B2 Grain Research Articles

Selective laser melting of the ternary NiTi+3Cu shape memory alloys with excellent properties via microstructural tailoring

The ternary NiTi+3Cu parts were produced using selective laser melting (SLM) with the addition of 3 wt% Cu nanoparticles. The incorporation of Cu nanoparticles resulted in the precipitation of nano-sized Ti2Cu second phase, the refinement of B2 matrix, the elimination of texture, and a transition from columnar to equiaxed grains. The SLM-fabricated NiTi+3Cu parts exhibited excellent antibacterial properties, achieving an antibacterial rate of 70.83 % due to the persistent release of Cu ions. The NiTi+3Cu deposits demonstrated stable tensile performance in both the transverse and longitudinal directions, and they performed better than their NiTi counterparts, primarily due to the effects of B2 grain refinement and the presence of nano-sized Ti2Cu precipitates. As the B2 phases transformed from columnar to equiaxed grains, the anisotropic rate of ultimate tensile strength and elongation to fracture was reduced from 16.3 % to 1.64 % and 16.1–3.07 %, respectively. Furthermore, the wear resistance was also enhanced by the incorporation of Cu nanoparticles, and the wear mechanism shifted from abrasive to delamination wear. These findings indicate that the crack-free ternary NiTi+3Cu shape memory alloys can be fabricated using SLM, synchronously achieving excellent antibacterial, mechanical, and wear properties through microstructural tailoring.

Atomic insights into shock-induced alloying reaction of premixed Ni/Al nanolaminates.

In material processing and handling processes, premixed interlayer often replace the ideal Ni/Al interface, which would become a new origin of alloying reaction. This work investigates shock-induced reaction mechanism and kinetics of premixed Ni/Al nanolaminates with molecular dynamics simulations and theoretical analysis. The reaction is found to be driven by the crystallization evolution in premixed interlayer and the diffusion of premixed atoms. Among them, multi-stage reaction patterns are strongly manifested by the crystallization evolution characteristics. Specifically, "crystallization-dissolution-secondary growth" and "crystallization-dissolution" of B2 phase respectively correspond to the solid-state and solid-liquid reaction cases, where crystallizations are fitted well by Johnson-Mehl-Avrami kinetics model. Interestingly, the different growth mechanisms of B2 grain are revealed, namely nuclei coalescence and atomic diffusion. Moreover, the analysis of microscopic diffusion theory indicates a certain non-random diffusion nature for solid-state reaction initiation, but near-purely random diffusion for solid-liquid reaction initiation. The diffused Al atoms possess a limited diffusion coefficient and enhanced diffusion correlation, resulting in extremely slow mixing rate in Ni layer. In addition, the influence law of Ni concentration in premixed interlayer on reactivity parameters can be quantitatively described by a quadratic function.

Simultaneous enhancements of strength and toughness by multiscale lamellar structure in Ti2AlNb based intermetallic

Multi-scale lamellar structure significantly improves toughness of Ti2AlNb based alloys, which are inherently brittle intermetallics, without compromising their strength. This structure was achieved through-B2-transus-forging (TBTF) combined with O + B2 two-phase region heat treatments. Various types of multi-scale lamellar structures were obtained by controlling the cooling rate after TBTF. These variations were mainly attributed to differences in the distribution, content, and size of the thick lamellar O phase and the size and crystallographic orientation of B2 grain. By analyzing the microstructural characteristics and crystallographic orientation near the crack propagation path, it was found that the crack propagation resistance of thick lamellae, sub grain and grain boundaries (GBs) O phase increased sequentially, accompanied by more tortuous crack propagation path. Moreover, B2 grains with high misorientation significantly deflected the crack propagation by cleavage ridges between adjoining cleavage planes. Additionally, the development of numerous secondary cleavage ridges, resulting from the transition through varying secondary cleavage planes in distinct sub B2 grains, further hindered the quick propagation of cracks. It was clarified that the cleavage planes were dominantly belonging to {110}. These findings provided valuable guidance for the design of damage tolerance strategies for Ti2AlNb-based intermetallics.

