Yield Strength Anisotropy Research Articles

Revealing the anisotropic response of grain structures in additive manufactured Al-Mn-Sc alloys: Modeling based on experiments

The anisotropy of yield strength of additive manufactured (AM) Al-Mn-Sc alloy is attributed to the diverse distribution characteristics of grain size, aspect ratio and orientation. This work focus on controlling the yield strength and anisotropy of AM aluminum through microstructure design. In this work, microstructures information of laser powder-bed fusion (LPBF) processed Al-Mn-Sc alloys were obtained by electron back scatter diffraction (EBSD) analysis. The impact of grain size, aspect ratio and orientation on strength and anisotropy are quantitatively researched through modeling. Base on quantitative effect of orientation on strength and anisotropy, all microstructure partition schemes are calculated by MATLAB. Microstructure partition schemes of alloy with low anisotropy and high yield strength were predicted and simulation was enforced to certify the credibility of predication. Local stress in grain and orientation variation during deforming were obtained by simulation. It is concluded that the impact of orientation is more obvious than grain size and aspect ratio. Compared with strength and anisotropy obtained by simulation, the prediction of calculation shows fine precision. The impact of microstructures on mechanical properties and the reliability of modeling were verified.

Anisotropy of microstructure, mechanical properties and thermal expansion in Invar 36 alloy fabricated via laser powder bed fusion

Invar 36 is a renowned iron-nickel alloy with an ultra-low coefficient of thermal expansion (CTE), which is widely used in aerospace and precision instruments. Laser powder bed fusion (LPBF) as a prevalent metal additive manufacturing technique could break through the limitations of traditional processes, such as low material utilization and difficulties in forming complex structures. In this work, Invar 36 alloy specimens were fabricated via LPBF process in 0° (H0 specimen), 45° (D45 specimen), and 90° (V90 specimen) build orientations. The microstructure, tensile properties, thermal expansion, and magnetic properties of the three orientations specimens were comprehensively investigated, and the anisotropy of microstructure, yield strength and CTE were analyzed. The V90 specimen exhibited the highest elongation (62.70%), while the D45 specimen owned the highest yield strength (496 MPa). The anisotropic mechanical properties can be attributed to variations in dislocation density, grain size, and intrinsic yield strength. The CTE of LPBF-fabricated Invar 36 alloy specimens were all lower than that of traditional manufacturing process at all temperature ranges. Since CTE is more related to magnetic properties, not sensitive to microstructure, the anisotropy of CTE is not significant in three build orientations. This study has significant implications to produce Invar 36 alloy devices with exceptional properties and 3D dimensional stability via the LPBF process.

Effect of microstructural inhomogeneity on mechanical anisotropy of Al–Cu–Li alloy plate

In order to reveal the relationship between microstructure inhomogeneity and mechanical anisotropy, the Al–Cu–Li alloy 100 mm thick plate for aerospace was evaluated. The grain structure, crystallographic texture, phase structure and fractography of different thickness positions and directions were observed by optical microscopy (OM), X-ray diffraction (XRD), scanning electron microscopy (SEM), electron backscatter diffraction (EBSD) and transmission electron microscopy (TEM). The in-plane and through-thickness anisotropy were characterized by tensile properties, fracture toughness and hardness tests. The results demonstrate a greater in-plane anisotropy in yield strength at the same thickness position of the plate. The Brass texture component increases successively at T, T/4, and T/2 positions, while the Cu texture component exhibits minimal variation. The in-plane anisotropy mainly arises from the Brass texture. Through-thickness anisotropy in the same direction is primarily influenced by the diameter, number density, and volume fraction of the T1 strengthening phase. The average diameter and number density of the primary strengthening phase T1 within grains are highest at the T/2 position and lowest at the T position. The plate contains unrecrystallized, elongated pancake-like grains along the rolling direction. Fracture toughness tests conducted in the L-T and T-L directions predominantly reveal dimple-type transgranular shear fractures, while the S-L direction exhibits intergranular fracture as the primary characteristic. The crack divider delamination toughening mechanism is the main contributor to higher fracture toughness values in the L-T and T-L directions compared to those in the S-L direction. The results reveal the corresponding relationship between the microstructure inhomogeneity and the mechanical anisotropy of Al–Li alloy ultra-thick plates, and provide ideas for the adjustment of process parameters during processing deformation and the regulation of microstructure during heat treatment of large-scale Al–Li alloy thick plates, which has engineering application reference value.

