B2 Matrix Research Articles

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.

High-strength and energetic Al2Ti6Zr2Nb3Ta3 high entropy alloy containing a cuboidal BCC/B2 coherent microstructure

The present work developed a novel high-strength and energetic Al 2 Ti 6 Zr 2 Nb 3 Ta 3 high entropy alloy (HEA) containing a BCC/B2 coherent microstructure. The microstructural evolution of BCC/B2 with both the aging temperature (873 ~ 1073 K) and aging time (up to 200 h) was investigated, from which it was found that the cuboidal BCC nanoparticles are coherently-precipitated into the B2 matrix in 873 K-aged state due to a moderate lattice misfit ( ε ~ - 0.76%). The increase of aging temperature could destroy the BCC/B2 coherency and accelerate the formation of brittle Zr 5 Al 3 phase. Especially, the BCC/B2 microstructure exhibits an excellent thermal stability, as evidenced by the fact that these cuboidal BCC nanoprecipitates were not coarsened obviously at 873 K with the particle size from r ~ 10 nm in 24 h-aged state to r ~ 20 nm in 200 h-aged state. This alloy possesses prominent mechanical properties with high yield strength at both room temperature ( σ YS = 1193 MPa) and elevated temperatures (1060 MPa at 873 K and 106 MPa at 1273 K), where the high strength was discussed in light of the precipitation strengthening mechanism. Importantly, the current alloy has an ultrahigh combustion calorific capacity with a value of 10240 J·g -1 , showing a great potential to be used as novel high-performance energetic structural materials. • A high-strength and energetic HEA was developed with the cluster formula approach. • High-strength of alloy was realized by forming a BCC/B2 coherent microstructure. • The BCC/B2 coherent microstructure exhibits a higher thermal stability at 873 K. • The alloy has an ultrahigh combustion calorific value of 10240 J·g -1 .

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.

Phase stability of a light-weight AlNb2TiV refractory high-entropy alloy at high temperature

• The phase stability of a new low-weight AlNb 2 TiV RHEA was firstly investigated. • A15-Nb 3 Al and σ -Nb 2 Al precipitates formed from B2 matrix when annealing at 1100 °C. • Rod-like Nb 3 Al phase original from GBs grow along parallel orientation. AlNb 2 TiV, a newly proposed low-weight refractory high-entropy alloy (LW-RHEA), shows good mechanical properties at high temperatures. However, the thermodynamic phase stability is still not clear. Here, we evaluated the phase stability of AlNb 2 TiV at the temperature of 700–1100 °C. The LW-RHEA is decomposed into B2, A15-Nb 3 Al and σ -Nb 2 Al phases after annealing at 1100 °C. Rod-like A15-Nb 3 Al precipitates from GBs grow along parallel orientation. Chain-like σ -Nb 2 Al particles precipitate along GBs.

Effect of Hf on the microstructure and martensitic transformation behavior in Ti–Pd–Hf alloy

We investigated the microstructure and the crystallography of martensite variants in Ti–Pd–Hf alloy using electron microscopy. The martensitic transformation temperature decreased with increasing amount of Hf substituted into the Ti50−xPd50Hfx alloy. No indication of a martensitic transformation was observed in the as-quenched Ti25Pd50Hf25 and Ti10Pd50Hf40 alloys, even when the samples were cooled to 150 K. The Ti40Pd50Hf10, Ti35Pd50Hf15, and Ti30Pd50Hf20 alloys were composed of a B19 orthorhombic phase having plate-like habit-plane variants consisting of both major and minor correspondence variants several hundred nanometers in width and several tens of micrometers in length at room temperature. Lattice-invariant shear of these alloys was a {111}B19 Type I twin. A solution-treated sample of the Ti25Pd50Hf25 alloy consisted of a B2 cubic matrix with local atomic displacement and lattice distortions, and rounded H-phase particles several tens of nanometers in diameter were precipitated inside the B2 matrix upon aging of the specimen at 823 K for 10.8 ks. The as-quenched Ti10Pd50Hf40 alloy contained rounded H-phase particles several tens of nanometers in diameter embedded in a B2 matrix at room temperature. The martensitic transformation temperature in the Ti50−xPd50Hfx alloy was drastically decreased by both the fully coherent nanosized H-phase particles and the short-range order originating from clustering of solute atoms.

