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In this work, the exceptional strength-ductility synergy of a dual-phase high entropy alloy (HEA) is achieved by architecting complex microstructures. The HEA rolled at 700 °C shows the yield strength and tensile strength as high as 1580 MPa and 1854 MPa with ductility of ∼18.4%. Different flow stress regions (face-centered cubic (FCC) and body-centered cubic (BCC) phases) divided by complex microstructures lead to strong hetero-deformation-induced strain hardening. The dispersed micro-strain band and the precipitates buffering the dislocation in front of the heterogeneous phase boundary also help to improve the ductility. In addition, utilizing the sensitivity difference between FCC and BCC phases for dislocation accumulation, more dislocations are accumulated in FCC phase to reduce the mechanical incompatibility with BCC phase and fully release the strain hardening ability. The strategy of architecting complex microstructures and selectively modifying phases will be beneficial to the development of high-performance dual-phase alloys. [ABSTRACT FROM AUTHOR]

A high strength maraging stainless steel (MSS) was developed utilizing multiple nanoprecipitates and reverted austenite. After aging treatment, this steel shows remarkable mechanical properties with a yield strength of ∼1750 MPa, a tensile strength of ∼1910 MPa, and a total elongation of ∼10.5 %. The precipitation and strengthening mechanisms of multiple nanoprecipitates were clarified. The precipitation sequence upon aging can be identified as: Ni-rich cluster→ Ni-rich cluster + Mo-rich cluster→ Ni 3 Ti + Mo-rich phase→ Ni 3 Ti + Mo-rich phase + α′-Cr. Ni-rich cluster and Mo-rich cluster is the precursor of Ni 3 Ti and Mo-rich phase, respectively. The Ni-rich clusters, Mo-rich clusters, Ni 3 Ti and α′-Cr, contribute to the precipitation strengthening mainly by shearing mechanisms, among them the ordered strengthening plays the most significant role. Upon aging, Ni 3 Ti is wrapped by Mo-rich phase, restraining their growth. In contrast, the Mo-rich phase can be significantly coarsened, leading to a shift in the precipitation strengthening from dislocation shearing to Orowan looping mechanism. The reverted austenite improves the ductility of the MSS via a transformation induced plasticity (TRIP) effect. [ABSTRACT FROM AUTHOR]

Compared with Ti–Ni-based and Cu–Al-based alloys, Fe–Ni–Co–Al-based superelastic alloys have received great attention in recent years because of their low cost and excellent performance. However, the precipitation of brittle precipitates on grain boundaries in Fe–Ni–Co–Al-based polycrystalline alloys significantly reduces the mechanical properties. In this paper, the effects of V on the formation of brittle precipitates on both grain boundaries and inside grains and mechanical properties in Fe–Ni–Co–Al–Nb alloy were investigated systematically. The results show that γ′(Ni 3 Al) precipitate with L1 2 structure precipitate out on grain boundaries and inside grains in the Fe–Ni–Co–Al–Nb alloy during aging without V addition. The γ′ precipitate with L1 2 structure on the grain boundaries grew to form a heavily-coarsened L1 2 precipitate (HCLP γ′ precipitate) due to the accumulation of Nb on the grain boundaries, resulting in a significant decrease in elongation. With the addition of V, intragranular Nb and V can combine together with γ′ to form Ni 3 (Al, Nb, V) (nano-sized γ′ precipitate) composite precipitates, which improves the strength. More important, since the diffusion coefficient of V was larger than that of Nb, V preferentially occupied the lattice positions on grain boundaries, repelling Nb from the grain boundaries to the matrix. Thus, the coarsening of HCLP γ′ precipitate on the grain boundaries was reduced, and the elongation of Fe–Ni–Co–Al–Nb–V alloy was significantly improved. [ABSTRACT FROM AUTHOR]

While a high density of nano twins can effectively enhance alloys, it also leads to a significant reduction in ductility, similar to other nanostructures. In this work, a deformation strategy combining cold rolling and warm rolling has been employed on a nanotwinned Fe-25Mn-0.5C steel to improve ductility. A high density of nano twins was introduced in the coarse-grained matrix first by cold rolling for high strength. Subsequent warm rolling at 500 °C sheared the large nano twin bundles, resulting in a mixed microstructure consisting of nano twins and more randomly oriented refined grains. Planar slipping of dislocations, instead of twinning, dominates the tensile processes, which brings strong pile-up and continuous work-hardening. Compared with the cold-rolled counterpart with full nano twins, the one with a mixed microstructure exhibits three times higher tensile elongation (21.3 % vs. 5.6 %) while maintaining similar tensile strengths (about 1440 MPa). The evolution of microstructures and the underlying strengthening mechanisms of the mixed microstructure were extensively discussed. Additionally, a cold-rolled and statically annealed sample was also included for comparison. [ABSTRACT FROM AUTHOR]