Revealing the microstructure evolution of the deformed superelastic NiTi wire by EBSD

The generation of irreversible macroscopic strains in superelastic NiTi alloys is an important issue closely related to the crystallography of the transformation from B2 austenite to B19’ martensite under stress. Although widely investigated at micro and nano scales by transmission electron microscopy (TEM), the observation of this phase transformation at the mesoscale is still lacking. In the present study, a superelastic NiTi wire was maintained in bending condition and investigated by electron backscattered diffraction (EBSD) technique. A strong variant selection and a formation of martensite texture were observed and quantified using the interaction work (IW) based on the distortion matrix of a single variant. A new habit plane between martensite and parent B2, {114}B2 || (201¯)M, was experimentally measured. It was explained by the Phenomenological Theory of Martensite Crystallography (PTMC) using dislocation slip {110}<001>B2 as lattice invariant shear (LIS). Some new B2 domains were also found in {114}B2 twinning relation with primary B2 grain. Two scenarios are proposed to explain them: a) the direct formation of {114}B2 deformation twin in B2, and b) a more complex one in which a primary B2 grain is transformed into a martensite variant, a (201¯)M deformation twin is formed inside this variant; then the (201¯)M twinned martensite reversely transforms into a new {114}B2 twinned B2 domain. The scenario b) permits to explain some uncommon (130)M, (201)M, and (101¯)M twins observed by EBSD between the martensite variants of the two {114}B2 twin-related B2 phases.

Enhanced high‐temperature strength in textured (Ti1/3Zr1/3Hf1/3)B2 medium‐entropy ceramics via strong magnetic field

AbstractThis study prepared textured (Ti1/3Zr1/3Hf1/3)B2 medium‐entropy ceramics for the first time that maintain enhanced flexural strength up to 1800°C using single‐phase (Ti1/3Zr1/3Hf1/3)B2 powders, slip casting under a strong magnetic field, and hot‐pressed sintering methods. Effects of WC additive and strong magnetic field direction on the phase compositions, orientation degree, microstructure evolution, and high‐temperature flexural strength of (Ti1/3Zr1/3Hf1/3)B2 were investigated. (Ti1/3Zr1/3Hf1/3)B2 grain grows along the a,b‐axes, resulting in a platelet‐like morphology. Pressure parallel and perpendicular to the magnetic field direction can promote the orientation degree and hinder the texture structure formation, respectively. Reaction products of W(B,C) and (Ti,Zr,Hf)C between (Ti1/3Zr1/3Hf1/3)B2 and WC additive can efficiently refine the (Ti1/3Zr1/3Hf1/3)B2 grain size and promote grain orientation. (Ti1/3Zr1/3Hf1/3)B2 ceramics doped with 5 vol.% WC yielded a Lotgering orientation factor of 0.74 through slip casting under a strong magnetic field (12 T) and hot‐pressed sintering at 1900°C. Furthermore, cleaning the boundary by W(B,C) and introducing texture can enhance the grain‐boundary strength and improve its high‐temperature flexural strength. The four‐point flexural strength of textured (Ti1/3Zr1/3Hf1/3)B2‐5 vol.% WC ceramics was 770 ± 59 MPa at 1600°C and 638 ± 117 MPa at 1800°C.

Study on annealing treatment of NbMoTaTiNi high-entropy alloy with ultra-high strength disordered-ordered transition structure for additive manufacturing

In this paper, an ultra-high-strength, fine-grained NbMoTaTiNi high-entropy alloy was formed by SLM(selective laser melting). The compressive yield strength at room temperature can reach 1728Mpa, the ultimate strength is as high as 2753 MPa, the strain is 21.75 %, and the tensile strength is 1205Mpa. The as-cast alloys were annealed at 600 ℃, 800 ℃, 1000 ℃, 1200 ℃, and 1300 ℃ to study their microstructure and mechanical properties. After annealing at 1200 ℃, the compressive plasticity of the alloy is as high as 33.55 %, an increase of 54.3 %. With the increase of annealing temperature, the alloy gradually transforms from dendrites to cellular crystals, and the XB phase, which is the grain boundary phase resists thermal stress crack defects through stacking faults. The calculation of alloy stability parameters shows that the alloy forms a disordered (matrix phase)-transition (XB grain boundary phase)-ordered (B2 grain boundary phase) structure, and with the increase of annealing temperature, the lattice distortion of the matrix phase decreases. The matrix phase tends to be more disordered and stable. The lattice distortion of the B2 phase increases and the B2 phase tends to be more ordered and stable, and the XB phase increases the stability of the disorder-ordered phase boundary of the alloy. In this paper, through the research on the annealing process of NbMoTaTiNi high-entropy alloy, the plasticity of NbMoTaTiNi high-entropy alloy is improved, and the engineering application of NbMoTaTiNi high-entropy alloy in aerospace, nuclear energy and other fields is proposed.

