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The α variant selection and microstructure evolution in a new metastable β titanium alloy TB17 were studied in depth by DTA, microhardness, XRD, SEM, and EBSD characterization methods. Under the rapid cooling rate conditions (150 °C/min–400 °C/min), only a very small amount of granular αWM (α Widmanstatten precipitates within the grains) precipitated within the grains. The secondary α phase precipitated in the alloy changed from granular to fine needle-like at moderate cooling rates (15 °C/min–20 °C/min). When continuing to slow down the cooling rates (10 °C/min and 1 °C/min), the αGB (α precipitates along the β grain boundaries), αWGB (α Widmanstatten precipitates that developed from β grain boundaries or αGB) and αWM grew rapidly. Moreover, the continuous cooling transformation (CCT) diagram illustrated the effect of cooling rate on the β/α phase transition. EBSD analysis revealed that the variants selection of α near the original β grain boundary is mainly divided into three categories. (i) The double-BOR (Burgers orientation relationship) αWGB colonies within neighboring β grains grow in different directions but have the same crystallographic orientation. (ii) The double-BOR αWGB colonies within neighboring β grains have different growth directions and different crystallographic orientations. (iii) The double-BOR αWGB colonies within the same grain have the same growth direction, but different crystallographic directions. And these double-BOR αWGB colonies correspond to two variants of the given {0001}α//{110}β. [ABSTRACT FROM AUTHOR]

The correlation of prior austenite grain (PAG) size, mechanical properties, and the variant selection in martensite transformation of a 2 GPa grade automobile steel has been systematically investigated by using a combination of scaning electron microscopy (SEM), electron backscatter diffraction (EBSD), and uniaxial tesile tests. As the austenitizing temperature decreases from 970 °C to 870 °C, the as-quenched microstructure of the studied steel is fully lath martensite. Meanwhile, the average size of PAG is refined from 7.2 ± 3.9 μm to 2.1 ± 1.2 μm, and the volume fraction of high angle grain boundaries (HAGB) is increased from 59.8% to 72.3%, accompanied with an increased number fraction of grains belonging to closely spaced planes (CP) group. With the refinement of the PAGs, the martensitic variant selectivity becomes strong, and the Bain group variant selection discretely transites to the CP group, resulting in a dominate CP group. Consequently, a tensile strength of 2014 ± 33 MPa, a tensile elongtation of 12.9 ± 0.5% and a maximum strength × ductility production of 26.0 ± 1.4 GPa·%, are obtained when the austenitization temperature is at 920 °C.

The morphology of Ni4Ti3 precipitates is important in tuning the martensitic transformation (MT) behavior and mechanical properties of nitinol. Constrained ageing is effective in engineering the morphology of Ni4Ti3 precipitates due to the variant selection effect of external load which is still lacking. In this work, maps of variant selection effect of external load applied along all crystallographic directions are obtained by using a combination of theoretical analyses and phase field simulations. It is found that maps produced by uniaxial tension and uniaxial compression are quite different. The number and types of Ni4Ti3 variants preferred by external load vary as the loading direction changes. Moreover, factors influencing the strength of variant selection effect are discovered. This work provides insights on understanding the Ni4Ti3 precipitation process and sheds light on the engineering of morphology of Ni4Ti3 precipitates for desired mechanical and functional properties. [ABSTRACT FROM AUTHOR]

In this work, two bainitic steels with medium carbon content were developed by adding different Si. The mechanical properties of the two test steels and the stability of their retained austenite were studied, and the influence of Si in the steels was discussed. The results show that the film-like retained austenite of the high Si bainitic steel is greatly increased compared with that of the low Si bainitic steel and that the C content is increased, effectively preventing crack propagation and significantly improving the comprehensive mechanical properties. Due to the different Si content in the test steels, the orientation relationship of austenite in the steels has changed, and when the Si content is increased from 0.7 wt% to 1.4 wt%, the variant selection of bainite becomes weaker. Moreover, the increase of Si content not only inhibits the precipitation of carbides but also promotes the accumulation of C in the austenite, which improves the stability of the retained austenite.

The phase transformation of β to stress-induced α″ martensitic progresses in a displacive mode. The associated transformation strain induced by β to α″ martensitic has an important influence on the resultant microstructure. In this work, the stress-induced α″ martensitic transformation and its variant selection behavior were investigated in a metastable β titanium alloy under an uniaxial compression. The lattice distortion of β to α″ martensitic indicated that the shear strain on {112} planes and in directions mainly dominate the transformation. During the slight compression, only a limited number of α″ variants were selectively formed within β grains, i.e. the occurrence of variant selection of α″ martensite. The results showed that the selected variants of α″ martensite are preferential those with high Schmid Factors on their corresponding {112}/ shear systems.

Controlling continuous cooling is considered an effective means of producing high-strength and high-toughness bainitic steel. A medium carbon bainite frog steel is designed. By combining a thermal dilatometer with in-situ high-temperature metallography, the influence of continuous cooling rate on the phase transformation of bainitic steel in the medium temperature range (500-365 °C) is studied. The microstructure and crystallographic orientation are characterized. The results indicate that with the increase of cooling rate in the temperature range for bainitic transformation, the initial temperature of bainitic transformation significantly decreases, and the transformation kinetics enhance, which causes the cooling rate for obtaining a maximum fraction of bainite significantly shifts to the left. As the cooling rate increases, the morphology of the bainite changes from a mixture of granular and coarse lath-like forms to fine lath-like forms. The volume fraction and average size of retained austenite as well as the size of bainite laths decrease with the increase of cooling rate. In addition, there are significant differences in the high-angle interface under different cooling rates, which is related to the variant selection mechanism of bainitic transformation at different cooling rates. The research results reveal the transformation process and microstructure evolution of bainite under different cooling rates in the medium temperature range, providing support for the microstructure control and performance optimization of bainite frog steel under continuous cooling control.