In this work, the reverse transformation of ε -martensite located inside nanotwin lamellae has been introduced to improve the ductility of a high-strength Fe–Mn–C steel with a high density of nanotwins. The steel was designed to have a low stacking-fault energy (SFE, 17.8 mJ/m2) to introduce high density of nanotwins through pre-deformation. Meanwhile, the fine ε -martensite was also obtained in nanotwin lamellae. The steel was strengthened significantly, with an increase in yield strength from 357 to 1275 MPa by the introduction of nanotwins. A subsequent intermediate annealing was applied to control the reverse phase transformation from ε -martensite to austenite to enhance the deformation ability of the nanotwinned steel (rising tensile elongation from 5 to 16%). The nanotwins with a width of 32 nm on average were stable during the heat treatment. The high strength was retained because the size of the austenite transformed from ε -martensite was restrained by the stable nanotwins. This strategy holds promise and can be widely applied to alloys with low SFE. [ABSTRACT FROM AUTHOR]

Large-size grain is one of the effective parameters to determine the superelasticity of Cu-based alloys. In this study, the effects of Ni addition on abnormal grain growth (AGG) and microstructure evolution of CuAlMnCoNi alloys are systematically investigated. The activation energy and driving force of grain boundary (GB) migration during AGG are evaluated. The results reveal that Ni significantly promotes AGG. Average grain size reaches 6.0 cm after three cyclic heat treatments (CHTs) in a CuAlMnCoNi alloy containing 2 at.% Ni. A maximum superelastic strain of 11.8 % can be obtained at 15 % applied strain. The addition of 2 at.% Ni increases the solvus temperature of the α-phase to about 818 °C and raises α-phase volume fraction at phase equilibrium. The precipitation of α-phase with high volume fraction causes high number density of dislocations. The misorientation angle of subgrain is increased by the addition of Ni, contributing to the AGG in CuAlMnCoNi alloys. The addition of Ni reduces the activation energy of GB migration and increases the GB migration driving force. [ABSTRACT FROM AUTHOR]

In this paper, the evolution of microstructure in Fe–Ni–Co–Al polycrystalline alloys and its effects on mechanical properties were systematically investigated. The results show that the proportion of high-angle grain boundaries (HAGBs) is 98.1 % with 51.2 % of Σ3 GBs during the solid solution (SS) process. The distribution of Σ3 GBs and other HAGBs (non-Σ3 HAGBs) are relatively dispersed, which prevents the formation of a specific orientation. In addition, there is no discernible anisotropy in GB migration during the SS process, ultimately resulting in the absence of texture. In contrast, during the directional recrystallization (DR) process, the directional migration of GBs is driven by the temperature gradient. Random high-angle grain boundaries (RHAGBs) with Σ values exceeding 29 possess higher GB energy and migration rate compared to low-angle grain boundaries (LAGBs) and HAGBs, making them more susceptible to GB migration. In the grain incubation area, the proportion of RHAGBs between grains oriented {111}<214> (grain 2 and grain 5) and their surrounding grains are 90 % and 75 %. These grains have the growth advantage and eventually form the main orientation {111}<214>. While the proportion of RHAGBs between grains oriented {114}<214> (grain 11) and its surrounding grains is only 65 %, characterized by lower GB energy and migration rate. Therefore, these grains eventually form the minor orientation {114}<214>. The coherent L1 2 -Ni 3 Al (γ′) precipitates were observed in both the SS-AG (SS after aging) and DR-AG (DR after aging) treatments. The DR-AG alloy, characterized by {111}<214> and {114}<214> orientations and γ′ precipitates, exhibited a 3.6 % recoverable strain and a 1.2 % superelastic strain, whereas no evident superelastic behavior was found in the SS-AG alloy, which featured a random texture and γ′ precipitates. [ABSTRACT FROM AUTHOR]

This study investigated the effects of aging and thermomechanical treatments on the microstructure evolution and mechanical properties of a nanoprecipitates strengthened ferritic steel. The toughness of steel at various temperatures was measured carefully and correlated with microstructural features. Tensile tests show that aging can improve the mechanical strength without scarifying the ductility. With high yield strength of ~1000 MPa, excellent low temperature Charpy impact energy more than 300 J at −80 °C can be obtained. The ductile brittle transition temperature (DBTT) is lower than −80 °C. The high strength can be contributed by the nanocluster precipitation as determined by small angle neutron scattering and transmission electron microscopy. The excellent low temperature toughness is attributed to the delamination structure of the steel, which blunts the cracks and restrains the crack propagation. [ABSTRACT FROM AUTHOR]

