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Ultra-coarse grained cemented carbides are often used under conditions of concurrently applied stress and high temperature. Improvement of high-temperature mechanical performance of ultra-coarse grained cemented carbides is highly desirable but still a big challenge. In this study, it is proposed that the high-temperature compression strength of ultra-coarse cemented carbides can be enhanced by modulating hard matrix grains by activated TaC nanoparticles, through solid solution strengthening of Ta atoms. Based on the designed experiments and microstructural characterizations combined with finite element simulations, the grain morphology, stress distribution and dislocation configuration were studied in detail for ultra-coarse grained cemented carbides. The mechanisms of Ta dissolving in WC crystal and strengthening ultra-coarse grains through interaction with dislocations were disclosed from the atomic scale. This study opens a new perspective to modulate hard phases of cemented carbides for improving their high-temperature performance, which will be applicable to a variety of cermet and ceramic-based composite materials.

Control of the formation and stability of reverted austenite is critical in achieving a favorable combination of strength, ductility, and toughness in high-strength steels. In this work, the effects of Cu precipitation on the austenite reversion and mechanical properties of maraging stainless steels were investigated by atom probe tomography, transmission electron microscopy, and mechanical tests. Our results indicate that Cu accelerates the austenite reversion kinetics in two manners: first, Cu, as an austenite stabilizer, increases the equilibrium austenite fraction and hence enhances the chemical driving force for the austenite formation, and second, Cu-rich nanoprecipitates promote the austenite reversion by serving as heterogeneous nucleation sites and providing Ni-enriched chemical conditions through interfacial segregation. In addition, the Cu precipitation hardening compensates the strength drop induced by the formation of soft reverted austenite. During tensile deformation, the metastable reverted austenite transforms to martensite, which substantially improves the ductility and toughness through a transformation-induced plasticity (TRIP) effect. The Cu-added maraging stainless steel exhibits a superior combination of a yield strength of ∼1.3 GPa, an elongation of ∼15%, and an impact toughness of ∼58 J.

Low-carbon advanced nanostructured steels have been developed for various structural engineering applications, including bridges, automobiles, and other strength-critical applications such as the reactor pressure vessels in nuclear power stations. The mechanical performances and applications of these steels are strongly dependent on their microstructural features. By controlling the size, number density, distribution, and types of precipitates, it is possible to produce nanostructured steels with a tensile strength reaching as high as 2 GPa while keeping a decent tensile elongation above 10% and a reduction of area as high as 40%. Besides, through a careful control of strength contributions from multiple strengthening mechanisms, the nanostructured steels with superior strengths and low-temperature impact toughness can be obtained by avoiding the temper embrittlement regime. With appropriate Mn additions, these nanostructured steels can achieve a triple enhancement in ductility (total tensile elongation, TE of ~30%) at no expense of strengths (yield strength, YS of ~1100 to 1300 MPa, ultimate tensile strength, UTS of ~1300 to 1400 MPa). More importantly, these steels demonstrate good fabricability and weldability. In this paper, the microstructure-property relationships of these advanced nanostructured steels are comprehensively reviewed. In addition, the current limitations and future development of these nanostructured steels are carefully discussed and outlined.

Structure, crystallization behavior, and magnetic properties of as-quenched and annealed Fe81.3Si4B13Cu1.7 (Cu1.7) alloy ribbons and effects of Nb alloying have been studied. Three-dimensional atom probe and transmission electron microscopy analyses reveal that high-number-density Cu-clusters and Pre-existing Nano-sized α-Fe Particles (PN-α-Fe) are coexistence in the melt-spun Cu1.7 amorphous matrix, and the PN-α-Fe form by manners of one-direction adjoining and enveloping the Cu-clusters. Two-step crystallization behavior associated with growth of the PN-α-Fe and subsequent nucleation and growth of newly-formed α-Fe is found in the primary crystallization stage of the Cu1.7 alloy. The number densities of the Cu-clusters and PN-α-Fe in melt-spun Fe81.3−xSi4B13Cu1.7Nbx alloys are gradually reduced with enriching of Nb, and a fully amorphous structure forms at 4 at.% Nb, although smaller Cu-clusters still exist. After annealing, 2 at.% Nb coarsens the average size (Dα-Fe) of the α-Fe grains from 14.0 nm of the Nb-free alloy to 21.6 nm, and 4 at.% Nb refines the Dα-Fe to 8.9 nm. The mechanisms of the α-Fe nucleation and growth during quenching and annealing for the alloys with large quantities of PN-α-Fe as well as after Nb alloying have been discussed, and an annealing-induced α-Fe growth mechanism in term of the barrier co-contributed by competitive growth among the PN-α-Fe and diffusion-suppression effect of Nb atoms has been proposed. A coercivity (Hc) ∝ Dα-Fe3 correlation has been found for the nanocrystalline alloys, and the permeability is inverse with the Hc.