This study demonstrates the successful fabrication of a multi-material 18Ni(300) maraging steel – CoCrMo alloy using laser powder bed fusion (L-PBF), which in its as-built state, displays suboptimal mechanical performance. Addressing this, we propose different heat treatments that mutually enhance the properties of both alloys. Comparative analysis of texture development, precipitation sequence and mechanical properties of the dual structures at different scales has been conducted. The results indicate the cooperative strengthening of intragranular γ–ϵ transformation in CoCrMo, and Ni3Ti precipitation in maraging steel. Adding the solution treatment also balanced the formation of acicular Ni3Ti clusters with (Fe, Ni, Co)2(Ti, Mo) precipitates, and revealed that chemical segregation influences austenite reversion. Initial evidence of local grain variant selection has been revealed in as-built samples due to thermal cycling and austenite reversion, which generates residual stresses, recoil forces and convective flow. Surprisingly, the missing variants can also be inherited after heat treatment with insufficient solution temperatures.

In this article, we subjected the Ti60 alloy to solid-solution treatment at 1020 °C and aging treatment at 600 °C, respectively, achieving a bimodal microstructure. The microstructures obtained after aging treatment showed no significant difference in the primary α-phase content, size, and width of the lamellar α phase. This suggests that the final microstructure morphology is primarily determined by the solid-solution temperature, with the aging process exerting less pronounced effects on microstructural alterations. Furthermore, we investigated the effect of solid-solution and aging treatment on the crystallographic orientation evolution of the secondary α phase (αs) in the near-α titanium alloy Ti60. The αs phase displays a random orientation in solid-solution treatment sample, while it demonstrated a preferential {0 1 −1 0} orientation after aging treatment. This interesting phenomenon is attributed to the enhanced variant selection resulting from the dissolution of variant near 60° and 90° during aging. Furthermore, the αs with {0 1 −1 0} orientation nucleated at the grain boundary and coalesced into larger αs lath with increasing aging time, further contributing to the αs {0 1 −1 0} texture. [ABSTRACT FROM AUTHOR]

Laser welding of the near α-phase titanium alloy Ti-4Al-2V, used for complex components in the nuclear industry, has been rarely reported. In this study, butt weld joints made of Ti-4Al-2V alloy plates under different parameters, including the laser power, the welding speed, and the defocus distance, were manufactured and analyzed. The results showed that adjusting the combination of 4.2 kW of laser power, a 20 mm/s welding speed, and a −2 mm defocus distance could achieve a penetration depth exceeding 6 mm. Porosity defects were prone to forming in the middle and bottom parts of the fusion zone, due to rapid cooling. The microstructure of the fusion zone was mainly needle-like α martensite, which precipitated in the form of specific clusters. The interior of a cluster was composed of three types of variants with <11−20>/60° phase interfaces to achieve the lower boundary's energy. Affected by the microstructure and welding defects, the strength of the weld joint was basically similar under different welding conditions, namely about 720 MPa, slightly higher than that of the base metal, while the rupture elongation at breaking decreased by more than 50%. The micro-Vickers hardness of the weld joints was about 50–60 HV higher than that of the base metal, while the impact toughness was about 40 KJ, almost half that of the base metal. This research lays a solid foundation for the engineering application of laser welding of Ti-4Al-2V alloys. [ABSTRACT FROM AUTHOR]

A near-β Ti alloy with a highly anisotropic single-crystal-like β grain structure was fabricated on a polycrystalline pure-Ti substrate by laser powder bed fusion (LPBF) using a flat-top laser with bidirectional scanning and 90° hatch rotation. The as-fabricated specimen had an acicular microstructure with 12 α variants. β-annealing followed by air-cooling revealed the growth of columnar β grains with a 〈001〉 orientation in the build direction from the center outward with increasing height, which formed a highly anisotropic β grain structure with a 〈001〉 orientation in the build direction and 〈100〉 or 〈010〉 orientation along the scan direction with low-angle grain boundaries (LAGBs) in the β matrix. The predominant 〈21¯1¯0〉-oriented and minor 〈101¯2〉-oriented α phases precipitated in the 〈001〉-oriented β matrix according to the Burgers orientation relationship. The 〈101¯2〉-oriented α phase was found to form on some LAGBs. Notably, a single α variant was widely distributed without forming multiple-variant clusters across the highly anisotropic β grain structure.

Acicular ferrite (AF) has been regarded as the optimal microstructure in steel welds and heat affect zones (HAZs), because of its superior combination of strength and toughness. Although various aspects of AF have been investigated, the crystallographic characteristics of AF, including orientation relationship, variant selection and variant pairing, are still not fully studied. In this research, a Ti-deoxidized high strength low alloy steel was prepared and subjected to HAZ thermal cycle simulation tests to attain AF microstructure. Optical micrographs were obtained and EBSD mappings of AF microstructures were conducted. Through crystallographic analysis, the orientation relationship between AF and austenite was found close to that of bainite transformed at a high temperature (580 °C). In the AF microstructure, all 24 variants are formed and variant selection is nearly random. Compared with bainite, the variant pairing of AF is more random despite the slight favour of certain variant pairs. Most of the adjacent AF laths are from different close-packed plane (CP) groups, leading to the classic interlocking morphology. Meanwhile, a large number of neighbouring AF laths belong to different Bain groups, resulting in a high density of high angle grain boundaries (HAGBs). Intragranular nucleation of primary AF laths on inclusions contributes both to the high HAGB density and to the interlocking morphology of AF, while the sympathetic nucleation of secondary AF laths only contributes to the latter. These results suggest that to increase the HAGB density in AF, promoting the intragranular nucleation on inclusions is more important than enhancing the sympathetic nucleation.