High-throughput experimental study on the microstructural and compositional variations of mechanical properties for AlCoCrFeNi high entropy alloys

To efficiently establish the relationship among microstructure, composition and mechanical properties of the AlCoCrFeNi high entropy alloys, a high-throughput experimental method has been developed combining diffusion couple with the nanoindentation technique. In this paper, B2(A2) denotes that A2 (disordered bcc) particles are coherently dispersed in the B2 (ordered bcc) matrix, and vice versa for A2(B2). A B2(A2)-A2(B2)-fcc sandwich microstructure was produced by annealing an Al1.1CoCrFeNi/CoCrFeNi diffusion couple. The following nanoindentation measurements found that the hardness of the A2(B2) structure is slightly lower than that of B2(A2), but the elastic modulus and ductility behave differently. The fcc region, on the other hand, has a larger ductility than both B2(A2) and A2(B2). In addition, the elastic modulus of fcc decreases with the increasing Al content, whereas the hardness does not significantly change with composition.

On the bcc/B2 interface structure in a refractory high entropy alloy

A refined microstructure consisting of bcc precipitates embedded in an ordered B2 matrix has been observed in the refractory high entropy alloy (RHEA) AlMo0.5NbTa0.5TiZr, resembling an “inverted superalloy-like” microstructure. A planar {100} bcc/B2 interface exists in this state. Coarsening of the microstructure occurs after aging at 1000 °C for 6 h, resulting in a faceted interface. In this case, the primary plane of the interface is parallel to a {100} plane, while the step associated with the facet is parallel to a {110} plane. Misfit dislocations at the interface were observed on the {110} interface plane, with Burgers vector of a/2〈111〉bcc, which extend into the bcc phase, and have an average spacing of 13.7 ± 0.6 nm. No lattice invariant deformation (LID) normal to the primary {110} planes was observed, such that the transformation between the B2 and bcc phases constitutes a pure expansion.

Effects of heat treatment on microstructure evolution and mechanical properties of Ti–22Al–24Nb-0.5Mo alloy

In this study, microstructure evolution, tensile and fracture behaviors of Ti–22Al–24Nb-0.5Mo alloy processed under different heat treatment processing routes were investigated systematically. The results revealed the existence of bimodal-size microstructures of the primary O lamellae and the secondary acicular O phase. The strength and ductility of alloy specimens were optimized by accommodating the volume fractions and morphologies of the precipitates. With the increase of the solution temperature, more acicular O phase precipitated out, which led to the increase of the yield strength and ultimate tensile strength. It was observed that the acicular O phase led to the increase of the strength at elevated temperature without sacrificing plasticity. Owing to the improvement of microscopic strain homogeneity under coupled actions of stress and high temperature, the tensile fracture mode changed from quasi-cleavage to ductile transgranular creep fracture. The fracture toughness of this alloy was determined by the B2 phase and the acicular O phase. It was evaluated that the fracture toughness reduced with a higher volume fraction of the acicular O phase, which in turn affected the contiguity of the B2 matrix. Considering this, balanced strength and toughness were achieved by tailoring the relative contents of the B2 and acicular O phase.

Effect of energy density on the superelastic property of Ni-rich NiTi alloy fabricated by laser powder bed fusion

Laser powder bed fusion (LPBF) has proved to be a promising process for tuning the microstructure and mechanical response in NiTi-shaped memory alloys using a varied energy density input by modifying the process parameters such as laser power or scanning speed. The microstructure, precipitation, chemical composition, and their relationship with austenite–martensite transformation temperatures (TTs) and superelastic properties have been extensively investigated in Ni-rich NiTi alloys processed at varying volume energy densities (52–110 J/mm3) through LPBF. A specific amount of Ni3Ti precipitation was observed and primarily located along the boundary and at the center of the melt pool, and nanoscale spherical NiTi2/Ni2Ti4OX precipitation was homogeneously dispersed in the B2 matrix and B19’ martensite, but no Ni4Ti3 was observed as frequently reported in the literature. Decreasing the volume energy density (VED) led to lower TTs and a higher recovery strain ratio. However, there is an exception for the sample built with the highest laser power of 250 W that possesses the largest Ni/Ti atomic ratio and therefore the highest superelastic property despite the mediated VED (∼87 J/mm3) employed. We concluded that the variation in TTs and superelastic properties among the samples is caused by the varying amounts of Ni, which causes competition among alloying elements in evaporation, condensation, and precipitation during LPBF.