Nano-lamellar materials with ultrahigh strengths and unusual physical properties are of technological importance for structural applications. However, these materials generally suffer from low tensile ductility, which severely limits their practical utility. Here we show that markedly enhanced tensile ductility can be achieved in coherent nano-lamellar alloys, which exhibit an unprecedented combination of over 2 GPa yield strength and 16% uniform tensile ductility. The ultrahigh strength originates mainly from the lamellar boundary strengthening, whereas the large ductility correlates to a progressive work-hardening mechanism regulated by the unique nano-lamellar architecture. The coherent lamellar boundaries facilitate the dislocation transmission, which eliminates the stress concentrations at the boundaries. Meanwhile, deformation-induced hierarchical stacking-fault networks and associated high-density Lomer-Cottrell locks enhance the work hardening response, leading to unusually large tensile ductilities. The coherent nano-lamellar strategy can potentially be applied to many other alloys and open new avenues for designing ultrastrong yet ductile materials for technological applications., Nano-lamellar materials with ultrahigh strengths are highly desirable for technological applications. Here the authors report a nanolamellar architecturing approach by utilizing coherent L12 structures to achieve ultrahigh strength and ductility in Ni-Fe-Co-Cr-Al-Ti multicomponent alloys.

The strength-ductility paradox is a long-sought challenge for all engineering materials. In this study, we escaped the strength-ductility trade-off by engineering nano-scale heterogeneities carefully in the advanced nanostructured Fe-based alloys through alloying with Cu and Mn additions. We demonstrated a triple ductility enhancement by 20% together with a strength improvement of 100MPa compared to the alloys with sole Cu additions, overturning a common understanding of the strength-ductility trade-off. The strength-ductility enhancement is attributed to the complex interplay between the transformation induced plasticity (TRIP) and the coherent nano-scale Cu precipitates as well as the resultant heterogeneous stress–strain partitioning and dislocation interactions.

We developed a novel high-performance L12-strengthened high-entropy alloy (HEA) in the multicomponent Ni-Co-Fe-Cr-Al-Nb system. The phase transformation, mechanical properties and associated deformation behaviors were systematically investigated through combinational analyses involving the three-dimensional atom probe tomography (3D-APT), transmission electron microscopy (TEM) and first-principles calculations. In contrast to conventional alloys that generally strengthened by Ni3(Al, Ti)-type precipitates, a high density of coherent L12 nanoprecipitates with a new chemical constitution of (Ni, Co, Fe, Cr)3(Al, Nb) can be controllably introduced via elaboratively tuning the content of Al and Nb, resulting in a large lattice misfit of ~0.78% that rarely achieved in previous HEAs. The newly developed (Ni2Co2FeCr)92Al4Nb4 HEA enables excellent tensile properties at a large temperature window from room temperature to 870 °C. More remarkably, an anomalous growth in yield strength can be observed at the temperatures above 600 °C, showing a peak yield stress over 720 MPa when deformed at 760 °C, which surpasses most of the previous L12-strengthened HEAs, as well as the commercial superalloys. Detailed TEM analyses revealed that the multicomponent L12 precipitates are mainly sheared by the super-partial dislocations, forming superlattice intrinsic stacking fault (SISF) loops coupled with antiphase boundaries (APBs). Such an interesting deformation substructure enables sustained work hardening and produces high tensile strengths at the high temperatures. The underlying mechanisms of those SISF loops were carefully discussed, which could be possibly ascribed to the local elemental segregation on the planner faults.

Titanium alloys have been widely used for medical devices and structural applications. However, conventional titanium alloys often suffer from low resistance to wear, particularly at elevated temperatures. Herein, an equiatomic TiMoNb compositionally complex alloy (CCA) is shown to exhibit wear resistance comparable to alumina at room temperature (RT). Even at 600 °C, the alloy still shows an extremely low wear rate of the order of 10−6 mm3/(N·m). The remarkable wear resistance is achieved via tuning the interfacial structure and chemistry in TiMoNb CCA, including nanostructuring, titanium segregation at the grain boundaries, and the formation of a high density of nanoscale coherent Ti-rich precipitates with cube-on-cube orientation relationship with the ultrafine-grained matrix. The present results provide significant insights into the design of novel alloys for service in harsh environments.