Microstructure, texture evolution and mechanical anisotropy of Mg-Gd-Y-Zn-Zr alloy sheets produced by unidirectional and cross rolling

The unidirectional rolling (UR) and cross rolling (CR) were used to process the Mg-4.7Gd-3.4Y-1.2Zn-0.5Zr (wt.%) alloy sheets. The effect of rolling routes on microstructure, texture evolution, and mechanical anisotropy was investigated. The deformed grains in UR alloy were gradually elongated along the RD, and the intragranular lamellar LPSO phase also transforms into a rod-shaped structure during rolling. For CR alloy, the occurrence of dynamic recovery results in the LPSO phase remaining lamellar shape during rolling, and the deformed grains are rounder than those in UR alloy. After rolling, UR alloy exhibits a texture with basal pole deviates from ND toward the location between TD and RD, which is caused by the synergistic effect of limited twinning, basal and prismatic slip. The CR alloy shows a texture with basal poles deviating from ND to RD 15.6°, which results from the activation of basal slip, pyramidal slip, and multiple twinning variants. The UR alloy exhibits approximately isotropic in yield strength and anisotropy in elongation, while the CR alloy presents a completely different result. The anisotropy in yield strength of CR alloys is related to their texture distribution and remarkably different basal slip Schmid factor in three tensile directions. The isotropic in elongation of CR alloys can be explained by the distribution frequency of tilt angles between basal poles and tensile directions, as well as the kink deformation of lamellar LPSO phase.

Effect of Ce, Sm and Sr micro-alloying on the microstructure, mechanical properties and anisotropy of rotated rolled Mg–4.5Al–1.5Sn–0.5Ca–0.2Mn alloy

Mg–Al series sheets usually exhibit an apparent mechanical anisotropy induced by strong basal texture. In this study, a near-isotropic Mg–4.5Al–1.5Sn–0.5Ca–0.2Mn (ATXM5210) alloy was developed via 0.05 wt.% Sr micro-alloying combined with rotated rolling, displaying an ultra-low difference in yield strength (∼5 MPa), ultimate tensile strength (∼2 MPa) and elongation (∼1 %) between the rolling direction (RD) and transverse direction (TD), in contrast to a strong anisotropy of Sr-free alloy. We comparatively investigated the effect of trace Ce, Sm and Sr addition on the microstructure, texture evolution and mechanical properties of rolled ATXM5210 alloy. Specially, Sr-containing alloy possesses a more uniform distribution of finer CaMgSn particles after rolling, whereas coarse particles and particle bands are still distinctive in Sr-free alloys. Furthermore, the addition of Sr contributes to form broader-distributed texture towards both RD and TD with reduced peak intensity (∼10.8), leading to an apparently reduced mechanical anisotropy. The quite low mechanical anisotropy and balanced combination of yield strength (∼220 MPa) and ductility (∼21 %) make the present ATXM5210-0.05Sr alloy suitable for producing intermediate or final products and competitive for various engineering applications.