High Cycle Fatigue and Very High Cycle Fatigue of Orthorhombic (O + α&lt;sub&gt;2&lt;/sub&gt;)-Type Ti–27.5Nb–13Al Alloy with and without B and Fatigue Striation Analysis in Comparison with (α + β)-Type Titanium Alloys

The lightweight and high strength Ti–27.5Al–13Nb intermetallic alloy, based on the orthorhombic Ti2AlNb phase (O phase) and the α2 phase incorporated, was previously developed by the authors. This alloy would seem to have good potential for applications where fatigue behavior is a main concern, such as automobile and aircraft engine parts. The minor addition of boron (B) is known to refine the ingot grain size and thus to improve the subsequent mechanical properties. For these reasons, the high cycle fatigue (HCF) and very high cycle fatigue (VHCF) properties of B-free and 0.1 pct B-modified Ti–27.5Al–13Nb alloy were examined in the present study. HCF tests were performed at room temperature (RT) in tension-tension mode at an R of 0.1 and a frequency of 10 Hz, while VHCF tests were performed using an ultrasonic resonance fatigue test machine at an R of −1 and a frequency of 20 kHz. In both fatigue tests, hourglass-shaped specimens were used. With the addition of 0.1 pct B, the prior B2 grain size of an ingot was reduced drastically, from 600∼1000 µm for the B-free alloy to 100∼250 µm. The 0.1 pct B-modified Ti–27.5Al–13Nb alloy with a duplex microstructure consisting of a globular α2 phase and a lamellar microstructure exhibited superior elongation of 6.1 pct at RT. The HCF curve for this alloy with a duplex microstructure was almost the same as that for a Ti–6Al–4V alloy with a fully lamellar microstructure. Although prolonged fatigue life was previously reported in the HCF region in the 0.1 pct B-modified Ti–6Al–4V alloy, the addition of 0.1 pct B to the Ti–27.5Al–13Nb, Ti–6Al–4V and Ti–4Al–2.5V–1.5Fe alloys had no such effect in the VHCF region. VHCF strength for a lamellar microstructure was ranked in the order of Ti–4Al–2.5V–1.5Fe, Ti–6Al–4V and Ti–27.5Al–13Nb from the highest. Well-defined striations were observed at the propagation stage area of the fatigue fracture surface of B-free Ti–27.5Al–13Nb, and the measured striation spacing kept a constant of 0.29 µm through the propagation distance of 300 µm. The calculation based on this observation showed that the fatigue life spent in the propagation stage was very short and thus almost 100 pct of HCF life was thought to be spent in the fatigue initiation stage. For the B-free Ti–6Al–4V alloy with an equiaxed microstructure, the striation spacing increased from 0.06 µm to 4 µm as the fatigue crack propagated for a distance of 1,000 µm. Calculation based on the striation spacing revealed that, similar to the case with the Ti–27.5Al–13Nb alloy, the fatigue initiation stage consumed almost 100 pct of fatigue life regardless of the B addition.

Deformation behavior and fracture mechanism of Ti-22Al-20Nb-7Ta alloy

Three different typical microstructures (lath, equiaxed and duplex microstructures) of Ti-22Al-20Nb-7Ta alloy were obtained by thermomechanical processing. In order to analyze the deformation behavior, the typical microstructures were studied by tensile test and the fracture mechanism was also discussed. The results showed that for equiaxed microstructure, there are three slip transmission modes between O phase and B2 phase, indicating that the two-phases had good deformation coordination relationship. The O phase and B2 phase formed different type dislocation configurations, which is an important deformation strengthening mechanism of the alloy. For duplex microstructure, the fine lath O phase and α2/O phase precipitated in B2 matrix, which enhanced the hindrance effect on dislocation motion. The crack initiation and crack propagation process were observed by in-situ tensile test. For lath microstructure, the coarse lath O phase precipitated near B2 grain boundary, and the crack was easy to generate at grain boundary and rapidly fracture. For equiaxed microstructure, the microcracks were easy to generate in the slip lines and were connected to each other to generate the maincrack by the plastic deformation of B2 phase. For duplex microstructure, the microcracks generated in the lath O phase, the internal of α2/O phase and the phase interface of α2/O phase and B2 phase and were connected to each other to generate the maincrack by the plastic deformation of B2 phase. Because the disordered distribution of α2/O phase and lath O phase, the direction of crack propagation is tortuous.

