B2 Particles Research Articles

Influence of nickel on microstructure and mechanical properties of medium-manganese steels

ABSTRACTTo combine the advantage of density reduction and excellent performance, nanometer-sized B2 particles were introduced into the δ ferrite matrix of high-aluminium low-density steel by the addition of nickel (Fe–0.2C–11Mn–6Al–4/8Ni). Compared to Fe–0.2C–11Mn–6Al (0Ni) steel, the hardness and tensile strength of 4Ni and 8Ni steels are significantly improved. The improvement of tensile strength in 4Ni and 8Ni steels was primarily contributed by the precipitation strengthening or solution strengthening of B2 particles in δ ferrite. At the higher annealing temperature, the original dislocation density in δ ferrite is lower. However, dislocation multiplication during tensile deformation was more significant in the sample annealed at higher temperature, which was responsible for a higher work hardening rate.

Mechanical, deformation and fracture behaviors of bulk metallic glass composites reinforced by spherical B2 particles

Ti45Cu40Ni7Zr5Sn2.5Si0.5 alloys were prepared under various cooling rate conditions during solidification. The alloys exhibited different volume fractions of B2 particles with 0 ~ 40 vol% in an amorphous matrix. Monolithic bulk metallic glass of 1 mm diameter showed no macroscopic plasticity and exhibited the typical vein patterns in a maximum shear stress plane on the fracture surface. However, a bulk metallic glass composite containing the B2 particles revealed obvious plasticity (~ 5.6%) with a remarkable work-hardening behavior that resulted from a stress-induced martensitic transformation of the B2 particles. Moreover, the composite displayed the complicated fracture morphologies containing of three types of fracture features. Through detailed investigations on the microstructural evolution, mechanical, deformation and fracture characteristics, the influence of B2 particle on overall behavior of the TiCu-based bulk metallic glass composites was elucidated.

Microstructure characterization of Cu-rich B2 intermetallic nanoprecipitates in an austenite-based High specific strength steel

Effects of Cu addition on microstructure and tensile property of a hot rolled austenite-based high specific strength (i.e. yield strength-to-mass density ratio) steel (HSSS) are investigated. The addition of Cu promotes the precipitation of intermetallic compounds (B2), resulting in higher volume fraction of nanoscale B2 particles in the steel. Addition of 2.8wt% Cu improves both the yield strength (by 53MPa) and tensile strength (by 138MPa) of the steel, and maintains high ductility. The yield strength enhancement is predominantly attributed to nanoscale B2 intermetallic precipitation. A high number density of ultrafine (2-10nm) B2 and Cu uniformly precipitates in the austenite martrixwith three different types of ultrafine nanoscale particles.

Microstructure and enhanced mechanical behavior of the Al7Co24Cr21Fe24Ni24 high-entropy alloy system by tuning the Cr content

The single-phase face-centered-cubic Al7Co24Cr21Fe24Ni24 (in atomic percent, at%) high entropy alloy lacks sufficient hardness and strength at room temperature, which restricts its structural application in the future. To strengthen this material, the (Al7Co24Cr21Fe24Ni24)100-xCrx (x = 0, 11, 20, and 26) high entropy alloys were prepared and investigated. We found that with increasing the Cr content, the volume fraction of the BCC phase enriched with Al, Fe, and Cr elements increased gradually. In the BCC-contained HEAs, ordered B2 particles were always dispersed in the BCC solid solution, which resulted in a dramatic enhancement of comprehensive mechanical properties. Nanoindentation measurements showed that both the nanohardness and the elastic modulus values of the BCC phase are much greater than that of the FCC phase. The room-temperature compression test showed that the yield strength significantly increased from 249 MPa to 1649 MPa with the slight sacrifice of the ductility, along with increasing the Cr content. More importantly, the (Al7Co24Cr21Fe24Ni24)74Cr26 alloy exhibits outstanding compression behavior, with the high yield strength (1649 MPa) and fracture strength (2830 MPa) as well as good compressive plastic strain of 24.9%. Furthermore, to facilitate the optimization of the comprehensive properties of alloys, the close correlation between the microstructures and mechanical properties was also discussed.