MECHANICAL ANISOTROPY OF COLD-ROLLED ST-37 STEEL PLATE UNDER HIGH STRAIN RATES LOADINGS

Cold rolling process in metal could increase its strength and hardness, but also induces mechanical anisotropy. This is caused by unresolved plastic strain and microstructural changes caused by plastic deformation exerted by pair of rolls at a relatively low temperature. This research aims to provide understandings on anisotropic and strain rate sensitive behaviour of St-37 mild steel. The microstructure of rolled and unrolled St-37 plate were observed in 3 directions relative to the rolling direction. The cylindrical specimens were then prepared from rolled plate with 33.3% reduction in the orientation of 0o, 45o, and 90o relative to the rolling direction. Specimens were also prepared from unrolled plate as comparison in the same directions. These specimens were then tested in compression loading, first in quasi-static condition, and then by using Split-Hopkinson Pressure Bar (SHPB) at the strain rate of 1200 s-1. The experimental results in the form of stress-strain curves are used to obtain the parameters of Simplified Johnson-Cook viscoplastic model. The anisotropy of yield strength in rolled specimens could be seen on both quasi-static and high strain rate condition, where the highest strength found on the specimens perpendicular to the rolling direction. In addition, the effect of cold rolling to the strain rate sensitivity of the material were also captured successfully in this study, where specimens from rolled plate show less strain rate sensitivity compared to the unrolled specimens.

Anisotropic microstructure and mechanical properties of as-forged (Ti, Nb)B/Ti2AlNb composites

In situ boride reinforced Ti2AlNb-based composites are a new type of lightweight material that has excellent high-temperature performance exceeding that of Ti-based composites. Forging is one of the effective methods to improve the microstructure and mechanical properties of Ti2AlNb-based composites. After forging, the mechanical properties of the composites exhibit evident anisotropy, but the anisotropic mechanism remains unclear. In this study, the anisotropic mechanical properties of the as-forged (Ti, Nb)B/Ti2AlNb composite and its formation mechanism were investigated. It was found that the as-forged composite possessed higher yield strength along the radial direction (RD) and better plasticity along the compression direction (CD). The microstructural analysis results demonstrated that the O phase formed [100]O//CD fiber texture, and the (Ti, Nb)B reinforcement exhibited [010](Ti, Nb)B//RD fiber texture in the as-forged composite. The analysis based on In-Grain Misorientation Axes (IGMA) suggested that the {1 1‾ 0}<110> was the main activated slip system of the O phase during the deformation. The difference in both the Schmid factor of {11‾ 0}<110> slip system caused by the [100]O//CD texture and the strengthening efficiency induced by [010](Ti, Nb)B//RD texture led to the anisotropy of yield strength. The anisotropy of prior particle boundaries (PPBs) contributed to the different crack propagation modes of the as-forged composite under different loading directions, resulting in plastic anisotropy.

Effect of building direction on anisotropy of mechanical properties of GH4169 alloy fabricated by laser powder bed fusion

Ni Base superalloy samples were fabricated by laser powder bed fusion in vertical, oblique, and horizontal building directions. The microstructures, tensile properties, and fracture morphologies of the three specimens were studied. The precipitated strengthening phase structure, grain characteristics, and geometric necessary dislocation density of the samples were characterized. The reason for the anisotropy of yield strength for the three specimens was discussed based on the strengthening theory. The results show that the microstructures of the as-built alloys are composed of coarse columnar grains, fine dendritic structures, and high-density dislocations. Partial recrystallization occurs with the precipitation of the ellipsoidal γ″ strengthening phase in the alloys after solution and double aging heat treatment. Compared with the vertical and horizontal specimens, the oblique sample has the highest yield and tensile strength and has a good elongation of 19%. The effective grain sizes of the horizontal, vertical, and oblique samples are 11.92 μm, 15.15 μm, and 11.67 μm, respectively. There are no significant differences in the average Taylor factors of the three samples. The geometrically necessary dislocation densities of the three samples are 4.021 × 1014 m−2, 3.078 × 1014 m−2, and 4.362 × 1014 m−2, respectively. The anisotropy of yield strength for the samples with different building directions is explained by the effect of dislocation strengthening, followed by precipitation strengthening and grain boundary strengthening which is essentially related to the dislocation density, Taylor factor and effective grain size.