The α variant selection and microstructure evolution in a new metastable β titanium alloy TB17 were studied in depth by DTA, microhardness, XRD, SEM, and EBSD characterization methods. Under the rapid cooling rate conditions (150 °C/min–400 °C/min), only a very small amount of granular αWM (α Widmanstatten precipitates within the grains) precipitated within the grains. The secondary α phase precipitated in the alloy changed from granular to fine needle-like at moderate cooling rates (15 °C/min–20 °C/min). When continuing to slow down the cooling rates (10 °C/min and 1 °C/min), the αGB (α precipitates along the β grain boundaries), αWGB (α Widmanstatten precipitates that developed from β grain boundaries or αGB) and αWM grew rapidly. Moreover, the continuous cooling transformation (CCT) diagram illustrated the effect of cooling rate on the β/α phase transition. EBSD analysis revealed that the variants selection of α near the original β grain boundary is mainly divided into three categories. (i) The double-BOR (Burgers orientation relationship) αWGB colonies within neighboring β grains grow in different directions but have the same crystallographic orientation. (ii) The double-BOR αWGB colonies within neighboring β grains have different growth directions and different crystallographic orientations. (iii) The double-BOR αWGB colonies within the same grain have the same growth direction, but different crystallographic directions. And these double-BOR αWGB colonies correspond to two variants of the given {0001}α//{110}β.

Periprosthetic joint infection (PJI) is a difficult complication requiring a comprehensive eradication protocol. Cure rates have essentially stalled in the last two decades, using methods of antimicrobial cement joint spacers and parenteral antimicrobial agents. Functional spacers with higher-dose antimicrobial-loaded cement and antimicrobial-loaded calcium sulphate beads have emphasized local antimicrobial delivery on the premise that high-dose local antimicrobial delivery will enhance eradication. However, with increasing antimicrobial pressures, microbiota have responded with adaptive mechanisms beyond traditional antimicrobial resistance genes. In this review we describe adaptive resistance mechanisms that are relevant to the treatment of PJI. Some mechanisms are well known, but others are new. The objective of this review is to inform clinicians of the known adaptive resistance mechanisms of microbes relevant to PJI. We also discuss the implications of these adaptive mechanisms in the future treatment of PJI. Cite this article: Bone Joint J 2022;104-B(5):575-580.

Cobalt–Chromium–Molybdenum (Co–Cr–Mo) alloys are commonly used for artificial hip and knee joint metallic implants. These components are subjected to repetitive loads during service. Therefore, materials used for such applications must exhibit a high fatigue crack resistance. In this research, Co–28Cr–6Mo (wt.-%) powder was utilized as a feedstock in a laser powder bed fusion process to produce test coupons. The coupons were then subjected to load-controlled cyclic material tests in the low cycle fatigue regime to study mechanical response and microstructural changes of the material. With the progressing number of cycles, a continuous increase in macroscopic plastic strain was observed. The electron backscattered diffraction analysis revealed that cyclic loading caused deformation-induced face-centered cubic (FCC) → hexagonal close-packed (HCP) phase transformation. In addition, the phase transition generated an accumulation of plastic strain at the FCC/HCP interface giving rise to crack nucleation. The crack propagation path along HCP orientation variants with high mechanical work and strain hardening mechanism is discussed.

Abundant {10–12} twins appeared in an extruded Mg-2.8Y (wt. %) sheet with rare-earth (RE) texture under compression along the transverse direction (TD). Twinning-detwinning behavior during compression-tension loading was quantitatively investigated via quasi-in-situ EBSD observation. The evolution of deformation mechanisms was quantitatively studied by calculation of the plastic strain fraction of twinning, detwinning, and dislocation slip. Twinning and detwinning were the dominant deformation modes in compression and reverse tension, respectively, owing to their relatively higher Schmid factors (SF) and lowest activation stresses (τ). Twin variants with SF ranking 1st and 2nd were activated preferentially while detwinning with low SF ranking showed high activation. The impact of texture and load path on variant selection was systematically discussed. Moreover, short twin chains only passing through two or three grains caused by twinning transfer were observed. Grain boundary misorientation angle (GBMA) and geometric compatibility factor (m’) were introduced to analyze the crystallographic characteristics of twinning transfer. GBMA ≤34.5° or m’ ≥ 0.80 is a necessary condition for most twinning transfer events. RE texture as well as the nearly uniform distribution of GBMA gave rise to short twin chains and restricted the formation of long twin bands. The relationship between GBMA and m’ was also discussed.

Heat treatments with controlled cooling rates have been used to investigate orthorhombic (O) phase precipitation and its variant selection (VS) in Ti–22Al–25Nb alloy, a latest Ti2AlNb-based intermetallic alloy with excellent comprehensive properties. This study uncovers the misorientation types of different O variants mainly belong to two types with a sharing angle of 90° and distinct rotated axis of or . A notable lamellar structure, colony side plates (CSPs), frequently develop on both sides of adjoining B2 grains, which always have a detrimental effect on the final properties. This structure significantly influenced by VS, variants having (001) planes that are paralleled to the common {110} plane of adjacent B2 grains are selected in colony plates on both sides. If the axis of the two B2 grains coincidentally close as well, the colony plates may appear to penetrate the GB. Moreover, the CSPs structure could form on condition of much slow cooling rate after holding in rather high temperature. These findings could provide insights on how to control processing conditions and tailor the lamellar structure to obtain the desired properties.