The effect of 0.02–0.37 strain on the precipitation law of O phase colonies in Ti2AlNb-based alloy during aging

O phase is the main strengthening phase of the lightweight high-temperature structural material—Ti2AlNb-based alloy, its morphology, size and distribution have significant impact on the mechanical properties of the alloy. This paper obtained different deformation areas with strains within the range of 0.02–0.37 in Ti2AlNb-based alloy sample through tensile deformation and precisely characterized strain values with DIC technique. The results indicate that while the strain was larger than 0.04, O phases would precipitate at the B2 grain boundary in the form of phase colony, whereas others precipitated in B2 grains appeared as fine granule. Phase colony structure, with its size reaching to dozens of micrometers, was composed of lamellar O phase. Within the strain range of 0.08–0.24, the precipitation amount of the O phase colonies was correlated linearly with strain level. It increased with the accumulating of strain. Within the strain range of 0.24–0.37, the amount of the O phase colonies no longer increased and stabilized at 15%. The mechanism of strain promoting precipitation of O phase colonies at the B2 grain boundary is shown as follow. Deformation made B2 matrix store a lot of geometrically necessary dislocations (GND), which led atoms in deformation area arrange in disorder and thus provided sufficient nucleate sites for O phase, promoting the concentrated nucleate of the O phase in this area and formed phase colony structure. The GND density at the B2 grain boundary was noticeable higher than that inside grain, thus O phase colonies primarily formed at grain boundary.

Investigation of the O phase in the Ti–22Al–25Nb alloy during deformation at elevated temperatures: Plastic deformation mechanism and effect on B2 grain boundary embrittlement

The Ti–22Al–25Nb alloy, which possesses a complex microstructure, is a new type of lightweight high-temperature structural material. Revealing the effects of microstructure and temperature on the mechanical properties of this alloy is crucial for applications in the aerospace industry. In this study, we designed three types of microstructures of the Ti–22Al–25Nb alloy, namely, single-phase B2, dual-phase α2 + B2, and triple-phase O + α2 + B2, and investigated their deformation and fracture behavior with increasing temperature (25,750,and 930 °C). The O + α2 + B2 sample possessed the best comprehensive mechanical properties, and the softening of the O phase at elevated temperatures was mainly due to massive {001}<110>O basal slip and twinning. The evolution of O phase twins with increasing temperature can be described as follows: (021) → (021) + (110) → (110). At 750 °C, (021) twins formed in the strip-shaped O phase in B2 grains, while (110) twins formed in the spheroidized O phase at B2 grain boundaries. In addition, B2 grain boundary embrittlement was observed at elevated temperatures, and its mechanism can be summarized as follows. Elevated temperatures induced element segregation at the grain boundaries between hypersaturated-state B2 grains, enriching Ti and Al and diffusing Nb into the B2 grains on the nanometer scale, leading to a thin-shell O phase with a thickness of tens of nanometers precipitating at the B2 grain boundaries. During elevated-temperature fracturing, cracks propagated quickly along the interface between the thin-shell O phase and B2 phase, leading to embrittlement of the B2 grain boundary. The precipitation of many fine granular O or α2 phases at the B2 grain boundaries can inhibit the precipitation of the thin-shell O phase and hinder the propagation of cracks along the grain boundaries. Therefore, the microstructure of this alloy should be controlled to precipitate many fine granular O or α2 phases to occupy the B2 grain boundaries and inhibit its elevated-temperature embrittlement.

Effect of heat treatment on microstructure and mechanical properties of Al1.2CoCrFeNi2.1 high-entropy alloy fabricated by powder plasma arc additive manufacturing

Powder plasma arc additive manufacturing (PPA-AM) is a promising method for fabricating high-entropy alloys (HEAs). However, the characteristics of thermal cycling during the PPA-AM process and the mechanism of subsequent heat treatment on the microstructure and mechanical properties are unclear. In this study, an Al1.2CoCrFeNi2.1 HEA with dual-phase (FCC + B2) was fabricated via PPA-AM. The microstructure and mechanical properties of the as-built and heat-treated samples were explored. The crystallographic orientations of the FCC and B2 phases were also determined. Because the alloy in the bottom region was affected by thermal cycling, B2 particles precipitated in the FCC phase, and FCC particles precipitated in the B2 phase. During the tensile test, the phase boundary between B2 matrix and FCC particles produced cracks preferentially. These cracks reduced the strength and elongation of the alloy. After heat treatment at 1200 °C, the FCC and B2 precipitates formed globular shapes inside the B2 and FCC grain matrix, respectively. The alloy showed a uniform structure and the ductility of the alloy was improved significantly. This work elucidates the mechanisms of thermal cycling and heat treatment on the microstructure and mechanical properties of the alloy.