In the present study, a nanoprecipitate-strengthened high-strength low-alloy steel was irradiated by high-energy Au2+ ions with a peak dose of ∼70 displacements per atom (dpa) at room temperature. The formation of nanoprecipitates, dislocation loops, elemental segregation and mechanical properties before and after irradiation were carefully characterized using atom probe tomography (APT), transmission electron microscopy (TEM) and nanoindentation. The mechanism of precipitation and coarsening of these nanoprecipitates and the effect of the dose on irradiation-induced solute capture were studied. The results show that, different from a simple aging-induced precipitation of Cu-rich nanoprecipitates, coprecipitation of Cu/Ni nanoprecipitates occurred under irradiation. With increasing dose, the nanoprecipitates coarsened. Although irradiation induced the formation of small dislocation loops together with new nanoprecipitates, the abnormal softening that occurred after irradiation can be attributed to the coarsening of the preexisting nanoprecipitates. These studies and discoveries are expected to provide unique and important information for the design of new structural alloys with enhanced radiation resistances. [ABSTRACT FROM AUTHOR]

The grain refinement efficiency of Mg–5Li–3Al alloy inoculated by Al–5Ti–1B master alloy and the influences of grain size on the mechanical properties were investigated. With the addition of 1.0 wt% Al–5Ti–1B, the average grain size of Mg–5Li–3Al alloy decreases markedly from ~120 μm to 50 μm, leading to significant increases in both strength and elongation. The tensile yield strength (YS), ultimate tensile strength (UTS) and elongation increased from 82 MPa, 154 MPa and 7.8% to 110 MPa, 216 MPa and 12.2%, respectively. XRD and EDS analysis show that Al–5Ti–1B master alloy is composed of α -Al, Al 3 Ti and TiB 2 . Crystallographic calculations and EDS analysis indicate that TiB 2 particles are the potent heterogeneous nucleating substrates for α -Mg grains, which is the main reason for grain refinement. In addition, solute element, Ti also plays an important role in grain refinement. [ABSTRACT FROM AUTHOR]

The effects of deformation at 900 °C and 1050 °C along with various cooling rates on the microstructure and mechanical properties of a 460 MPa grade low carbon bainitic ferrite steel are investigated. Without prior high temperature deformation, the microstructures can be polygonal ferrite, pearlite, granular ferrite, acicular ferrite or bainite ferrite, depending on the cooling rate. High temperature deformation moves the transformation curves toward the left side in the continuous cooling transformation (CCT) diagram and promotes the formation of polygonal ferrite, granular ferrite, and acicular ferrite, which leads to a lower hardness. Electron backscattering diffraction (EBSD) results indicated that with a cooling rate of 40 °C/s, high proportion of low angle grain boundary (LAGB), large orientation gradient, and high dislocation density and residual stress can be obtained by deformation at high temperature before cooling. Low cooling rate reduces the proportion of LAGB, orientation gradient, dislocation density, and residual stress due to the release of residual stress and the consumption of dislocations. Deformation before cooling slightly lowers the transformation start temperature (Ar 3) while accelerating the transformation speed of the supercooled austenite. The yield and tensile strength increase with the increase in cooling rate. Furthermore, the impact energy value increases with the increase of cooling rate, and it slightly decreases when the microstructure fully changes to bainite ferrite owing to the significant increase in strength. Impact toughness is more sensitive to strength at low temperatures. [ABSTRACT FROM AUTHOR]

Unlike many Mg alloys that exhibit limited ductility, duplex Mg–Li alloys possess enhanced ductility but the underlying mechanisms are unclear. Using real-time in situ neutron diffraction measurements, we show that the underlying deformation modes in this duplex microstructure include an early yield of body-centered-cubic (BCC) β-Li phase and the later elevated hardening in this phase at large macroscopic plastic strain, and the sequential activation of basal and pyramidal < a > slip systems in hexagonal-close-packed (HCP) α-Mg phase. The latter relieves the Mises constraint for deformation compatibility, and the successive yielding sequence promotes the overall work hardening rate, both of which are beneficial for ductility enhancement. No obvious twinning activities were found and correspondingly the hysteresis loops were symmetric upon a full loading cycle within ±1% applied strain. [ABSTRACT FROM AUTHOR]