Acicular ferrite has appeared in various low carbon microalloyed steel welds (e.g., laser-arc hybrid welds) due to its high grain boundary density and toughness. However, the transformation kinetics and crystallographic structure characteristics of acicular ferrite in laser-arc hybrid welds are still unclear. In this paper, the nucleation and growth kinetics of acicular ferrite in laser-metal active gas hybrid welds with different cooling rates (1–150 K/s) were investigated and their crystallographic characteristics were revealed from the perspective of variant. The result showed that acicular ferrite can nucleate at all the test cooling rates in weld metal, and the microstructure coexisting with acicular ferrite changes from polygonal ferrite and granular bainite with a low cooling rate to lath martensite with a high cooling rate. As the cooling rate increases, the phase transformation temperature of acicular ferrite decreased, and the phase transformation mode changed from partitioning local equilibrium to negligible partitioning local equilibrium. The variety of the phase transformation mode and the higher phase transformation driving force led to an increase in the growth rate of acicular ferrite. Compared with the bainite and martensite in the base metal, the acicular ferrite had the characteristics of light variant selection, uniformly distributed operating factor and small variant size, which resulted in high-density effective grain boundaries. To obtain more acicular ferrite, the cooling rate should be increased as much as possible on the premise that the phase transformation termination temperature was higher than the Ms. [ABSTRACT FROM AUTHOR]

In this study, the coarse grained heat affected zone (CGHAZ) of Cu-containing/free steels was simulated at various heat inputs by thermal simulator. A comparative study was conducted to reveal the role of Cu addition and heat input in toughness variation by microstructural observation and crystallographic analysis. The results showed that the toughness exhibited a plateau-like variation with heat input increasing, which was mainly determined by the distribution of high angle grain boundary (HAGB). Also, a considerably higher toughness was achieved in Cu-containing steel irrespective of heat input. This attributed to the grain boundary distribution and variation selection. Cu addition weakened the variation selection and promoted the formation of more Bain groups, which helped to increase the fraction of HAGB and improve the impact toughness. In addition, Cu addition resulted in a higher fraction of boundary length in V1/V2, V1/V3, V1/V5 variant pair, which was beneficial to accommodate the transformation strain and release the residual stress, leading to a higher toughness in Cu-containing steels.

In this article, we subjected the Ti60 alloy to solid-solution treatment at 1020 °C and aging treatment at 600 °C, respectively, achieving a bimodal microstructure. The microstructures obtained after aging treatment showed no significant difference in the primary α-phase content, size, and width of the lamellar α phase. This suggests that the final microstructure morphology is primarily determined by the solid-solution temperature, with the aging process exerting less pronounced effects on microstructural alterations. Furthermore, we investigated the effect of solid-solution and aging treatment on the crystallographic orientation evolution of the secondary α phase (αs) in the near-α titanium alloy Ti60. The αs phase displays a random orientation in solid-solution treatment sample, while it demonstrated a preferential {0 1 −1 0} orientation after aging treatment. This interesting phenomenon is attributed to the enhanced variant selection resulting from the dissolution of variant near 60° and 90° during aging. Furthermore, the αs with {0 1 −1 0} orientation nucleated at the grain boundary and coalesced into larger αs lath with increasing aging time, further contributing to the αs {0 1 −1 0} texture.

Laser welding of the near α-phase titanium alloy Ti-4Al-2V, used for complex components in the nuclear industry, has been rarely reported. In this study, butt weld joints made of Ti-4Al-2V alloy plates under different parameters, including the laser power, the welding speed, and the defocus distance, were manufactured and analyzed. The results showed that adjusting the combination of 4.2 kW of laser power, a 20 mm/s welding speed, and a −2 mm defocus distance could achieve a penetration depth exceeding 6 mm. Porosity defects were prone to forming in the middle and bottom parts of the fusion zone, due to rapid cooling. The microstructure of the fusion zone was mainly needle-like α martensite, which precipitated in the form of specific clusters. The interior of a cluster was composed of three types of variants with /60° phase interfaces to achieve the lower boundary’s energy. Affected by the microstructure and welding defects, the strength of the weld joint was basically similar under different welding conditions, namely about 720 MPa, slightly higher than that of the base metal, while the rupture elongation at breaking decreased by more than 50%. The micro-Vickers hardness of the weld joints was about 50–60 HV higher than that of the base metal, while the impact toughness was about 40 KJ, almost half that of the base metal. This research lays a solid foundation for the engineering application of laser welding of Ti-4Al-2V alloys.

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The objective of this study is to establish a quantitative relationship between the strengthening effects of different phases and yield strength, and also to gain a new insight into the relationship between crystallographic variants and impact toughness in the simulated inter-critical heat affected zone (ICHAZ) of 690 MPa grade thick-plate medium Mn steel. The results indicated that the heterogeneous microstructure consisted of completely tempered martensite (CTM), partly tempered martensite (PTM), fresh martensite (FM) and retained austenite (RA) was formed in ICHAZ. As the peak temperature increased from 700 to 800 °C, the relative percentage of FM significantly increased from 5.5 to 79.2 vol %, while that of RA, PTM and CTM decreased from 20.3 to 11.3 vol %, from 65.5 to 9.3 vol %, and from 8.7 to 0.2 vol %, respectively. The yield strength correspondingly increased from 800 to 1200 MPa as the peak temperature rose due to the increasing contribution of various strengthening mechanisms of distinct phases. Furthermore, impact toughness decreased paradoxically from 184.9 to 47.8 J with increasing density of high angle boundary (HAB) from 0.806 to 1.47 μm−1 and decreasing size of prior austenite grains (PAGs) from 12.5 to 4.1 μm because of the variable crack propagation path induced by different variant selection mechanisms. In concluding, this research proposed a feasible strategy to improve the welding performance of medium Mn steel.