Microstructure evolution and mechanical properties of a high-strength Ti2AlNb alloy processed by large-strain two-pass hot extrusion

The microstructure evolution and tensile behavior of Ti2AlNb alloy (Ti-22Al-23Nb-1Mo-1Zr) processed by two-pass hot extrusion and two-step heat treatment were investigated. After extruding deformation in the B2 phase field, the B2 grains were refined from about 1500–50 µm through dynamic recrystallization. The microstructures subjected to solution treatment in the B2 + α2 phase field and aging treatment in the B2 + O phase field include B2 matrix, lamellar O, rim O and lamellarα2/O phases. The microstructure and tensile properties are sensitive to aging temperature. As the increase of the aging temperature, the content of the B2 phase and the size of the lamellar O phase increase, resulting in low strength and high ductility. The alloy aged at 800 °C has a good match of strength and ductility. It has an elongation of 6.3% with a tensile strength as high as 1225 MPa due to the precipitation strengthening of numerous lamellar O phase and fine grain strengthening. The tensile fracture mode predominantly shows quasi-cleavage characteristics with typical cleavage facets and massive dimples, which is consistent with the good balance of tensile strength and ductility. An in-depth understanding of the relationship between microstructure and tensile properties in this study can be helpful for the engineering application of the Ti2AlNb alloy.

Investigation of acoustic softening and microstructure evolution characteristics of Ti3Al intermetallics undergoing ultrasonic vibration-assisted tension

Ultrasonic vibration-assisted deformation (UVAD) technology has been widely utilized in the metallic materials forming and manufacturing process owing to its advantages of effectively reducing deformation stress. However, the intrinsic mechanism of UVAD involved in Ti3Al intermetallics has not yet been clarified. In this research, an ultrasonic vibration tension device for Ti3Al was designed. The mechanical properties under ultrasonic vibration-assisted tension (UVAT) were investigated. Subsequently, the rheological behavior under the effect of ultrasound was analyzed. The influences of the vibration amplitudes and vibration modes on the microhardness and microstructure were explored. The results indicated that Ti3Al behaved obviously an acoustic softening after introducing ultrasonic energy, and the degree of softening was positively correlated with amplitude. There was a “competitive” relationship between the acoustic softening and acoustic residual hardening effects. The maximum stress and microhardness reductions were 206.13 MPa and 6 % when the amplitude was 6.55 μm and 1.31 μm, respectively. The fracture mode evolved from ductility dimple fracture to a mixed-mode of cleavage pattern and brittle fracture. The metallographic organization revealed a “defective phase” structure in the α2 phase and B2 matrix. The amounts of defective phases enlarged with amplitude increasing and extended away from the vicinities of the fracture cross-section.

Microstructure and mechanical property of novel nanoparticles strengthened AlCrCuFeNi dual-phase high entropy alloy

High entropy alloys are considered to be the best candidates for structural materials because of outstanding comprehensive properties resulted from their unique microstructure. Clarifying the microstructural evolution and controlling the content and morphology of the second phase are of great significance in the design of high-performance HEAs. Along this line of thinking, we report a research of adding little Mo(0–0.5 at%) to a novel Co-free AlCrCuFeNi alloy to tailor microstructure by introducing large lattice distortion and precipitation strengthening to achieve excellent comprehensive properties. The AlCrCuFeNi alloy exhibits BCC+FCC dual-phases, typical solid solution structures (B2 matrix, A2 flap-like and Cu-rich structure) and moderate mechanical properties. An ultrahigh hardness of 643 HV was achieved at the composition of Mo reaches 0.5 at%, which is approximately 1.5 times higher than AlCrCuFeNi alloy. A large amount of nano-sized σ precipitates with various sizes and morphologies were formed when the content of Mo was more than 0.3 at%. Most importantly, the Mo0.3 alloy exhibits an increase of ~18% in yield strength and an increase of ~30% in fracture strength accompanied by a reasonable plasticity which possesses potential application prospects in industrial areas. The solid solution strengthening and the formation of σ phase are the main factors contributing to the strengthening.

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 <a> type slip, the basal <a> type slip and the pyramidal <a+c> type slip. The prism <a> type slip is most likely to be activated, followed by the basal <a> type slip and finally the pyramidal <a+c> type slip. In addition, the critical resolved shear stress (CRSS) for the pyramidal <a+c> 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 <a> type slip and the basal <a> type slip.

Highly complex magnetic behavior resulting from hierarchical phase separation in AlCo(Cr)FeNi high-entropy alloys

SummaryMagnetic high-entropy alloys (HEAs) are a new category of high-performance magnetic materials, with multicomponent concentrated compositions and complex multi-phase structures. Although there have been numerous reports of their interesting magnetic properties, there is very limited understanding about the interplay between their hierarchical multi-phase structures and the resulting magnetic behavior. We reveal for the first time the influence of a hierarchically decomposed B2 + A2 structure in an AlCo0.5Cr0.5FeNi HEA on the formation of magnetic vortex states within individual A2 (disordered BCC) precipitates, which are distributed in an ordered B2 matrix that is weakly ferromagnetic. Non-magnetic or weakly ferromagnetic B2 precipitates in large magnetic domains of the A2 phase, and strongly magnetic Fe-Co-rich interphase A2 regions, are also observed. These results provide important insight into the origin of coercivity in this HEA, which can be attributed to a complex magnetization process that includes the successive reversal of magnetic vortices.

