructured Al alloys.

Investigation of Ti–45Nb alloy's mechanical and microscopic behaviors under transverse ultrasonic vibration-assisted compression

Numerous researches have been implemented to disclose the improvement mechanism of plasticity and surface quality by ultrasonic vibration “volume effect” and “surface effect”. In this study, a novel transverse ultrasonic vibration-assisted compression (TUVAC) system is developed to improve the plasticity of titanium alloy, an interesting phenomenon was found that the micro-hardness of Ti–45Nb specimen occurred first rise and then fall with the amplitude increase, which is contrary to the work hardening mechanism and acoustic residual hardening mechanism. To reveal the mechanism of micro-hardness transformation, a series of studies were implemented, such as temperature measurement, microstructure observation et al. The EDS, XRD and EBSD were combined to analyze and reveal the micro-hardness evolution mechanism. The experimental results show that the element distributes in a segregation state, the number of XRD peaks remain unchanged, and grain is refined with the amplitude increase. In addition, the Ti-alpha phase, low-angle boundary fraction, sub-grain, grain size and texture have changed with the amplitude increase.

Strength and microstructure evolution in nickel during large strain wire drawing

This study deals with the evolution of strength and microstructure in pure nickel during wire drawing from an initial diameter of 1.74 mm to 30 μm, corresponding to a total true strain of 8.1. Electron backscattering diffraction (EBSD) and X-ray diffraction (XRD) have been used for microstructural characterization. In the later stages of deformation, the fraction of low angle boundaries decreased as did the misorientations within a grain or cell. The dislocation density stabilized at ∼2 × 1015 m−2, and the activation volume decreased from ∼100 b3 at a strain of 1 to ∼ 15 b3 at strains ≳4, where b is the magnitude of the Burgers vector. Cross-sectional EBSD of the wires revealed that a core region had <111> fiber texture, whereas a peripheral shell region had a complex fiber texture due to redundant shear strain. Viscoplastic self-consistent (VPSC) simulations were in good agreement with the crystallographic texture evolution in the wires during drawing. The wire drawing data is compared with other deformation techniques, to provide a broad framework for analyzing microstructure-strength-ductility relationships. At large strains of >2, with an essentially constant dislocation density, the strength can be related solely to Hall-Petch strengthening by a refinement of the transverse grain size.

Effect of Mg content on the dislocation characteristics and discontinuous dynamic recrystallization during the hot deformation of Al-Mg alloy

The effect of Mg content on the dynamic recrystallization behavior during the hot deformation of Al-Mg alloys were investigated. Three different Mg contents of Al-6Mg, Al-8Mg, and Al-9Mg were hot-torsioned under the deformation temperature of 723 K and a strain rate of 0.1 s−1. Using and electron backscatter diffraction, the dislocation characteristics during the hot deformation of each alloy were evaluated. The results of X-ray line profile analysis showed that the strain anisotropy parameter and dislocation arrangement parameter decreased when the strain increased, and when the fraction of edge dislocation and screening of dislocation field (interaction between dislocations) increased. However, the variation in Mg content affected the strain dependence of the dislocation parameters, which means that the Mg content in the Al-Mg alloy affected dislocation characteristics and dynamic recrystallization. In addition, the fraction of low-angle boundaries was accelerated when the strain increased at lower Mg content, while the formation of serrated grain boundaries was remarkably observed. Consequently, considering the absence of twinning, at all strains, higher Mg content increased the dynamic recrystallization rate and induced the flow softening at relatively low strains.

Effect of extrusion ratios on hardness, microstructure, and crystal texture anisotropy in pure niobium tubes subjected to hydrostatic extrusion

Nb tubes were fabricated through hydrostatic extrusion at extrusion ratios of 3.1 and 6.1 at ambient temperature, and then their microstructure, texture, and Vickers hardness were investigated based on electron back-scattered diffraction (EBSD) data. The fraction of low-angle boundaries (LABs) largely decreased with a sharp decrease in mean grain sizes after hydrostatic extrusion and was not proportional to extrusion ratios, assuming that mixed-asymmetrical junctions forming LABs dissociate at high extrusion ratios from the external stress (>981 MPa) with thermal activation by the generated heat. The correlation between grain size and Vickers hardness followed the Hall−Petch relationship despite the texture gradient of the 〈111〉 cyclic fiber textural microstructure at low extrusion ratios and the 〈100〉 true fiber textural microstructure at high extrusion ratios. The increase in hydrostatic pressure on the Nb tubes contributed to texture evolution in terms of extrusion ratios due to the difference between {110}〈111〉 and {112}〈111〉 components based on EBSD data.