A crystallographic analysis was conducted of the upper bainite nucleated at the austenite grain boundary in Fe-0.6C-0.8Mn-1.8Si (in mass %) steel by the EBSD analysis. The effect of the character of the prior austenite grain boundary (PAGB) on the formation of upper bainite was investigated from several perspectives: PAGB plane, grain boundary energy, and so on. BFs form on both sides of the high-angle PAGBs, while BFs do not form at twin boundaries. It is suggested one of the reasons for the suppression of BF formation at twin boundaries is the lower grain boundary energy. At high-angle grain boundaries, there is no difference in the potency for BFs' nucleation between the tilt-like PAGBs and twist-like PAGBs, and the formation of BF is not affected by the angle between the rotation axis, the PAGB plane, and grain boundary energy. The variant selection of BFs was investigated. The BFs pair, whose misorientation across the PAGB is small, is formed preferentially. When several variant pairs can form having small misorientation across the PAGB, the variant pair that can reduce the elastic strain energy preferentially forms to accommodate the shape strain. [ABSTRACT FROM AUTHOR]

The study of orientation variant selection helps to reveal the mechanism and dynamic process of martensitic transformations driven by temperature or pressure/stress. This is challenging due to the multiple variants which may coexist. While effects of temperature and microstructure in many martensitic transformations have been studied in detail, effects of stress and pressure are much less understood. Here, an in situ variant selection study of Mn2O3 across the cubic-to-orthorhombic martensitic transformation explores orientation variants at pressures up to 51.5 GPa and stresses up to 5.5 GPa, using diamond anvil cells in radial geometry with synchrotron X-ray diffraction. The diamonds not only exert pressure but also impose stress and cause plastic deformation and texture development. The crystal orientation changes were followed in situ and a {110} c 〈001〉 c // (100) o 〈010〉 o relationship was observed. Only the {110} c plane perpendicular to the stress direction was selected to become (100) o , resulting in a very strong texture of the orthorhombic phase. Contrary to most other martensitic transformations, this study reveals a clear and simple variant selection that is attributed to structural distortions under pressure and stress.

Three-dimensional microstructures of bainitic ferrites and prior austenite grains (PAGs) were observed in the initial stage of upper bainite transformation by using a serial sectioning technique and orientation analysis by electron back scattering diffraction (EBSD). The formation site of the bainitic ferrites was quantitatively evaluated by three-dimensional observation. It was revealed that the bainitic ferrites mainly form at the planes rather than the edges of prior austenite grain boundaries (PAGBs) and form on both sides of the PAGB plane. The effect of the orientation of the PAGs on the formation of the bainitic ferrites was also investigated. The bainitic ferrite has a small misorientation with the bainitic ferrite in the adjacent PAG across the PAGB. It is suggested that the reason for the formation of bainitic ferrite at the planes rather than edges of PAGBs is because it is difficult for bainitic ferrite to have a small misorientation with the bainitic ferrites in adjacent PAGs at edges. [ABSTRACT FROM AUTHOR]

The dissolution behavior of carbides in martensitic stainless steel and its effect on microstructure and hardness were investigated by using X-ray diffractometer (XRD) and field emission scanning electron microscopy (FE-SEM) with energy dispersive spectrometer (EDS) and electron backscattering diffraction (EBSD). The results indicated that the microstructure after austenitizing heat treatment and oil quenched consisted of martensite, M23C6 carbides and retained austenite. The temperature and particle size had great influence on the dissolution of carbides. The EBSD results showed that the twin-related variant pair V1/V2 governed the phase transformation. Meanwhile, the density of high-angle grain boundaries (HAGBs) increased with the increase of austenitizing temperature from 950 to 1150 °C. The hardness test results indicated that the hardness first increased and then decreased with the increase of the austenitizing temperature, and the peak appeared at 1050 °C with a Rockwell hardness value of 59.8 HRC. A model was established to quantitatively explain the contribution of different microstructures to hardness. The contribution to hardness came mainly from martensite. The retained austenite had a negative effect on hardness when the volume fraction was more than 10%. In contrast, carbides contributed less to hardness due to their small content. [ABSTRACT FROM AUTHOR]

{332}<113> twinning is an important deformation mode for metastable β–Ti alloys. However, little is known on the reverse detwinning process and its hardening effect. Herein, detwinning behavior in Ti–15Mo alloy and its effect on the microstructure evolution and yield strength is investigated. Twins are generated during pre‐tension along the rolling direction. Then, the pre‐twinned samples are compressed in the same direction by different strains. It is shown that compressive yield strength is significantly improved by pre‐tension compared with the as‐received alloy. Microstructure characterization reveals that almost all the twins generated in pre‐tension were detwinned during the reverse compression. New twins appear in some β grains by increasing the compression strain. The variant with the highest Schmid factor (SF) is likely selected for twinning. Some paired twins formed at grain boundaries do not have the highest SF, but have a large strain compatibility factor (m′), implying a strong influence from local stress. Profuse slip traces are observed, which transferred across the detwinned regions during the reverse compression, causing severe stress concentrations in the primary twin domains. Based on the experimental observations, a possible strengthening mechanism by the twinning–detwinning behavior is proposed and discussed. [ABSTRACT FROM AUTHOR]