Influence of pre-stretching on the tensile strength, fatigue properties and the in-plane anisotropy in Al-Cu-Li alloy AA2099

The influence of pre-stretching prior to artificial aging on microstructure, tensile properties, and fatigue behavior of AA2099-T8 sheets was investigated. The results showed that increased pre-strain level from 1% to 6% resulted in a more uniform distribution and increasing density of the T1 phase. The localized plastic deformation during pre-stretching led to the formation of dense precipitation bands (DPBs) and precipitation free bands (T1-PFBs) in the sheets with the 1% pre-strain. The non-uniform distribution of the T1 phase with T1-PFBs and DPBs resulted in a decrement in elongation and a strong in-plane anisotropy of tensile properties. An appropriate level of pre-strain should be executed aiming to improve elongation and reduce the tensile anisotropy by promoting a uniform distribution of the T1 phase. In-plane anisotropy of yield strength due to the presence of DPBs and T1-PFBs was quantitatively analyzed by the modified Hall-Petch formula. The results of the calculation for the modified model showed good agreement with the experiment. In addition, resistances of fatigue crack propagation (FCP) in the same orientation were gradually decreased with the increment of pre-strain due to the increment in yield strength and more uniform distribution of the T1 phase. For the in-plane anisotropy of FCP resistance, the main reasons were the development of rough fracture surfaces and the transgranular fracture modes in L-T orientation.

A universal shear-lag model for accurate assessment of whisker load-transfer strengthening in metal matrix composites

Accurate assessment of the strengthening contribution of whiskers is crucial for the design, development, and application of high-performance whisker reinforced metal matrix composites (WRMMCs). The present work presents a novel universal shear-lag model to accurately calculate the load-transfer strengthening of whiskers in all WRMMCs regardless of matrix and reinforcement. This model considers the tensile strength of whiskers and the stress transfer from the matrix to the whisker ends. The present model provides a facile way to allow consideration of the influence of whisker orientation and aspect ratio distributions on whisker load-transfer strengthening by introducing probability density functions. Their distribution characteristics have a significant effect on the predicted results, which cannot be simply replaced by their average values. The present model was verified by the reported experimental results and comparison with other models, and it shows better agreement. Moreover, titanium matrix composite reinforced with misaligned TiB whiskers was prepared by vacuum arc remelting and hot-forging. TiB whisker aspect ratio and orientation distributions were quantified by image statistics and pole figure analysis. The anisotropy of yield strength was evaluated under monotonic tensile loading in various directions of the composite. Different shear-lag models were applied to evaluate the load-transfer strengthening of TiB whiskers at different angles relative to the tensile loading direction. The present model gave a more accurate prediction of the anisotropy of yield strength than other shear-lag models.

Effects of rolling method on the microstructure and anisotropy of mechanical properties of Cu–15Cr in-situ composites

The effects of unidirectional rolling (UR) and cross rolling (CR) on the Cr phase morphology, texture, and anisotropy of mechanical properties of Cu–15Cr in situ composites were investigated by using optical microscope, x-ray diffraction, and quasi-static compression techniques. The results showed that the elongation of the Cr phase in the UR specimen (49.2 μm) was greater than that of the CR specimen (18.6 μm) in the rolling direction after rolling with a reduction of 86.7%, while the elongation of the Cr phase in the UR specimen (10.9 μm) was slightly lower than that of the CR specimen (11.7 μm) in the transverse direction. The volume fraction of the Goss texture component of the Cu matrix was the largest in the UR specimen (31.3%), while that of the Brass texture component was the largest in the CR specimen (28.0%). The approximate ∼ (102)[2‾2‾1] texture component (14.2%) was present in the CR specimen. The results of Sachs model calculations indicated that the anisotropy of the compressive yield strength of the CR specimen (10.8%) was greater than that of the UR specimen (6.8%) due to the difference in texture components. The critical stress required to cross the Cu/Cr phase interface was calculated and indicated that the presence of the fibrous Cr phase made the anisotropy of the compressive yield strength of the CR specimen (21.9%) lower than that of the UR specimen (32.7%). The difference in the anisotropy of the compressive yield strength of the two specimens was reduced by the competition between these two factors, and eventually, the anisotropy of the compressive yield strength of the UR specimen (17.7%) was slightly higher than that of the CR specimen (14.1%), indicating that the effect of the fibrous Cr phase on the compressive yield strength anisotropy was greater than that of the texture.