Effect of liquid nitrogen and warm deformation on the microstructure and mechanical properties of 321-type metastable austenitic steel

New thermomechanical treatments combining plastic deformation with cooling in liquid nitrogen followed by warm deformation and annealing are applied to form fine-grained structure in the 321-type metastable austenitic steel. The structural-phase transformations during these treatments are studied by Transmission Electron Microscopy (TEM), Scanning Electron Microscopy with Electron Backscatter Diffraction (SEM EBSD), X-ray Diffraction (XRD) and magnetic investigations. During liquid nitrogen deformation (LND) with a true strain of e = 0.2 a band-like deformed structure with a volume fraction of α′-martensite ≈ 70% is formed. The subsequent warm deformation (WD), with e = 0.4, i.e. LND + WD treatment at 600 °C, leads to a partial reverse α′ → γ transformation under plastic deformation. A fine-grained structure with high fraction of low-angle boundaries was formed by the shear reversion mechanism. The α′-martensite content decreases to ≈50%. The mechanical properties of the material in different stages of the treatments are studied under static tensile testing. It is demonstrated that these treatments give rise to an increase in the yield strength of more than 1250 MPa. Additional annealing provides a more complete (up to 85–93%) martensite-to-austenite transformation and allows maintaining a fine-grained structure. After annealing, the fraction of high-angle and twin boundaries in the austenite increases. The diffusion-type mechanism of austenite reversion is involved in the formation of microstructure features under these conditions. Annealing allows changing tensile and ductile properties (yield strength up to ≈ 830–945 MPa at elongation of ≈10–19%) through modifying the microstructure and phase composition of fine-grained steel.

Development of grain size/ texture graded microstructures through friction stir processing and subsequent cold compression of a rare earth bearing magnesium alloy

The present work deals with the effect of mechanical post-treatment on the microstructure and texture evolution of friction stir processed Mg-4.37 wt%Y-2.9 wt%RE-0.3 wt%Zr alloy. In this respect, the stir processed specimens were compressed down to engineering strain of 10 and 20%, and the resultant microstructures were characterized. Through applying subsequent compressive loading, fraction of low angle boundaries was increased, substructure was developed, the grain size was refined, and specified misorientation angles/texture components were faded or appeared. The degrees of such evolutions were different between the surface layer and stir zone, and resulted in development of graded microstructures. The fraction of strain-less and fresh grains was considerably decreased; but the fraction of coarse grains holding high grain orientation speared (GOS) was remarkably increased specially by moving away from material surface. Schmid analysis indicated the higher probability of non-basal slip within the unstable coarse grain in the course of subsequent loading. This was led to the occurrence of post-dynamic recrystallization which was traced through the sequence of substructure development and transformation of the low angle boundaries in to the high angle ones. Such gradual microstructure/texture evolution indicate the stir processing plus compression loading as capable method for fabrication of micro-scale functionally graded materials. • Mechanical post-treatment of friction stir processed WE43 magnesium alloy. • Fabricating graded microstructures regarding grain size, substructure and texture. • Contribution of continuous dynamic recrystallization during friction stir processing. • High potential of unstable coarse grains for refinement during subsequent loading. • Post-dynamic/Meta-dynamic recrystallization activates during subsequent compression.

The role of Mg2Si additive in sintering silicon carbide

A novel Mg2Si sintering additive was introduced to improve the relative density of SiC ceramic. The SiC samples were hot press sintered at 0.5, 2 and 4 h. The microstructure and relative density were measured, the distribution, microstructure and orientation of Mg2Si were characterized, and the interface of SiC/Mg2Si was observed. Results show that relative density of the ceramics increased with the prolongation of sintering time, and densification fulfilled when sintered at 2 h, resulting in the high relative density of 96.86%. With the prolongation of sintering time, the pores become coarsening and the densification process was protracted. Some Mg2Si react with SiC matrix, and the Mg element distributes into the SiC matrix. The fraction of deformed grains and the geometrically necessary dislocations decreased initially then increased slightly with the prolongation of sintering time. The fraction of low angle boundaries is small in all the samples and the transition from low angle boundaries to high angle boundaries is not smooth. The Mg2Si grains locate on the grain boundaries of SiC with little misorientation, resulting in a perfect lattice coherence between SiC and Mg2Si.