Low-temperature densification of high entropy diboride based composites with fine grains and excellent mechanical properties

High entropy diborides require high temperature (>2000 °C) for densification, leading to grain coarsening and deteriorated mechanical properties. Herein, fully dense and fine-grained (Hf, Zr, Ta, Nb, Ti)B2 ceramics were successfully fabricated by spark plasma sintering at 1500 °C using metallic cobalt as additives. The result showed that the relative density of (Hf, Zr, Ta, Nb, Ti)B2 increased tremendously from 49.3% to 99.5% as 5 vol% cobalt (HEB5C-15) were introduced. The Co–B based liquid phase was reactively formed during sintering, accelerating the densification by wetting grain boundaries and resulting the presence of Co segregation. Due to the significantly reduced sintering temperature, sluggish diffusion characteristic of the high entropy matrix and the segregation drag effect, the resulting (Hf, Zr, Ta, Nb, Ti)B2 grain size was as small as 0.5 μm. Vickers hardness of HEB5C-15 densified at 1500 °C was 24.90 ± 1.39 GPa, at least 20% higher than the corresponding value in the cobalt free counterpart densified at 2000 °C.

Variation in Tocochromanols Level and Mycotoxins Content in Sweet Maize Cultivars after Inoculation with Fusarium verticillioides and F. proliferatum.

A major problem in maize production is the contamination of the grain with Fusarium spp., mainly F. proliferatum and F. verticillioides and their secondary metabolites—mycotoxins. Under biotic stress conditions, caused by a fungal pathogen, plants initiate a series of defense mechanisms that may cause quantitative and qualitative changes in the composition of phenolic compounds. We analyzed the resistance of four sweet maize cultivars (Syngenta Group: Overland, Sweetstar, GSS 8529, Shinerock) to the infection with Fusarium verticillioides and F. proliferatum isolates, along with fumonisins B1, B2, and B3 grain contamination and the levels of tocopherols and tocotrienols accumulated. Differences in ear rot levels were found between the cultivars and isolates used. The phenotypic evaluation positively correlated with the concentrations of fumonisins. The results obtained also indicate a significant dependence on tocochromanols content in sweet maize cultivars tested on the infection of plants with Fusarium isolates and fumonisin biosynthesis. Further studies are needed to investigate the mechanisms of the plant reaction and the effect of different levels of tocopherols and tocotrienols on Fusarium resistance and grain contamination with mycotoxins.

Structural characterization of a composite joint prepared during laser welding of Ti–22Al–27Nb intermetallic alloy with an interlayer of Cu-Hf-Ni-Ti-Zr high entropy bulk metallic glass

The advent of high entropy alloys (HEAs) has provided scientific solutions to material related problems of grave importance. High-temperature embrittlement (HTE) in the laser welded joint (LWJ) of titanium aluminides is one of the challenges, known to impede the prevalent applications of this novel alloy system. In the current research work, a sandwich-structured composite joint of an intermetallic based alloy (Ti–22Al–27Nb) with a high entropy bulk metallic glass (HE-BMG) composed of Cu, Hf, Ni, Ti and Zr was welded through laser beam. The adulteration of LWJ with HE-BMG resulted into evolution of finite microregions in the fusion zone (FZ) through heterogeneous nucleation. The FZ comprised fine-grained B2 phase with greater fractions of high angle grain boundaries (HAGBs) and lower degree of local area misorientations (LAM). The crystal texture of B2 phase in the FZ abetted the plastic deformation at room as well as high temperature (650 °C), as the Schmid factor “m” for ≥ 99% of {1 1 0} <1 1 1> slip system of B2 remained greater than 0.38. The high-resolution transmission electron microscopy revealed the presence of nano-sized composite regions (NCRs), composed of crystalline/non-crystalline structure. These NCRs remained stable at 650 °C and inhibited the nucleation of O phase along B2 grain boundaries (GBs). The percentage elongation of HE-BMG supplemented LWJ of Ti–22Al–27Nb was improved by 28.19 and 31.34% at room temperature and 650 °C, respectively. The dilution of FZ with HE-BMG contributed a significant increase in tensile ductility at room as well as high temperature.