Non-toxic and biocompatible Ti alloys with low modulus are promising biomaterials for implants. Except from the intensely studied metastable β Ti-Nb-based alloys, the α′-type Ti alloys could also present low modulus. But due to the low β stability, the microstructures of these α′ Ti alloys are sensitive to the heat-treatments especially to the cooling rates, which could lead to very different deformation mechanisms and mechanical properties. Thus, the phase transformations and the 'microstructure–mechanical property' correlation in the α′ Ti alloys need to be established. In this study, a novel Ti-12Nb wt% alloy was prepared and cooled from the β region temperature to obtain the water-quenched (WQ), air-cooled (AC), and furnace-cooled (FC) samples. Systematic investigations revealed the displacive β → α′ martensitic transformation in the WQ and diffusional β → β + α transformation in the AC and FC samples, in which the α′/α show evident variant selections. The inter-variant orientation relationships (ORs) among the α′/α variants and their ORs with the retained β phase, were found to be agree well with the classical Burgers OR. The high-resolution transmission electron microscope (HRTEM) studies show coherent α′/α′ interfaces in the WQ samples and α/β phase boundaries in the AC and FC samples, the abnormal β nanodomains inside the α′ martensites in WQ samples and the ω nanoparticles in the β phase of AC and FC samples were also presented. The tensile tests show increasing Young's modulus of 61.8 GPa, 77.1 GPa and 108.6GPa in the WQ, AC and FC samples, respectively. The difference in the modulus of these samples was explained by the Nb dependency of Young's modulus in the Ti-Nb alloy. The correlations between the microstructures, deformation mechanisms and mechanical properties were established. The { 10 1 ¯ 1 } α′ and { 10 1 ¯ 2 } α′ deformation twins inside the α′ martensites are believed to contribute to the good plasticity in the WQ samples, while the fine α + β structures and severely piled-up dislocations should account for the high yield strength and the poor plasticity in the AC samples. The abundant mobile dislocations in the α bands and deformation-induced ω needles in the surrounding β phase led to the best plasticity in the FC samples among all Ti-12Nb samples. This study systematically studied the displacive and diffusional transformations and the plastic deformation in a Ti-12Nb alloy, the results might pave the way for designing low-modulus biomedical α′ Ti alloy with low content of β stabilizing elements and provide insights into tailoring of the phase transformations and mechanical properties in these Ti alloys. • The displacive and diffusional phase transformations in a Ti-12Nb wt% alloy were presented. • The α′-type Ti-12Nb alloy showed low modulus, high yield strength and good plasticity. • The abnormal β nanodomains in α′ martensites and ω particles in β phase were revealed. • The microstructure-deformation mechanism-mechanical property correlations were established. [ABSTRACT FROM AUTHOR]

Digital holographic microscopy (DHM) was employed to investigate the formation process and shape evolution of the most frequently observed bainite variant pairs in low-carbon alloy steel. Through in-situ observation coupled with theoretical predictions, the continuous changes in surface relief corresponding to bainite and adjacent austenite were measured. Additionally, the crystal orientations of the observed variant pairs were determined using electron backscattered diffraction (EBSD). The results indicated that at the onset of the transformation, the bainite variant shape agreed with theoretical predictions, while the adjacent austenite experienced plastic deformation. However, as the transformation proceeded towards completion, the shape of some bainite variants deviated from the theoretical predictions, while their adjacent austenite remained undeformed. This shift of deformation strain between austenite and bainite suggests a change in strain accommodation mechanism. Furthermore, the observed 'tent-shaped' surface relief was attributed to back-to-back mutually accommodating variant pairs, resulting in reduction of the total shape strain, compared to the theoretical prediction. [Display omitted] [ABSTRACT FROM AUTHOR]

The mixed structure of lath bainite and martensite was obtained by isothermal transformation tests at 698 K, 673 K and 623 K in ultra-high-strength steel. The interweaving mode of lath bainite and martensite was revealed by colored metallography and band contrast maps based on Gaussian fit. The crystallographic characteristics were analyzed in terms of variant pairing. The twin-related V1/V2 variant pairs were found in the region with high band contrast (BC) values, inferred as lath bainite. While in the low-BC value region, which was inferred as lath martensite, besides the dominated twin-related V1/V2 variant pair, V1/V4 variant pairs with low-angle grain boundaries were occasionally revealed. According to the classification by Takayama et al., both variant pairings of lath bainite and martensite in this work correspond to a type II mode. The present work has confirmed, even with complex microstructure, i.e., mixed lath bainite and martensite were formed depending on the processing route, bainite and martensite still follow their own rules of variant pairing. [ABSTRACT FROM AUTHOR]

Crystallographic relationships between α- and β-phases resulting from phase transformations, which took place during the continuous water quenching (WQ), air cooling (AC) and furnace cooling (FC) of a Ti6Al4V plates solution treated at 1065 °C, were investigated by methods of electron backscatter diffraction (EBSD) and transmission electron microscopy (TEM). WQ, AC and FC resulted in typical martensite, basket-weave and parallel-plate Widmanstatten structures, respectively. The experimental distribution of α/β-misorientations deviated from BOR at set discrete angles close to 22, 30, 35 and 43°. The experimental spectra of angles were confirmed by theoretical calculations of the possible misorientations between the α and β phases through the βo→α→βII –transformation path based on Burgers orientation relationship (BOR). Joint analysis of the experimental data and theoretical calculations revealed that the secondary βII-phase was precipitated according to the sequence βo→α→βII during continuous cooling from the single-phase β-region. Similar spectra for α/β-phase misorientations for all investigated cooling rates acknowledged the similar transformation mechanisms and dominant shear component of the phase transformations. [ABSTRACT FROM AUTHOR]

In this work, the variant selection of martensite in a stainless maraging steel was investigated by electron backscattering diffraction and a new protocol of parent phase reconstruction. The reconstruction protocol enables digital austenite reversion into prior austenite microstructure and provides information of variant selection from a large number of austenite grains. It was found that strong variant selection occurred when the prior austenite grains were significantly refined in annealing or severely deformed by ausforming. When the prior austenite grain size was finer than 20 μm, it was found that a pair of twinned variants dominated in one packet, which dominates the prior austenite grain. This finding is explained by the inefficient space left by the early transformed martensite in the dominant packet. In contrast, variants with the same Bain orientation occupied most of the space of the austenite when the strain of the austenite exceeded 50%. The accumulated microbands on the 1   1   1 plane acted as nucleation sites of specific variants of martensite. This work provides statistical results to revisit the variant selection of martensitic transformation with the assistance of computational crystallography. [ABSTRACT FROM AUTHOR]

The mechanisms driving variant selection phenomena in ausformed bainitic microstructures are not fully controlled yet. Previous results are not congruent among them, as they cannot be explained by the same rule. In this work, we aimed to understand the mechanisms governing variant selection in a medium carbon-high silicon steel subjected to ausforming treatments. To do so, the microstructural characterization in different regions of barreled samples has been combined with the simulation of the stress and strain state at the macro and micro level, by finite elements simulations and crystal plasticity simulations, respectively. The main conclusion is that the variant selection shown in these microstructures can be explained by understanding the distribution of the plastic strains at the macro and micro level. The selection of crystallographic variants is more pronounced in the regions which have been more deformed. Also, the deformation distributes along deformation microbands, where the most promoted variants seem to grow, in good agreement with previous results in ausformed lath martensite.