Aging treatments at 400–550 °C are commonly used to attain a peak strengthening for the Cu-rich nanocluster-strengthened high-strength low-alloy (HSLA) steels. However, these temperatures fall within the dangerous 300–600 °C temper-embrittlement regime, leading to poor impact toughness. On the other hand, aging at temperatures above the embrittlement regime can improve the impact toughness but at a great expense of strength. In this work, the strengthening mechanisms as well as the toughening of a low cost weldable HSLA steel with a low content of carbon (C ∼0.08 wt.%), nickel (Ni = 0.78 wt.%), and copper (Cu = 1.3 wt.%) were carefully investigated. Our findings show that the low-C-Ni-Cu HSLA steel is insensitive to the aging temperatures and can achieve a yield strength (YS) and ultimate tensile strength (UTS) over 1000 and 1100 MPa, respectively, with tensile ductility >10% (reduction of area >60%) at a heat-treat temperature of 640 °C through multiple strengthening mechanisms. Besides, a good low-temperature (−40 °C) impact performance (∼200 J) with high YS (∼900 MPa) and UTS (∼1000 MPa) can be obtained by seeking a strength balance among the fine grain size (∼2.5 μm), medium-sized (∼14 nm) overaged Cu-rich precipitates, tempered martensite, and fresh martensite (or carbides). Moreover, a relatively lower YS (∼800 MPa) and UTS (∼900 MPa) useful for steel manufacturing can be attained by a prolonged aging at 640 °C. In addition, the dislocation-precipitate interactions were also explored based on the dislocation theories in this study.

In this paper, G-phase precipitation and the resulting hardening effect on Fe-20Cr-3Ni-1Mn-3Si ferritic alloys by the addition of Ti, Nb, Ta and Zr were individually studied by electron microscopy and atom probe tomography, combined with thermodynamic and first principle calculations. The high resolution scanning electron and transmission electron microscopy observations confirmed that four kinds of Ni16M6Si7 (M=Ti, Nb, Ta and Zr) G-phase particles were distributed uniformly in the matrix of the four alloys aged at 560–860 °C. The 3D-APT results revealed the formation of the four different nanoscale precipitates, with the highest number density (6.05 × 1023 m−3) and smallest radius (1.64 ± 0.45 nm) in the Ti added alloy. The nanoscale sized precipitates stability (~3 nm for Ti added alloy; ~5 nm for Nb added alloy and Zr added alloy; ~25 nm for Ta added alloy) was attributed to their particular cube-cube orientation relationships, which led to a very low interfacial energy. The effects of the G-phase on precipitation hardening and quasi steady-state deformation resistance were examined. The 560 °C-aging hardening experiments suggested a rapid precipitation process of the nano-particles. The peak hardness values of the four alloys are in the descending order: Ti ˃˃ Nb > Ta ˃˃ Zr. The 660 °C quasi steady-state deformation experiments showed threshold stresses of 110 and 140 MPa in cases of the Nb and Ti added alloys, which were higher than that of the previously reported B2-NiAl strengthened steels and commercial heat-resistant steels. By using the formation of nanoscale G-phase precipitates, a new ferritic steel family with very high strength and creep resistance has been proposed. Further alloy optimization is in progress.

In past decades, molybdenum (Mo) has demonstrated the potential to replace tungsten (W) in designing lighter nickel-based single crystal superalloys with enhanced creep resistance. In this research, three groups of nickel-based single crystal superalloys are designed and prepared by tuning the Mo:W ratio. The experimental observations show that the primary and secondary dendrite arm spacings are both independent on Mo:W ratio, but the γ/γ′ eutectic volume fraction decreases as Mo:W ratio increases. The electron probe microanalysis reveals that Re, W and Co segregate to dendrite core regions, while Al, Ti, Cr, Mo and Ta to interdendritic regions. The microsegregation of Ta, W and Re are gradually relieved with increasing Mo:W ratio. Atom probe tomography results further indicate that Cr and Mo are depleted in γ/γ′ eutectics, leading to the enrichment of Cr and Mo at interdendritic regions away from γ/γ′ eutectics. Creep rupture results show that the rupture life at 1040 °C/145 MPa is improved as Mo:W ratio increases.