In Situ Observation of Tensile Deformation of Ti-22Al-25Nb Alloy and Characterization of Deformation in α2 Phase

The room temperature tensile deformation of Ti-22Al-25Nb alloy with an equiaxed α2 phase microstructure and the activated slip system of α2 particles were investigated by a combination of in situ tensile tests and electron backscatter diffraction experiments. The results demonstrate that only a few wide and long slip bands occur in the B2 matrix in the initial stage of yielding. With the tensile displacement increases, a large number of slip bands, including multiple- and cross-slip bands, appear in the B2 matrix and the distance between two adjacent slip bands decreases significantly. Meanwhile, the movement of the slip bands is hindered by the α2 particles and the B2 grain boundaries, and the slip bands appear only in a small number of the α2 particles. From the beginning of the tensile process to the final fracture, there are lots of α2 particles without slip bands. The slip bands penetrate the needle-like lamellar O phase without changing the slip direction. Compared with the α2 particles, the hindering effect of needle-like O phases on the motion of the slip bands is quite small. The microcracks nucleated at the α2/B2 phase boundaries or within the α2 particles, and microcracks propagated along the α2/B2 phase boundaries or across the α2 particles. The fracture surface shows the quasi-cleavage feature, which contains a large number of small and shallow dimples on planar facets. The analysis indicates that the plastic deformation of the alloy is mainly contributed by the B2 phase. For room temperature tensile deformation of α2 phase, there are three types of slip systems that can be activated, including the prism &lt;a&gt; type slip, the basal &lt;a&gt; type slip and the pyramidal &lt;a+c&gt; type slip. The prism &lt;a&gt; type slip is most likely to be activated, followed by the basal &lt;a&gt; type slip and finally the pyramidal &lt;a+c&gt; type slip. In addition, the critical resolved shear stress (CRSS) for the pyramidal &lt;a+c&gt; type slip is the highest among the three types of slip systems. Therefore, the deformation in the α2 phase is mainly contributed by the prism &lt;a&gt; type slip and the basal &lt;a&gt; type slip.

Achieving high ductility of Ti2AlNb sheet without sacrificing the tensile strength without post heat treatment

So far, one main shortcoming restricting the wide application of Ti2AlNb sheet is the poor ductility. To solve this problem, we hot packed rolled the Ti2AlNb compact prepared by spark-plasma-sintering the pre-alloyed powders at the α2+B2+O phase field in this work. The sheets were rolled by different thickness reductions and passes. Results showed that the packs made of 304 stainless steel effectively protected the Ti2AlNb sheet from cracking, and the obtained sheets had good surface formation quality. B2 grain size and the texture intensity increased with increasing the rolling pass. The sheets rolled by all schemes had excellent ductility and relative high strength without applying any post heat treatment. An extreme high elongation of 13.91% with a tensile strength of 1063.27 MPa was obtained after four rolling passes. The nano O phases had excellent ability to coordinate transformations and account for the excellent ductility of the sheet. The α2 phases mainly strengthen the strength of the sheet; maintaining its high strength. This work may provide a new insight to develop commercial high ductility Ti2AlNb alloys.

Precipitation Behavior of O Phase during Continuous Cooling of Ti-22Al-25Nb Alloy