Effect of reduction ratio during cold rolling and final annealing temperature on the properties and microstructure of Al–Mg–Sc alloy sheets

The study covers the effect of the reduction ratio during cold rolling (εh) and the final annealing temperature of sheets rolled with different reduction ratios on the microstructure and the complex of mechanical and processing properties of cold-rolled sheets made of the V-1579 aluminum alloy of the Al–Mg–Sc system. It was established that as εh increases, the nature of plastic anisotropy changes slightly, and an increase in tensile strength and yield strength with a decrease in relative elongation is observed. In this case, the ultimate strength and yield strength anisotropy is practically absent. As the reduction ratio increases to 30–40 %, the relative elongation anisotropy increases, and its value in the rolling direction decreases more rapidly. However, after rolling with εh &gt; 50 %, the relative elongation anisotropy practically disappears. Regardless of the annealing temperature, samples rolled with a higher reduction ratio have better strength properties. It was found that as the annealing temperature increases, the ultimate strength and yield strength decrease, and the relative elongation increases. In this case, softening with an increase in the annealing temperature occurs more intensively for samples rolled with a lower reduction. After annealing, the distribution nature of anisotropy indices in the sheet plane does not decrease and corresponds to the deformation type of textures for all analyzed modes. Moreover, the value of the in-plane anisotropy coefficient decreases in comparison with a cold-rolled sample. At the same time, processing properties of samples rolled with a higher degree of deformation after annealing are higher than those of samples rolled with a lower reduction, regardless of the annealing temperature.

A new modeling framework for anisotropic yield strength of Al-Li alloy sheet with inhomogeneous plate-like T1 precipitates

The high strength of Al-Li alloy is mainly attributed to precipitates, which has attracted extensive investigations on precipitation dynamics and interaction with dislocations. However, the influence of plate-like T1 precipitate on mechanical anisotropy is still not fully clarified, and an accurate corresponding modeling framework considering inhomogeneous distribution and various configurations of T1 precipitate is also needed. Here, the evolution of mechanical anisotropies in 2198 Al-Li alloy sheets is systematically studied with different distributions and thicknesses of T1 precipitates obtained via changing pre-deformation direction and artificial aging temperature. A new modeling framework based on visco-plasitic self consistant (VPSC) is proposed to predict the anisotropic yield strength under different processing conditions. The resistance of four variants of T1 precipitates which form a 'T1 precipitate forest' to dislocation movement is considered. According to the geometric relationship with slipping systems, T1 precipitates can be divided into three types: type I is parallel to slipping plane, type II is parallel to slipping direction but not slipping plane, and type III has a certain angle to both slipping plane and direction. The critical resolved shear stress (CRSS) of each slipping system can be calculated using a linear mixture law considering volume fraction and shear strength of these three types. It is found that T1 precipitate of type I plays an important role in the strengthening of 2198 Al-Li alloy sheet. The shear strengths of types I, II and III are determined to be 400 MPa, 8 MPa and 8 MPa, respectively, at artificial aging temperature of 155 °C, and reach 400 MPa, 40 MPa and 40 MPa due to the increased T1 thickness when aging temperature increases to 162 °C. The calculated anisotropy of yield strength is in good agreement with experimental results. Furthermore, a reasonable correlation between slipping activation and T1 precipitate is revealed. That is, the activation fractions of different slipping systems exhibit how the varying distribution and thickness of T1 precipitate lead to different mechanical anisotropy of 2198 Al-Li alloy sheet.

Effect of electropulsing on anisotropy in strength of laser metal deposited Ti−6Al−4V alloy

The effect of electropulsing treatment on microstructure and mechanical strength of laser metal deposited Ti−6Al−4V alloy was investigated in order to eliminate the anisotropy in strength of laser metal deposited Ti−6Al−4V alloy by tensile tests, optical microscopy, scanning electron microscopy, electron back-scattered diffraction analyses and transmission electron microscopy. With increasing applied voltages from 0 to 130 V, the evolution of microstructure within columnar β grains followed the sequence of α′ martensite → colony α structure → basket-weave α structure. The electropulsing treated at 130 V weakened the texture of martensite within β grains. The as-built Ti−6Al−4V alloy showed an anisotropy in yield strength (6.2%). After processing at 130 V, the anisotropy in yield strength was reduced to 0.6%, which was attributed to the almost equivalent distribution of Schmid factor in the samples deformed along different orientations.