Mechanical properties of a new high entropy alloy with a duplex ultra-fine grained structure

A new approach to increase the tensile performance of high entropy alloys (HEAs) by producing a duplex ultrafine-grained (UFG) structure was reported in this work. A novel HEA based on the CoCrFeNiMn system with substantial amounts of Al and C was used for the illustration of this approach. In the as-cast condition the alloy had almost entirely a single face-centered cubic (fcc) phase structure with an insignificant amount of M23C6 carbides. After cold rolling and annealing at 800–1000 °C an increased amount of fine second phases, namely M23C6 carbides and B2 phase, effectively pinned boundaries of recrystallized fcc grains. As a result, a duplex UFG structure composed of the recrystallized fcc grains and M23C6 and B2 particles was produced. The alloy with the UFG structure demonstrated attractive mechanical properties. For example, after annealing at 900 °C the alloy had the yield strength of 785 MPa, the ultimate tensile strength of 985 MPa, and elongation to fracture of 32%. The phase composition of the alloy in different conditions was compared with the equilibrium phase diagram obtained using a Thermo-Calc software. Strengthening mechanisms were qualitatively analyzed, and some possibilities for further improvement of strength of the alloy were discussed.

Nucleation and growth crystallography of Al8Mn5 on B2-Al(Mn,Fe) in AZ91 magnesium alloys

Manganese is widely added to Mg-Al-based alloys to form Al-(Mn,Fe) intermetallics that combat impurity Fe. Here EBSD, deep etching and FIB-tomography are combined to study the nucleation and growth crystallography of Al-Mn-Fe particles in Mg-9Al-0.7Zn-0.15Mn-0.002Fe (wt%) solidified at 1 K⋅s−1. It is shown that Al8Mn5 nucleates on B2-Al(Mn,Fe) particles and an incomplete peritectic transformation results in a Fe-richer B2-Al(Mn,Fe) core enveloped by a low-Fe Al8Mn5 shell. A reproducible orientation relationship (OR) is measured, and the close lattice match and OR are discussed in terms of the group-subgroup relationship between these phases. It is found that most rhombohedral Al8Mn5 particles are twinned, consisting of four orientations related by ∼90° rotations around three common <11¯02>, which is discussed in terms of the pseudo-cubic <100> axes of the Al8Mn5 rhombohedral gamma brass. Al8Mn5 grew as equiaxed particles that are explained by polyhedron models based on {100}, {110} and {112} facets of the pseudo-cubic Al8Mn5 cell. The results indicate that, at low Fe:Mn ratio, most impurity Fe is dissolved in B2 particles that are encapsulated by low-Fe Al8Mn5 which may be important for corrosion resistance.

Controlled formation of coherent cuboidal nanoprecipitates in body-centered cubic high-entropy alloys based on Al2(Ni,Co,Fe,Cr)14 compositions

Microstructures and mechanical properties of Al-Ni-Co-Fe-Cr high-entropy alloys (HEAs) were investigated by systematically varying transition metals instead of Al, within the chemical formula of Al2M14 (M represents different mutations of transition metals). The formation of different crystal structures (FCC, BCC, or FCC+BCC mixture) and its effects on the resulting mechanical properties of this series of HEAs, both in tension and compression, were evaluated. It was found that, in the BCC-dominated HEAs, ordered B2 precipitates were always coherently dispersed in the BCC solid-solution matrix. The shape of these B2 precipitates was strongly affected by the lattice misfit between the disordered BCC and ordered B2. A uniform distribution of cuboidal B2 particles could be obtained by properly adjusting M, thus the lattice misfit, in a manner similar to that in Ni-based superalloys. Strengthening effects caused by different BCC/B2 morphologies were also estimated and compared with experimental measurements. The optimal strengthening as a function of the shape and size of the coherent precipitates was discussed in light of the lattice misfit in these HEAs.

Strain Rate Effect on Tensile Behavior for a High Specific Strength Steel: From Quasi-Static to Intermediate Strain Rates

The strain rate effect on the tensile behaviors of a high specific strength steel (HSSS) with dual-phase microstructure has been investigated. The yield strength, the ultimate strength and the tensile toughness were all observed to increase with increasing strain rates at the range of 0.0006 to 56/s, rendering this HSSS as an excellent candidate for an energy absorber in the automobile industry, since vehicle crushing often happens at intermediate strain rates. Back stress hardening has been found to play an important role for this HSSS due to load transfer and strain partitioning between two phases, and a higher strain rate could cause even higher strain partitioning in the softer austenite grains, delaying the deformation instability. Deformation twins are observed in the austenite grains at all strain rates to facilitate the uniform tensile deformation. The B2 phase (FeAl intermetallic compound) is less deformable at higher strain rates, resulting in easier brittle fracture in B2 particles, smaller dimple size and a higher density of phase interfaces in final fracture surfaces. Thus, more energy need be consumed during the final fracture for the experiments conducted at higher strain rates, resulting in better tensile toughness.