Novel precipitation and enhanced tensile properties in selective laser melted Cu-Sn alloy

The current work reports nano-precipitation and exceptional combination of high strength and ductility in a Cu-13Sn alloy, fabricated by selective laser melting (SLM). The SLM samples revealed competitive unidirectional columnar grains with a fine cellular structure (~600 nm) having Cu-rich spherical nano-precipitates (~3 nm), and δ-phase (Cu41Sn11). Besides, the SLM samples presented a spatially heterogeneous microstructure with both microstructural and chemical heterogeneities in the length scales ranging from micron to nano-scale. The microstructural heterogeneity arises from the heterogeneous grain structure (alternate layers of coarse and fine grains) and dislocation density variations (regions with high and low fractions of low-angle boundaries). Further, the chemical heterogeneity is from the segregation of Sn to the cellular boundaries. The multi-phase hierarchy and spatially heterogeneous microstructure with fine cellular structure significantly contributed to an exceptional combination of high strength and ductility in three orthogonal directions of the SLM processed Cu-13Sn alloy. The back stress due to the heterogeneous structure and the effective stress due to the nano-precipitates, fine grains, and friction stress contribute to realizing exceptional strength. The presence of cellular structure and geometrically necessary dislocations contribute to achieving high uniform elongation in the SLM processed Cu-13Sn alloy. Overall, this work demonstrates the capability of the SLM process in developing novel heterogeneous materials with minimum tensile anisotropy and an exceptional combination of high strength and ductility.

Brittle-ductile transition temperature of recrystallized tungsten following exposure to fusion relevant cyclic high heat load

The lifetime of tungsten (W) monoblocks under fusion conditions is ambivalent. In this work, the microstructure dependent mechanical behaviour of pulsed high heat flux (HHF) exposed W monoblock is investigated. Two different microstructural states, i.e. initial (deformed) and recrystallized, both machined from HHF exposed monoblocks are tested using tensile and small punch tests. The initial microstructural state reveals a higher fraction of low angle boundaries along with a preferred orientation of crystals. Following HHF exposure, the recrystallized state exhibits weakening of initial texture along with a higher fraction of high angle boundaries. Irrespective of the testing methodology, both the microstructural states display brittle failure for temperatures lower than 400∘C. For higher temperatures (>400∘C), the recrystallized microstructure exhibits more ductile behaviour as compared to the initial state. The observed microstructural state-dependent mechanical behaviour is further discussed in terms of different microstructural features. The estimated brittle-to-ductile transition temperature (BDTT) range is noticed to be lower for the recrystallized state as compared to the initial state. The lower BDTT in the recrystallized state is attributed to the high purity of the W in combination with its low defect density, thereby preventing segregation of impurities at the recrystallized boundaries and the related premature failure. Based on this observation, it is concluded that the common opinion of the aggravation of BDTT in W due to recrystallization is not unerring, and as a matter of fact, recrystallization in W could be instrumental for preventing the self-castellation of the monoblocks.

EBSD study of superplastically strained Al-Mg-Li alloy

In this study, electron back scatter diffraction (EBSD) was employed to examine the microstructure evolved during superplastic deformation of advanced Al-Mg-Li alloy. In contrast to the widely-accepted conception of superplasticity, the microstructure was found to be characterized by elongated grains, a notable fraction of low-angle boundaries, and a distinct (though a very weak) crystallographic texture. All these observations suggested a significant activity of intragranular slip.

Recovery and recrystallisation during creep exposure of cold worked Ti-modified 14Cr–15Ni austenitic stainless steel

ABSTRACT Creep behaviour of Ti-modified 14Cr–15Ni austenitic stainless steel in solution annealed, 20%, 30%, and 40% cold-worked conditions, was studied at 973 K and 175 MPa stress level in the current investigation. Increase in cold-work level resulted in denser dislocation network resulting in prominent low-angle grain boundaries. Low-angle boundary fraction increased as the amount of cold working increased from 20% to 40% prior to creep loading. The variation in the fraction of low-angle boundaries impacted the creep rupture strength of the steel. During creep exposure of the cold-worked steels, softening occurred due to both recovery and recrystallisation. The extent to which recrystallisation and recovery occurred during creep was dependent on the level of cold work. Subgrain formation and recrystallisation resulted in enhanced recovery and decreased the creep strength of 40% cold-worked steel. The creep rupture life was highest for 30% cold-worked steel.

Annealing behavior in a high-pressure torsion-processed Fe–9Cr steel

A Fe–9Cr steel containing second-phase particles was processed by ten rotations of high-pressure torsion (HPT) to produce a microstructure consisting of pancake-shaped nanoscaled grains with dominantly high-angle boundaries. Annealing was carried out on the HPT-processed Fe–9Cr steel from 500 to 700 °C up to 48 h. During the annealing, grains grew in a continuous manner. During high-temperature annealing (above 600 °C), a higher fraction of low-angle boundaries was observed when texture J became the dominant texture component. The annealing behavior of the HPT-processed Fe–9Cr steel was compared to that of the equal-channel angular pressing (ECAP)-processed Fe–9Cr steel. It was found ECAP Fe–9Cr showed a different grain growth mode, i.e., discontinuous growth, and a higher degree of thermal stability. Our studies provide insights into how the annealing behavior is affected by the microstructure and texture evolution.