Good combination of strength and mechanical isotropy in AA6082 alloy processed by high temperature intermediate annealing

The microstructure, textures and mechanical property anisotropy of AA6082 alloy sheets after intermediate annealing were investigated. The result shows that the anisotropy of final cold-rolled and T6 alloy sheets decrease and the strengths increase as the intermediate annealing temperatures rise from 400 °C to 550 °C. Higher intermediate annealing temperature promotes the coarsening of soluble Mg2Si particles, which influences subsequent recrystallization and precipitation. The final cold-rolled sheet intermediately annealed at 550 °C presents weak rolling textures, thereby decreasing in-plane anisotropy (IPA) of yield strength from 4.4% to 2.5%. The T6 sheet intermediately annealed at 550 °C exhibits high yield strength (326 MPa), which is ascribed to the synergistic effects of precipitation strengthening, texture strengthening and grain refinement. Meanwhile, it presents high volume fractions of CubeND and random textures. This causes that r̅ value increases by 9.2% and ∆r value decreases by 80%.

Texture evolution during sub-critical annealing and its effect on yield strength anisotropy of laser directed energy deposited Ti-6Al-2Zr-1Mo-1V alloy

Titanium alloys fabricated by laser directed energy deposition (LDED) are usually subjected to sub-critical annealing to improve mechanical properties. This work mainly investigated the texture evolution of LDEDed Ti–6Al–2Zr–1Mo–1V alloy during sub-critical annealing and its effect on the anisotropy of yield strength. Results indicated that <0001> of α phase in the as-deposited sample exhibited two types of circle-shaped texture, which were perpendicular to the deposition direction (VD) and 45° away from VD, respectively. The intensities of these two types of texture were approximately equal because the α texture in the as-deposited sample were transformed from the strong <001>β//VD fiber texture following Burgers orientation relationship with weak variant selection. During sub-critical annealing, the formation of primary α phase, which tended to be transformed from the {110}β parallel to VD, led to the intensity of circle-shaped texture perpendicular to VD greater than that 45° away from VD. The calculation results suggested that all basal and prismatic slip systems of some α variants transformed from the {110}β parallel to VD were in hardly-activated orientation with Schmid factor (m) equal to zero for loading along the horizontal direction (HD), the slip systems of some α variants transformed from the {110}β 45° to VD were in hardly-activated orientation with m = 0 for loading along 45° direction (OD), while the systems of all α variants were in easily-activated orientation with high Schmid factors for loading along VD. After sub-critical annealing, the content of α phase in hardly-activated orientation increased for loading along HD but decreased for loading along OD, resulting in a smaller difference of yield strength between OD and VD or HD.

Effect and mechanism of inter-layer ultrasonic impact strengthening on the anisotropy of low carbon steel components fabricated by wire and arc additive manufacturing

The problem of anisotropy in the microstructure and mechanical property of the fabrications limits the application and development of wire and arc additive manufacturing (WAAM). By introducing inter-layer ultrasonic impact (UI) strengthening in the low carbon steel WAAM process, the anisotropy can be effectively improved. After being treated by UI, the typical columnar microstructure with obvious direction is transformed into a uniform and fine equiaxed microstructure. Digital image correlation results show that the stress concentration in the tensile process is significantly improved. The anisotropy of tensile strength and yield strength is reduced from 4.2% to 1.6% and 10.1%–2.3%, respectively. Scanning electron microscope (SEM) and electron backscattering diffraction (EBSD) are applied to investigate this phenomenon. The results indicate that UI can break the constraint for dislocations to move, promoting dislocation merging and annihilating. And then, numerous substructures are formed, partially recrystallized by the subsequently deposited layers' thermal effect. The transformation can block the columnar microstructure growth and divide the columnar microstructure into the cellular or equiaxed microstructure. In addition, the results show that inter-layer ultrasonic impact strengthening can result in local misorientation reduction, grain refinement (average grain size reduction from 2.18 μm to 1.65 μm), and texture orientation strength weakening.