Influence of spherical particles and interfacial stress distribution on viscous flow behavior of Ti‐Cu‐Ni‐Zr‐Sn bulk metallic glass composites

Bulk metallic glass composites containing micro-scale B2 particles are subject to investigation with regards to the influence of B2 particles and interfacial stress and strain distribution on the viscous flow behavior at a super-cooled liquid state. An increased volume fraction of B2 particles leads to an increase of minimum viscosity and influences viscous flow behavior before crystallization. In high temperature deformation, the bulk metallic glass shows homogeneous deformation feature. However, the heterogeneous deformation feature is found in thermoplastically deformed bulk metallic glass composite. The strong stain accumulation and sluggish viscous flow occur around B2 particles, which are caused by the heterogeneous stress distribution linked to stress concentration and shear stress impediment around B2 particles. The sluggish viscous flow of super-cooled liquids around B2 particles during high temperature deformation induces an increase of viscosity and strongly affects the viscous flow behavior of Ti-based bulk metallic glass composites containing micro-scale spherical B2 particles.

Microstructures and mechanical properties of mechanically alloyed and spark plasma sintered Al0.3CoCrFeMnNi high entropy alloy

The present study focuses on phase evolution in Al0.3CoCrFeMnNi high entropy alloys (HEAs) during mechanical alloying and after spark plasma sintering. Aluminium addition hardens and induces ordered precipitates in a soft fcc alloy based on CoCrFeMnNi. Mechanical alloying of the alloy powders resulted in a single fcc phase. However, ordered B2 precipitates and chromium carbide precipitates were observed after spark plasma sintering. Sintering temperature optimization was done and maximum densification and hardness were obtained at 900 °C. High compressive yield strength of 979 ± 20 MPa and compressive ductility of 39 ± 3% were observed for the SPS processed alloy. Significant contributions from grain boundary strengthening coupled with dispersion strengthening via carbides and B2 particles appear to be major contributors to alloy strengthening. These hard intermetallic particles not only keep the grain growth in check but also increase the cumulative (fcc + B2) strength of the material.

Direct versus indirect particle strengthening in a strong, ductile FeNiMnAlTi high entropy alloy

In this paper, we show that the strengthening from (Ni, Al, Ti)-rich B2 particles in the recrystallized (Fe, Mn)-rich f.c.c. high entropy alloy (HEA) Fe42Ni12Mn36Al8Ti2 arises indirectly by reducing the grain size thus providing increased Hall-Petch strengthening, rather than directly through dislocation pinning, a concept first introduced by P.M. Hazzledine (Scripta Metall. Mater. 26 (1992) 57–58). Thus, the yield strength increased from 200MPa for the as-cast HEA to 509MPa for the recrystallized HEA. The contributions to the increase of the yield strength were calculated to be 28MPa from particle pinning and 281MPa from Hall-Petch strengthening. We also show using high angle angular dark field imaging in a scanning transmission electron microscope that Fe42Ni12Mn36Al8Ti2 exhibits no evidence of the severe lattice distortion that is a core tenet of HEAs. Both as-cast and recrystallized HEAs showed ductile fracture modes.

Novel ultra-high-strength Cu-containing medium-Mn duplex lightweight steels

High strength and ductility and vehicle's weight reduction are key issues for improving fuel efficiency in automotive industries. In addition, structural reinforcement components require high yield strength because the prevention or minimization of deformation is more important than the absorption of impact energy. In this study, new ultra-high-strength duplex lightweight Fe-0.5C-12Mn-7Al-(0,3)Cu (wt%) steels have been developed by varying annealing temperature. Here, Cu, an austenite stabilizer, not only raises the austenite volume fraction but also delays the recrystallization due to a solute drag effect, while it promotes the formation of Cu-rich B2 particles and Cu-segregated interfacial layers. The steels show the planar slip and fine dislocation substructures (Taylor lattices) as desirable deformation mechanisms. Non-shearable Cu-rich B2 particles, solid solution hardening of Cu, and delayed recrystallization greatly improve the yield strength (∼1 GPa) and strain hardening. Through these unique and excellent tensile properties together with weight saving of 10.4%, the present work provides a desirable possibility for applications to automotive reinforcement components preferentially requiring an excellent yield-to-tensile ratio.