Microstructure and micro-texture evolution in a Fe 45 Co 35 Cr 10 V 10 medium-entropy alloy under 10‐–88% cold rolling was investigated by electron backscattering diffraction. The variant graph method reconstructed parent γ grain orientations at 20, 30 and 40% thickness reduction to determine the operative orientation relationships (ORs) with the child ε and α ′ -martensite phases and to study variant selection. The parent γ phase accommodates deformation by faulting, slip, shear banding and martensitic phase transformation via the γ → ε → α ′ and γ → α ′ pathways. The ε and α ′ -martensite variant selection occurs within individual parent γ grains but not in the bulk microstructure. Variant pairing in α ′ -martensite shifts from a preference for the same crystallographic packet group to the same Bain group with increasing thickness reduction. The development of the Brass-type micro-texture in the parent γ phase beyond 40% thickness reduction is attributed to Cu γ and S γ orientations transforming to ε and α ′ -martensite, along with shear banding. The micro-texture of ε -martensite records maximum intensities for ∼ 01 1 ¯ 5 3 7 ¯ 42 ε and ∼ 11 2 ¯ 6 1 ¯ 1 ¯ 21 ε on the hkil -fibre and originating from ∼ Cu γ and S γ via the Shoji-Nishiyama OR. The α ′ -martensite micro-texture is characterised by the α α ′ , γ α ′ and β α ′ fibres. The β α ′ -fibre forms between 20% and 60% thickness reduction and rotates towards stable orientations along the α α ′ -fibre at higher thickness reductions. Modelling shows that orientations along the β α ′ and γ α ′ - fibres stem from ∼ Br γ and ∼ S γ and those along the α α ′ -fibre originate from G γ , Br γ , Cu γ , and S γ. [Display omitted] • Traces the origins of child ε and α ′ -martensite within parent γ via the γ → ε → α ′ and γ → α ′ pathways. • Copper and S oriented γ grains exhibit preferential martensitic transformation. • Variant selection significantly influences ε and α ′ -martensite micro-texture formation. • Variant pairing in α ′ -martensite shifts from the preference for same crystallographic packet to same Bain group with thickness reduction. [ABSTRACT FROM AUTHOR]

Ru is a common addition to the Co-binder in WC Co hardmetals for advanced cutting tool applications. Internal Co-Co interfaces control many properties such as hot-hardness, toughness, and creep resistance. Hence, phase transformations determining the internal interface character can be harnessed to achieve superior properties. We investigate the γ (face-centred cubic) to α (hexagonally closed packed) martensitic phase transformation of a model Co-Ru binder alloy. We describe the crystallography of γ to α phase transformations and the resulting α/γ and α/α interface character distributions. The stabilisation of the γ-phase results in the formation of low energy (0 0 0 1) α/γ interphase boundary planes. Preferred formation of α/α intervariant and twin-related boundaries with symmetrical tilt configurations are also observed. We discuss how crystallographic constraints of the transformation promote the formation of grain boundary planes other than those of the lowest energy configurations. [Display omitted] [ABSTRACT FROM AUTHOR]

[Display omitted] • Fabrication of single-crystal-like anistropic β grain structures of a near-β Ti alloy. • Flat-top laser used in laser powder bed fusion without single-crystal seeds. • Homogeneous 〈0 0 1〉-oriented β texture with low-angle grain boundaries. • Precipitation of 〈 2 1 ¯ 1 ¯ 0 〉 -oriented α phase from 〈0 0 1〉-oriented β matrix predominant. • Single α variant widely distributed without any multiple-variant clusters. A near-β Ti alloy with a highly anisotropic single-crystal-like β grain structure was fabricated on a polycrystalline pure-Ti substrate by laser powder bed fusion (LPBF) using a flat-top laser with bidirectional scanning and 90° hatch rotation. The as-fabricated specimen had an acicular microstructure with 12 α variants. β-annealing followed by air-cooling revealed the growth of columnar β grains with a 〈0 0 1〉 orientation in the build direction from the center outward with increasing height, which formed a highly anisotropic β grain structure with a 〈0 0 1〉 orientation in the build direction and 〈1 0 0〉 or 〈0 1 0〉 orientation along the scan direction with low-angle grain boundaries (LAGBs) in the β matrix. The predominant 〈 2 1 ¯ 1 ¯ 0 〉 -oriented and minor 〈 10 1 ¯ 2 〉 -oriented α phases precipitated in the 〈0 0 1〉-oriented β matrix according to the Burgers orientation relationship. The 〈 10 1 ¯ 2 〉 -oriented α phase was found to form on some LAGBs. Notably, a single α variant was widely distributed without forming multiple-variant clusters across the highly anisotropic β grain structure. [ABSTRACT FROM AUTHOR]