In the present work, three kinds of precipitates with different morphologies, structures, sizes, and volume fractions were obtained via energetically-tuning the microstructures of the nano-precipitated CoCrFeNiTi0.2 high-entropy alloy (HEA). Subjected to the heavy cold rolling immediately after homogeneous precipitation, L12 structured spherical nano-particles with an average size of 16.5 nm rapidly grow into 200 nm-sized spherical ones due to Ostwald ripening. On the other hand, superfluous mechanical energy storage energetically facilitates the phase transformation from spherical L12 to rod-shaped D024 structures for initially formed nano-precipitates. Besides, some other newly formed nano-precipitates with an average size of 6.5 nm are available, originating from heavily plastically deformed-induced nucleated sites. Multi-scale precipitates interact with dislocations in different ways. The strengthening provided by dislocations cutting through smaller nano-particles and bypassing grown ones account for 57.7% and 42.3% of precipitation strengthening, respectively, while rod-shaped precipitates can act as equivalent interfaces to hinder dislocation movement. Their synergistic effect has achieved remarkable strengthening and toughening. Specially, dislocation slips dominate at 298 K, while stacking faults (SFs) assist plastic deformation at 77 K. Compared with 298 K, the yield strength (YS) and ultimate tensile strength (UTS) of the current HEAs at 77 K are increased by 38.9% and 38.2% to 1 GPa and 1.5 GPa, respectively, and the tensile strain is slightly increased to 35% instead of loss, realizing excellent strength and plasticity combination. Theoretically established strengthening models agree well room-temperature and cryogenic yield strengths experimentally. Moreover, the tensile elongation is effectively predicted by the Whitehouse-Clyne model. This strengthening strategy of energetically-tuning polymorphous precipitates provides the basic guidance to develop high-performance nano-precipitated alloys. The current strengthening and plasticity models can be employed to well predict the mechanical properties of such kinds of alloys at cryogenic temperatures.

A series of G-phase strengthened ferritic stainless steels Fe-20Cr-3Ni-3Si-X (X = 2Mn, 1Mn-2Ti, 1Mn-2Nb and 1Mn-2Ta) are characterized after aging using experimental (microhardness, TEM and APT) and theoretical (DFT) techniques. The results indicate that the Ni16Mn6Si7 G-phase shows sluggish precipitation during aging treatment. This was attributed to the small difference in the enthalpy of formation between the Ni16Mn6Si7 G- and BCC phase and the requirement of high Ni:Fe ratio. A superfine Ni16Ti6Si7 G-phase was found to precipitate as a core accompanied with an “envelope” of Fe2TiSi-L21 Heusler phase during early aging (≤24 h) in the Ti containing alloy. This morphology is predicted to occur due to early Ni clustering in ferrite and a negative Ni concentration gradient away from the cluster that favors Fe2TiSi formation. The G-phases show only particle coarsening without obvious chemical composition evolution for further aging up to 96 h. A prominent hardness increase of 100-275 HV was also observed during aging. These findings provide valuable insight into methods for precipitating low lattice mismatch silicide phases for the development of future high strength steels.

The failures of conventional pearlitic rail steels are strongly affected by the formation of hard and brittle White and Brown Etching Layer (WEL and BEL) on the rail raceway during service. The bainitic rail steels with excellent strength-toughness balance are promising candidates for the next generation of rail steels. It is generally stated that the hard and brittle WEL/BEL is difficult to generate in the bainitic rails because of low carbon content. However, we find a distinct multilayer WEL/BEL composite structure on the worn surface/subsurface of the bainitic rail after in-field testing. The multi-scale characterizations from the microscopic to the atomic scale reveal that the WEL and BEL exhibit different microstructural, chemical, and mechanical features. The oxide particles preferentially form in the BEL rather than the WEL. The local oxidation generates the Mn, Si, and Cr depletion but C enrichment in the BEL matrix. Consequently, the BEL exhibits hard but brittle features due to the formation of brittle twinning martensite, cementite, and oxide particles. In contrast, the WEL is relatively soft and ductile. Under plastic deformation and temperature rise upon wheel-rail contact, the heterogeneous microstructure and chemical micro-segregation in parent material lead to the inhomogeneous microstructural refinement, oxidation, and localized embrittlement, which is responsible for the formation of multilayer WEL/BEL structure in bainitic rail steels.