Effect of Ni alloying on the microstructural evolution and mechanical properties of two duplex light-weight steels during different annealing temperatures: Experiment and phase-field simulation

This paper presents a study of two lightweight steels, Fe-15Mn-10Al-0.8C-5Ni and Fe-15Mn-10Al-0.8C (in wt.%) where strength is dependent upon the microstructure of 2nd phase precipitates. We investigate the effects of annealing temperature from 500 °C to 1050 °C on the precipitation of ordered phases size and morphology through phase-field modelling and experimental studies based on laboratory scale annealing and characterization. The chemical composition of carbides and B2 compounds as a function of isothermal annealing temperature and the matrix within which they formed are elucidated in this study. It is found that nano-sized disk-shaped B2 particles form at higher annealing temperatures (e.g. 900 °C and 1050 °C). The simulation results on carbides demonstrated the effects of energetic competition between interfacial energy and elastic strain energy on the morphological evolution of carbides. In addition to that, different ordering behaviours observed depending on the Ni content into the steel. The results demonstrate processing route designed through the phase-field simulations led to a better combination of strength and ductility. The tensile testing results indicate an increase in the strength and elongation when B2 precipitate morphology changes from micro-size faceted shape to nano-size disk-like particles.

Novel Fe36Mn21Cr18Ni15Al10 high entropy alloy with bcc/B2 dual-phase structure

Abstract A novel Fe 36 Mn 21 Cr 18 Ni 15 Al 10 high entropy alloy composed mostly of inexpensive components was produced by vacuum arc melting. In the as-cast condition the alloy had a dual-phase structure consisted of a bcc matrix and homogeneously distributed in the matrix cuboidal B2 ordered particles. The matrix phase was enriched with Fe and Cr, and B2 particles were composed mostly of Ni and Al. The alloy had high compression yield strength of 940–990 MPa at temperatures up to 400 °C. Further increase in temperature resulted in pronounced softening. The alloy could be compressed to 50% reduction without fracture at room temperature, but had a limited tensile ductility, especially at room temperature – elongation to fracture was only 2.5%. Annealing at 1200 °C did not change the structure of the alloy noticeably, but after annealing at 1000 °C an additional fcc phase appeared. The formation of the fcc phase caused pronounced softening. The comparison of the experimental results on the phase composition of the alloy with CALPHAD predictions showed reasonable agreement. Possibilities to increase stability of dual bcc-B2 structure were discussed.

Crystallization and phase transformation behavior of TiCu-based bulk metallic glass composite with B2 particles

Abstract Crystallization and phase transformation behaviors of Ti 45 Cu 40 Ni 7.5 Zr 4 Sn 2.5 Si 1 bulk metallic glass composite with B2 particles were investigated by isothermal heat treatment with systematic TEM analysis. The crystallization of the amorphous phase occurs in two steps with the formation of nanocrystalline followed by the precipitation of γ-TiCu phase. On the one hand, the metastable B2 phase still maintained until crystallization process is completed. However, when the sample was heat treated longer times, the cubic B2 phase are transformed into the tetragonal γ-TiCu phase with highly lattice distortion.

Control of intermetallic nano-particles through annealing in duplex low density steel

In high Al-low-density steels for future vehicle light weighting, it is vital to design a thermal profile to form and retain the uniformly dispersed nanosize B2-type intermetallic precipitates that are crucial for the material strength. In this paper, the influence of heating rate, during annealing to 1050°C was simulated in a Au-image furnace. The post annealing structure was then characterized and two different morphologies of B2 particles were observed: triangle-like with a few micrometres (≈1.4μm) and disk-like precipitates with a diameter of around a few hundred nanometres. It was found that a slower heating rate (2.5°C/s) led to an increase in the volume fraction and to uniform distribution of particles within the microstructure and considerably affected the shape and size of the precipitates.