In this study, we investigated the effect of strain rate on the work hardening, strain-induced martensite transformation, strain partitioning, and crystallographic variant selection in a medium-Mn steel. Uniaxial compression tests were performed under quasi-static and dynamic loading conditions at the strain rates of 1 0 − 3 , 1 and 1 0 3 s − 1 . Besides, to determine the contribution of temperature rise due to adiabatic heating at a higher strain rate, compression tests were carried out at 409 K (estimated temperature rise) under quasi-static conditions. The work hardening rate was higher at high strain rates and it rapidly decreased at all strain rates up to a true plastic strain of ∼ 0.05. Beyond the true plastic strain of ∼ 0.05, the work hardening rate increased at a low strain rate up to a true plastic strain of ∼ 0.15 while at a high strain rate, it remained relatively constant and gradually decreased beyond a true plastic strain of ∼ 0.15. From the analysis of microstructures obtained from electron backscatter diffraction, it was observed that the strain-induced transformation of retained austenite to α ′ -martensite is significantly impeded at higher strain rates and in the sample tested at 409 K at low strain rates. The higher stability of retained austenite is attributed to the adiabatic heating which decreases the chemical driving force for retained austenite to martensite phase transformation and increases the stacking fault energy. Analysis of the misorientation axis and angle evolution revealed that while the Kurdjumov–Sachs relationship was followed under quasi-static conditions, it deviated at higher strain rates. Analysis of crystallographic variants related to strain-induced martensitic transformation at a true plastic strain of ∼ 0.15 revealed that the samples deformed at a lower strain rate did not show any noticeable variant selection. However, in samples deformed at a higher strain rate, a significant variant selection was observed. It is reasoned that the martensitic transformation nucleates initially on favorably oriented habit planes of retained austenite grains at all strain rates and other variants get activated with an increase in strain. As the transformation is impeded beyond a critical strain at a higher strain rate, the variants activated at lower strains remains predominant. [Display omitted] • Post-critical strain, lower work hardening is observed in high strain rate condition • The strain induced α ′ martensite transformation is suppressed at higher strain rates • High strain rates and the associated adiabatic heating stabilize retained austenite • Adiabatic heating lowers the driving force for transformation and enhances the SFE • At high strain rates, transformation is restricted to favorably oriented habit planes [ABSTRACT FROM AUTHOR]

The precipitation behavior and microstructural evolution of α phase in a novel metastable β-type Ti alloy, Ti-10Mo-6Zr-4Sn-3Nb (wt.%), during isothermal compression are investigated in this study through the use of SEM (scanning electron microscope), TEM (transmission electron microscope) (HRTEM) (high-resolution transmission electron microscopy) and EBSD techniques. The results show that some finer α precipitates are randomly distributed within the β matrix during hot deformation. The morphological characteristics of α precipitates are distinctly different from those of α precipitates during the same solution-plus-aging treatment. The volume fraction of α precipitate gradually increases with increased true strain. A large proportion of precipitated α phases are prone to be precipitated at HAGBs (high-angle grain boundaries) and LAGBs (low-angle grain boundaries) during isothermal deformation. On the contrary, only a small amount of spherical α phases is precipitated within the β grain. The crystallographic orientation relationships for most spherical α precipitates formed at LAGBs and within the β grains are similar, whereas the crystallographic orientation relationships for α precipitates at grain boundaries are significantly different. The precipitation behavior of α phase in the Ti-B12 alloy during hot compression is considerably influenced by the density of dislocations. [ABSTRACT FROM AUTHOR]

Deformation and phase transformation are two major topics in the study of titanium alloy materials. Titanium alloys usually require a series of complex thermo-mechanical treatments to obtain a microstructure corresponding to service performance. Its hot deformation behavior is a typical thermo-mechanical coupling process. Deformation and phase transformation may occur simultaneously and affect each other. It is one of the current research hotspots. However, due to the inevitable existence of dynamic recovery / recrystallization, microstructure fragmentation / spheroidization, a large number of deformation defects, deformation texture, and stress-induced phase transformation during hot deformation, coupled with the changes of the phase transformation kinetics characteristics and the crystallographic mechanism of the variant selection under thermo-mechanical coupling, directly leads to the extremely complicated dynamic phase transformation behavior of the titanium alloy during the hot deformation process, and it becomes quite difficult to reveal its evolution law in depth. This paper summarizes the main characteristics and laws of titanium alloy deformation and phase transformation in view of the dynamic phase transformation behavior of titanium alloy during hot deformation,mainly introduces the research progress of the dynamic phase transformation of the titanium alloy under the thermo-mechanical coupling from the three aspects of precipitation phase morphology characteristics, variant selection mechanism and phase transformation kinetics characteristics, and summarizes and prospects its research and development trend.

A crystallographic analysis was conducted of the upper bainite nucleated at the austenite grain boundary in Fe-0.6C-0.8Mn-1.8Si (in mass %) steel by the EBSD analysis. The effect of the character of the prior austenite grain boundary (PAGB) on the formation of upper bainite was investigated from several perspectives: PAGB plane, grain boundary energy, and so on. BFs form on both sides of the high-angle PAGBs, while BFs do not form at twin boundaries. It is suggested one of the reasons for the suppression of BF formation at twin boundaries is the lower grain boundary energy. At high-angle grain boundaries, there is no difference in the potency for BFs’ nucleation between the tilt-like PAGBs and twist-like PAGBs, and the formation of BF is not affected by the angle between the rotation axis, the PAGB plane, and grain boundary energy. The variant selection of BFs was investigated. The BFs pair, whose misorientation across the PAGB is small, is formed preferentially. When several variant pairs can form having small misorientation across the PAGB, the variant pair that can reduce the elastic strain energy preferentially forms to accommodate the shape strain.

Three-dimensional microstructures of bainitic ferrites and prior austenite grains (PAGs) were observed in the initial stage of upper bainite transformation by using a serial sectioning technique and orientation analysis by electron back scattering diffraction (EBSD). The formation site of the bainitic ferrites was quantitatively evaluated by three-dimensional observation. It was revealed that the bainitic ferrites mainly form at the planes rather than the edges of prior austenite grain boundaries (PAGBs) and form on both sides of the PAGB plane. The effect of the orientation of the PAGs on the formation of the bainitic ferrites was also investigated. The bainitic ferrite has a small misorientation with the bainitic ferrite in the adjacent PAG across the PAGB. It is suggested that the reason for the formation of bainitic ferrite at the planes rather than edges of PAGBs is because it is difficult for bainitic ferrite to have a small misorientation with the bainitic ferrites in adjacent PAGs at edges.