Alloys that have high strengths at high temperatures are crucial for a variety of important industries including aerospace. Alloys with ordered superlattice structures are attractive for this purpose but generally suffer from poor ductility and rapid grain coarsening. We discovered that nanoscale disordered interfaces can effectively overcome these problems. Interfacial disordering is driven by multielement cosegregation that creates a distinctive nanolayer between adjacent micrometer-scale superlattice grains. This nanolayer acts as a sustainable ductilizing source, which prevents brittle intergranular fractures by enhancing dislocation mobilities. Our superlattice materials have ultrahigh strengths of 1.6 gigapascals with tensile ductilities of 25% at ambient temperature. Simultaneously, we achieved negligible grain coarsening with exceptional softening resistance at elevated temperatures. Designing similar nanolayers may open a pathway for further optimization of alloy properties.

It is known that nanoscale precipitates strongly affect the precipitation hardening of structural materials. In this study, we report the precipitation kinetics and thermostability of Cu-rich and NiAl nanoprecipitates in nanostructured steels with Mo additions. Atom probe tomography and first-principles calculations revealed that the addition of Mo effectively decreased the diffusion coefficients of Cu, Ni and Al atoms, leading to the change in the precipitation mechanisms from NiAl prior-precipitation with an instantaneous-nucleation mechanism to the Cu prior-precipitation with a continuous-nucleation mechanism. The decreased diffusion coefficient significantly improves the thermostability of nanoprecipitates and strongly enhances the strength of the nanostructured steels. The precipitation mechanisms and thermostability of nanoscale precipitates in nanostructured steels are strongly dependent on the diffusion coefficient of elements, which can be effectively tuned by alloying with Mo additions.

Alloying plays an important role in determining the phase stability and mechanical behavior of medium/high-entropy alloys (M/HEAs). In this work, the effects of Al, Ti, Mo, and W additions on the phase stability, strengthening behavior, and stacking fault energies of CoCrNi alloys are quantitatively investigated by using first-principles calculations. Our results reveal that the Al, Ti, and W additions enhance the structural stability of metastable face-centered cubic structures, whereas Mo is in favor of the formation of hexagonal close-packed structures at low temperatures. Through analyzing the elastic moduli and lattice mismatch based on a Labusch-type model, we show that the solute strengthening effect decreases in the order W > Mo > Ti > Al. The compositional dependence of intrinsic stacking fault energy (ISFE) and unstable stacking fault energy (USFE) of CoCrNi MEAs was calculated, and the results indicate that the Al, Ti, Mo and W additions significantly reduce the USFE, leading to a reduction in the energy barriers of dislocation slips. The alloying effects on the deformation behaviors of CoCrNi MEAs are discussed in terms of the ratio of ISFE to the energy barrier of dislocation slips. The present study not only sheds light on the fundamental understanding of phase stability and deformation mechanisms of M/HEAs but also provides useful guidelines for the alloy design of advanced M/HEAs with superior mechanical properties.

Nanostructured W-Cu-Cr composites were fabricated, which exhibit exceptionally high hardness (~1000 HV) as compared with those of the conventional W-Cu composites, due to the combined advantages of Cr dissolution, precipitate formation and grain refinement. Nonmonotonic variation of the wear resistance with hardness was discovered. With an appropriate content of Cr, the preferential oxidation reduced the oxidation of W. Moreover, it facilitated formation of a stable protective film with fish-scale morphology and also refined the structure, leading to a high microhardness at the worn surface. The wear rate (1.65 × 10−6 mm3 N−1 m−1) was reduced by an order of magnitude compared with that of the conventional counterpart. However, excessive addition of Cr may deteriorate the wear resistance in spite of a higher hardness due to the embrittlening of W phase and difficulty to form a stable protective film at the worn surface. This study provides a new strategy for developing W-Cu composites with outstanding wear resistance.

The Cu and NiAl nanoscale precipitation and interfacial segregation in the martensite and austenite phases within a high-strength steel were studied by atom-probe tomography (APT). In the martensite phase, APT reveals the precipitation of isolated NiAl nanoparticles and NiAl/Cu co-precipitates, indicating that NiAl nanoparticles form first in the precipitation sequence. In comparison, the austenite phase contains only Cu nanoparticles with Ni segregation at the particle/matrix interface, in which the Ni segregation reduces the Cu nanoparticle interfacial energy. In addition, Mn and C exhibit an enrichment at the martensite/austenite interface, and the mechanism for the interfacial segregation was also discussed.