High-entropy Al0.3CoCrFeNi alloy fibers with high tensile strength and ductility at ambient and cryogenic temperatures

High-entropy alloys (HEAs) are multi-component systems based on novel alloy composition designs with entropy maximization. They feature an array of unique mechanical properties when compared with traditional alloys. In this study, HEA fibers with diameters ranging from 1 to 3.15 mm in diameter, with the composition of Al0.3CoCrFeNi (atomic percent, at.%), were successfully fabricated by hot-drawing, followed by microstructural characterization using scanning-electron microscopy (SEM) and transmission-electron microscopy (TEM). The compositional variations within and between fibers were determined using energy-dispersive X-ray spectroscopy in TEM along with atomic-probe tomography (APT). These analyses revealed a homogeneous face-centered cubic (FCC) structure in the as-cast material, while post processing (e.g., forging and wire drawing) produced nanosized B2 particles in an FCC matrix. Electron back-scatter diffraction (EBSD) was used to determine the evolution of the texture and grain boundary character after processing of the fibers. The tensile strength and plasticity of the fibers were determined at both 298 K (1207 MPa/7.8%) and 77 K (1600 MPa/17.5%). Detailed TEM analyses revealed that the improvement of mechanical properties at 77 K (i.e. increased strength and ductility) is due to a change in deformation mechanisms from the planar slip of dislocations to nano-twinning. Such properties could be beneficial for cryogenic applications.

Transformation-mediated plasticity in CuZr based metallic glass composites: A quantitative mechanistic understanding

In this paper, we present a thorough stress analysis of the CuZr metallic-glass composite with embedded B2 particles subject to a martensitic transformation. Within the framework of the Eshelby theory, we are able to explain, in a quantitative manner, (1) the formation of three types of shear bands with distinct morphologies as observed experimentally in the severely deformed CuZr metallic-glass composite and (2) the work hardening ability of the CuZr metallic-glass composite as related to the coupled effects of elastic back stress and elastic mismatch caused by the martensitic transformation. Furthermore, we also discuss the issues about the stress affected zone of the individual B2 phase and the stability of the crystalline–amorphous interface. Given the general agreement between the theoretical and experimental findings, we believe that the outcome of our current work can lead to a deeper understanding of the transformation-induced plasticity in the CuZr based metallic glass composites, which should be very useful to the design of the metallic-glass composites with improved ductility.

Gradual martensitic transformation of B2 phase on TiCu-based bulk metallic glass composite during deformation

In this study, we investigate the progress of stress-induced martensitic transformation depending on the stress state on TiCu-based bulk metallic glass composite via systematical transmission electron microscopy analysis. On the stage of elastic deformation (before yielding), the martensitic transformation from austenite B2 phase to martensite B19′ phase occurred from interface between B2 particle and amorphous matrix with ∼50 nm region. Just after yielding, the martensitic transformed region was extended toward B2 particle with distance of ∼130 nm from the interface. Moreover, the deformation twinning of martensite B19′ phase in martensitic transformed region was also observed. When 1% plastic deformation occurred, the martensitic transformed region reached up to ∼600 nm. Based on these results, it is believed that the high level of stress can be concentrated around the interface due to the elastic mismatch of B2 and amorphous phases. As a result, structural gradient morphology originated from phase transition was formed into the B2 particle during early stage of deformation.

Turning machining induced microstructural stability of a high Nb-containing TiAl alloy during high temperature exposure

Turning machining induced microstructural instability was investigated in a fully lamellar Ti–45Al–8.5Nb–(W, B, Y) alloy during high temperature exposure. After turning machining followed by thermal exposure at 900 or 1000 °C for 100, 300 and 500 h, a depth-dependent gradient microstructure with random orientations was produced in the region close to the machining surface. Two typical layers, a fine-grained (FG) layer with equiaxed grains and a coarse-grained (CG) layer with elongated grains, are formed in this region in transversal direction. The thickness of the two layers is up to 120 μm after thermal exposure at 1000 °C for 500 h, which is less than the depth of the hardened region (200 μm) after turning machining. Most of the new grains in FG and CG layers are constituted of γ single phase, while short α2 segments and few B2 particles are precipitated at the γ/γ interface or inside the γ grains. Recrystallization and phase boundary bulging are found to be the major mechanisms responsible for lamellar degradation in FG layer and CG layer, respectively. The residual deformation energy stored is considered to be the main driving force of this process.