A quantitative study on planar mechanical anisotropy of a Mg-2Zn-1Ca alloy

A TD-tilted basal texture in Mg alloys often generates a strong planar mechanical anisotropy. Unfortunately, there has been not a quantitative study on this issue. In the present study, the mechanisms for the anisotropy of tensile yield strength in RD-TD plane were quantitatively studied for a Mg-2Zn-1Ca alloy with different grain sizes and textures. Here, RD and TD refer to the rolling direction and transverse direction of the plate. The results show that there is a strong planar yield anisotropy (∼24 ± 5 MPa), regardless of grain size, while this yield anisotropy is absent in texture with a circular distribution of basal poles. The examination of microstructure and X-ray diffraction analysis reveals that, beside basal slip as the predominant mode under both RD-tension and TD-tension, prismatic slip in RD-tension and both prismatic slip and extension twinning in TD-tension are also important. The parameters of Hall-Petch relation are also used to further analyze the mechanism for yield anisotropy. The changeable intercept, σ0, and invariant Hall-Petch slope, k, between RD-tension and TD-tension are rationalized accounting for this anisotropy. The reasons why there are different σ0 and similar k between TD-tension and RD-tension are quantitively discussed. At last, a texture design for decreasing the planar yield anisotropy is given.

Effects of granular bainite and polygonal ferrite on yield strength anisotropy in API X65 linepipe steel

Linepipe steels fabricated using thermo-mechanically controlled processes exhibit microstructural inhomogeneity and a characteristic texture; therefore, they often lead to anisotropic yield strength properties. Although yield strength anisotropy is considered as a major guaranteed property, particularly in pipe forming processes, systematic verification of the dominant microstructural component affecting the yield strength is challenging owing to complicated microstructures and inhomogeneous distributions. In this study, the microstructures were controlled by varying the rolling reduction ratios and start rolling temperatures, specifically for polygonal ferrite (PF) and granular bainite (GB); the Taylor factor (TF) and orientation distribution function (ODF) of each microstructure were quantitatively analyzed by electron backscatter diffraction to study the effects of microstructural characteristics on the yield strength. The deviation of TF with tensile direction in GB was larger than that in PF; the intensity of {113}<110> components in ODF maps was greater in GB than in PF. The results indicated that GB induced more yield strength anisotropy than PF; thus, steels with a greater GB fraction exhibited greater yield strength anisotropy. This study can be used for lowering the yield strength anisotropy in linepipe steel plate design, with promising prospects for wider industrialization of high-strength linepipe steels.

Effect of regression and re-aging treatment on tensile yield strength anisotropy of 7050 aluminum alloy

In this study, the evolution rule of yeild strength and microstructure along the axial, hoop and radial directions of 7050 aluminum alloy ring forging during Regression and re-aging (RRA) heat treatment was studied by using the experimental technology of tensile property test and transmission electron microscope (TEM) and Electron backscatter diffraction (EBSD) observation. During the RRA process, the evolution rule of intragranular and intergranular precipitates along axial, hoop and radial directions of 7050 aluminum alloy ring forging was described by establishing a model. After the pre-aging stage, the yield strength of samples along the axial, hoop and radial directions of forgings shows obvious anisotropy, and the anisotropy decreases gradually after regression and re-aging treatment. The results show that the cause of the larger yield strength anisotropy is the inhomogeneous size and distribution of precipitates after the pre-aging stage; After the regression stage, the cause of the reduction of the yield strength anisotropy is due to the homogeneous distribution of intragranular precipitates; After re-aging treatment, the reasons of further reduction of the yield strength anisotropy are because of more homogeneous distribution of intragranular precipitates and the discontinuous of intergranular precipitates.