The microstructure evolution and formation mechanism of the O phase in a Ti-22Al-25Nb (at.%) orthorhombic alloy resulting from different cooling rates were investigated. The results show that the morphology of the precipitated O phase is significantly affected by the cooling rate. As the cooling rate decreases, the floccular O, composed of many fine acicular O phases, gradually grows into the lamellar O phase. When the alloy is cooled from the B2 phase region, the grain boundary O (OGB) preferentially nucleates at the triple junctions and grain boundaries and forms the flat and the zig-zag OGB according to different cooling rates. The OGB consists of separated, flat OGB parts and unconnected, zig-zag OGB composed of multiple short, separated, flat OGB at a higher cooling rate. The zig-zag OGB presents a connected state due to the sufficient diffusion time at a lower cooling rate. When the alloy is cooled from the (B2 + α2) phase region, the increase of the phase boundary provides favorable conditions for the nucleation of the O phase due to the presence of α2 particles. The precipitated rim O phase appears on the periphery of the α2 particles at lower cooling rates. The analysis indicates that the Widmanstätten intragranular O (OWI) precipitated directly from the B2 phase maintains the plane relation with the parent B2 phase, and the Widmanstätten grain boundary O (OWGB) holds the specific orientation relationship with one of the two adjacent B2 grains. The OGB keeps the specific orientation relationship with one of the B2 grains as much as possible. When it cannot maintain the specific orientation relationship with one of the B2 grains, the OGB maintains a near-orientation relationship with B2 grains on both sides to reduce the nucleation activation energy. Moreover, there can be more than one nucleation site for the O phase on a single B2 grain boundary to form the OGB. The rim O phase formed through a decomposition reaction of α2→α2 (Nb-lean) + O (Nb-rich) is controlled by a diffusional mechanism and maintains a specific orientation relationship, i.e., {001} O//{0001} α2 and &lt;110&gt;O//&lt;112¯0&gt; α2, with the parent α2 particles.

Phase decomposition and precipitation of metastable A2 phase in B2 ordered Co–Al–Fe alloys

Abstract Phase decomposition in rapidly solidified Co–Al – Fe B2 alloys during isothermal ageing at 923 K has been investigated by means of X-ray diffraction and transmission electron microscopy. At the early stage of ageing, A2 disordered particles precipitate inside B2 matrix grains, being aligned along the &lt;100&gt; direction of the matrix, and equilibrium A1 phase (α-Co) is also formed mostly on grain boundaries. With further ageing, the plate-like A2 particles increase in size. On the other hand, the A1 phase on grain boundaries increases in size and also spreads into the B2 grain like tree branches as so-called Widmanstätten precipitate. Finally, the A2 phase disappears. The A1 phase remains on grain boundaries and also inside the B2 grains as spherical particles. Using the composition gradient method, the metastable precipitation limit of the A2 phase, that is the metastable phase boundary B2/(A2 + B2), was experimentally determined at 923 K. It extends smoothly into the stable A2 + B2 two-phase field on the Co–Fe side of the ternary Co–Al –Fe system.

Significant enhancement in tensile strength and work hardening rate in CoCrFeMnNi by adding TiAl particles via selective laser melting

In this study, 4 at.% Ti–48Al–2Cr–2Nb (TiAl) powder particles were added into CoCrFeMnNi high entropy alloy (HEA) and processed by selective laser melting (SLM) followed by hot isostatic pressing and ageing with the aim of enhancing its strengths. It was found that the as-developed HEA-TiAl sample shows a complex microstructure which is composed of near-equiaxed γ grains embedded with long-range ordered (LRO) L12 domains, various intragranular precipitates including Al2O3, B2, Heusler particles and γ-TiAl particles and grain boundary precipitates such as σ. With such a unique microstructure, the samples show remarkably enhanced 0.2% yield strength and significantly improved ultimate tensile strength and strain hardening rates. Precipitates such as Al2O3, Heusler and σ were found to have acted as effective dislocation motion obstacles. Dislocations tended to cut through the LRO L12 domains by forming stacking faults with a number of them intersecting to form immobile Lomer-Cottrell locks which are beneficial for strain hardening.

Hierarchical phase evolution in a lamellar Al0.7CoCrFeNi high entropy alloy involving competing metastable and stable phases

Guided by solution thermodynamic modeling coupled with detailed experimental characterization, the present study establishes that the alternating FCC and BCC lamellar microstructure in the Al0.7CoCrFeNi high entropy alloy, is a result of non-equilibrium partitionless solidification from the liquid to single B2 phase, followed by solid-state decomposition. Widmanstätten FCC lamellae form from the allotriomorphic FCC precipitates at the B2 grain boundaries, leading to a lamellar microstructure, divided into two distinct sub-systems. Isothermal annealing further drives these individual sub-systems towards equilibrium via precipitation of ordered intermetallic phases. The transformation in FCC lamellae initiates by the formation of metastable L12 precipitates at shorter annealing times, which are eventually replaced by the equilibrium BCC and B2 phases, forming composite B2+BCC laths, on long term annealing. These results further exemplify that interesting transformation pathways lead to hierarchical microstructures within HEAs, and the fact that as processed conditions in these alloys are often far-from equilibrium.