Unusual through-thickness variations of microstructure and texture in heavily rolled and annealed Al–0.3%Cu

The evolution of microstructure and texture has been investigated in coarse-grained Al–0.3%Cu subjected to 98% cold rolling and subsequent annealing for 1 h at temperatures up to 300 °C. In the rolled material the distribution of crystallographic orientations is found to be highly non-uniform through the sample thickness, with the center layer being strongly dominated by the Bs component, whereas no single texture component dominates in the subsurface. The rolled microstructure contains lamellar structures and a large number of shear bands in all layers. The shear bands promote nucleation of Goss-oriented and other grains in the center layer. Most grains nucleated in the subsurface have random orientations. In the fully recrystallized microstructure obtained after annealing at 225 °C for 1 h, a strong Goss texture is formed in the center layer, while random orientations constitute the greatest area fraction of all orientations in the subsurface. Grain growth taking place after complete recrystallization at 250 °C and 300 °C leads to unusually high fractions of the Goss texture and high fractions of low angle boundaries in the center layer. In the subsurface layers, grain growth results in increased fractions of random orientations, and consequently most grain boundaries in these layers are high angle boundaries. The sample annealed at 300 °C for 1 h exhibits a pronounced sandwich-type structure, with a Goss-oriented band in the center layer enclosed by subsurface layers containing coarser grains of many different orientations.

Microstructural development and its effects on tensile properties of a high Ni TRIP steel produced by repetitive corrugation and straightening via rolling (RCSR)

Microstructural development and its effect on tensile properties of a Fe- 24Ni-0.3C Transformation Induced Plasticity (TRIP) steel subjected to repetitive corrugation and straightening via rolling (RCSR) process was investigated in this study. Microstructure developed during the application of one to 45 cycles of RCSR on TRIP steel sheets were analyzed by optical microscope and scanning electron microscope (SEM) equipped with electron backscatter diffraction detector and X-ray diffraction techniques. Results indicated that the volume fraction of martensite formed due to the deformation-induced by amplification of applied strain during RCSR process was about 75% accompanied with the formation of a large fraction of low angle boundaries. To study, tensile and hardness tests were carried out on several deformed samples their mechanical properties. The results also showed that after 40 cycles of the RCSR process, yield strength increased from 150 MPa in the starting material to 1157 MPa in the deformed state. As well, the uniform elongation reduced from 170% to 30% and the hardness value increased from 130 to 340 HV. This amount of improvement in tensile strength for this type of alloy by the application of RCSR has not been reported to date. Therefor the technique used in this research for the improvement of tensile properties can be very helpful for an alloy designer when very high tensile properties for a particular application is required.

Microstructure gradient characteristics and mechanical properties of friction stir welded high strength QP980 steel

Friction stir welding (FSW) was carried out on 1.2 mm thick QP980 steel plate and W-Re stirring tool was used for FSW. This study focused on the microstructure gradient characteristics and mechanical properties of QP980 FSW joints. The FSW joints without defect could be obtained under welding parameter of 400 rev min−1–400 mm min−1. The martensite was the mainly microstructure in the stir zone (SZ) and the microhardness of the SZ was increased to about 480 HV. The martensite of the subcritical heat affected zone (SCHAZ) underwent tempering transformation and decomposition, small amount of carbides were precipitated from the martensite, so the microhardness of the SCHAZ was slightly decreased. The microstructure in the SZ had the highest dislocation density and the highest fraction of low angle boundaries. The tensile test results indicated that the joints were broken in the base metal (BM), which indicated that the FSW joints had excellent performance.

Effect of ultrasonic treatment on the structure of coarse-grained nickel

Commercially pure coarse-grained nickel was studied. On the electrically polished surface of four samples an area 1 × 1 mm2 in size was marked by a special indenter and studied by EBSD analysis. All the samples were subjected to ultrasonic treatment (UST) with stress amplitudes of 20–80 MPa. The marked area was studied after that. In all the samples, X-ray diffraction analysis, TEM study, microhardness measurements and surface roughness analysis were carried out before and after the UST. It was shown that the UST results in an increase in dislocation density, an increase in the fraction of low-angle boundaries, an increase in microhardness and root-mean-square microstrain. The values of these parameters depend on the UST amplitude.