Phase transformation behavior of Ti–40Al–8Nb alloys with a submicron (ω0+γ) microstructure during tempering at 1000 °C

In this work, the phase transformation behavior of Ti–40Al–8Nb (at. %) alloys with a submicron (ω0+γ) microstructure during tempering at 1000 °C was investigated, and special attention was paid on the ω0-assisted α2 nucleation. Results indicated that the ω0+γ→B2→α2 transformation dominated the precipitation of α2 phase. The ω0 and γ phases first dissolved into the B2 phase within a short time, and then the B2→α2 transformation occurred. The α2 phase could nucleate in the interior of B2 matrix and precipitate at the B2/γ interfaces by following the Burgers orientation relationships (ORs), <11–20>α2//<111>B2 and (0001)α2//{110}B2. With the increase of isothermal holding time, the γ phase reprecipitated through two phase transition routes, including B2→γ and α2→γ transformations by following the Kurdjumov-Sachs ORs, <111>B2//<110>γ and {110}B2//{111}γ, and Blackburn ORs, i.e., (0001)α2//{111}γ and <11–20>α2//<011>γ, respectively. Finally, a (α2+γ+B2) microstructure was obtained after isothermal holding at 1000 °C for 300 min.

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.

On the yield stress anomaly in a B2-ordered refractory AlNbTiVZr0.25 high-entropy alloy

High-entropy intermetallics have a great potential for high-temperature applications. However, there is relatively limited information about the plastic flow behaviour and deformation mechanisms of these materials. Here we reported a yield strength anomaly (YSA) in a refractory AlNbTiVZr0.25 high-entropy alloy with a multicomponent B2 matrix phase. The YSA was detected during the compression tests at 22–900 °C with a stress peak at 700 °C. Detailed analysis suggested that the YSA could be connected with the glide of dissociated a 〈111〉 superdislocations.

High temperature phase stability of the compositionally complex alloy AlMo0.5NbTa0.5TiZr

Recently, a class of refractory high-entropy alloys has been developed with a refined microstructure consisting of an ordered B2 matrix with cuboidal BCC precipitates resembling an “inverted superalloy-like” microstructure. In this paper, we have studied the evolution of the duplex microstructure of AlMo0.5NbTa0.5TiZr during aging at elevated temperatures, particularly with regard to the nature of the B2 and BCC phases. Samples were aged at 1000 °C for 6 h, followed by a water quench. It was found that while the relative volume fractions of the B2 and BCC phases remained nearly constant after ageing, compositional changes of both the B2 and BCC phases were determined. In the aged condition, there is evidence from high-resolution STEM HAADF imaging that suggests that the B2 phase in the aged condition may undergo a spinodal reaction, with the sub-lattice occupancies differing within the ordered precipitates. A change in size and shape of the BCC precipitates was also noted, and this was accompanied by a difference in the nature of the B2/BCC interfaces. Thus, a step-like B2/BCC interface is evidenced in the aged condition, in contrast with the planar {100} interfaces in the “superalloy-like” microstructure, likely adopted to accommodate a change in coherency resulting from an increase in misfit between the two phases from compositional changes occurring during the coarsening of the BCC precipitates.

Hot pack rolling of spark-plasma-sintered pre-alloyed powders to fabricate Ti2AlNb sheets

In this work, spark-plasma-sintered Ti-20.3Al-24.7Nb (at.-%) compact was pack rolled in different phase fields to fabricate Ti2AlNb sheets with high strength and ductility without post-heat treatment. The nano O phases in the B2 matrix contributed to the high tensile strength and ductility in a single B2 phase field. In the O + B2 phase field, the O phases remarkably increased in size and resulted in high tensile strength of 1407.1 MPa. Maintaining the O phases to nano sizes and cooperation with micro-sized α 2 phases resulted in extremely high elongation of 13.91% in the α 2+O + B2 phase field. The , , , and strengthening effects of the α 2 and O phases under different conditions were analysed in detail. Highlights Spark-plasma-sintered pre-alloyed powders are used as the raw compact before hot pack rolled Ti2AlNb sheet. Ti2AlNb sheets with good mechanical properties are obtained without any post-heat treatment. This ‘SPS pre-alloyed powder + hot pack rolling’ method has great potential in breaking the limit in sheet properties. The strengthening effects of different factors are analysed.