Inspired by the gradient structures of biological materials, researchers have explored compositional and structural gradients for about 40 years as an approach to enhance the properties of engineering materials, including metals and metallic alloys. The synthesis of various gradient nanostructured materials, such as gradient nanograined, nanolaminated nd nanotwinned metals and alloys, has provided new opportunities to understand gradient-related mechanical behaviour. These emerging gradient materials often exhibit unprecedented mechanical properties, such as strength–ductility synergy, extraordinary strain hardening, enhanced fracture and fatigue resistance, and remarkable resistance to wear and corrosion, which are not found in materials with homogeneous or random microstructures. This Review critically assesses the state of the art in the field of gradient nanostructured metallic materials, covering topics ranging from the fabrication and characterization of mechanical properties to underlying deformation mechanisms. We discuss various deformation behaviours induced by structural gradients, including stress and strain gradients, the accumulation and interaction of new dislocation structures, and unique interfacial behaviour, as well as providing insight into future directions for the development of gradient structured materials. Gradient nanostructured metals and alloys are an emerging class of materials that exhibit a combination of excellent mechanical properties that are not possessed by their homogeneous counterparts. This Review assesses the fabrication, characterization and deformation behaviour of these materials, as well as the challenges and future directions of the field.

Mechanical properties and deformation mechanisms of gradient nanostructured metals and alloys

The transformation-induced plasticity (TRIP) in advanced high-strength steels (AHSS) is reviewed, where the main concepts and the recent progress in the processing and properties of AHSS are introduced. The metastable austenitic stainless and multiphase TRIP-assisted steels, as well as the more recent third generation AHSS grades, namely the medium-Mn and quenching and partitioning (Q&P) steels, are critically discussed. These steels utilize the TRIP effect and the enhanced work-hardening rate through the transformation of (retained) austenite in their microstructures to martensite during plastic deformation for the improvement of strength-ductility balance, which make them especially suitable for the automotive industry to be used in the lightweight car body for addressing the safety, fuel consumption, and air pollution issues. The kinetics of strain-induced martensitic transformation (SIMT) as well as the effects of chemical composition, grain size, deformation temperature, strain rate, and deformation mode on the austenite stability are reviewed. The effects of holding temperature and time during the isothermal bainitic transformation (IBT) in TRIP-aided steels, during the austenite-reverted-transformation (ART) annealing in medium-Mn steels, and during the quenching and partitioning steps in the Q&P steels are critically discussed towards enhancement of the amount of retained austenite and optimization of strength-ductility trade off. The alternative thermomechanical processing routes as well as the modified grades such as δ-TRIP and quenching-partitioning-tempering steels are also introduced.

Transformation-induced plasticity (TRIP) in advanced steels: A review

Thermomechanical processing of advanced high strength steels

New insights into intragranular ferrite in a low-carbon low-alloy steel

Effect of acicular ferrite formation on grain refinement in the coarse-grained region of heat-affected zone

The microstructure–property relationship of Nb-free and Nb-bearing ultrahigh strength steels were studied in this work. Martensitic steels with 1800 MPa tensile strength were obtained by conducting reheated quenching and tempering after thermomechanical rolling. The microstructures of the steels were investigated by scanning electron microscopy, transmission electron microscopy electron backscattering diffraction technique. The results show that with the addition of Nb, the prior austenite grain growth was impeded, which leaded to the further refinement of final martensite microstructure. Randomly distributed Nb-rich (Nb,Ti)C in Nb bearing steel played an important role in the strengthening of the steel. As revealed by dilatometry and differential scanning calorimetry, with the increase of tempering temperature the decrease of mechanical properties mainly initiated by the decomposition of retained austenite during low temperature tempering, and the formation of harmful iron carbides was retarded when the steel microalloyed with 0.021% Nb.

Effect of Nb addition on the microstructure and mechanical properties of an 1800 MPa ultrahigh strength steel

High strength and high ductility are critical to metallic materials and have always been pursued. A novel strategy to produce a medium Mn stainless steel with high dislocation density and heterogeneous nano/ultrafine-grains was designed and outstanding mechanical properties were achieved. The special heterogeneous microstructure was fabricated by cold rolling followed by rapid reverse annealing at intercritical temperature and a final step of low-temperature tempering. The results of this study demonstrated that the heterogeneous microstructure consisted of 74% reversed austenite, 16% deformed austenite and 10% tempered martensite. The dislocation density in austenite of the final microstructure had a high value of 1.12 × 1015 m−2 and the average grain size was 0.63 μm. The high yield strength (1324 MPa) and tensile strength (1401 MPa) of the steel were achieved and combined with high uniform elongation of 34.7%. The combination of high dislocation density (dislocation strengthening) and heterogeneous nano/ultrafine grains (fine-grain strengthening) resulted in such high yield strength. Simultaneously, twinning-induced plasticity (TWIP) and transformation-induced plasticity (TRIP) effect occurred within the heterogeneous nano/ultrafine grains leading to high strain hardening rate and good uniform elongation in the steel.

Strong and ductile steel via high dislocation density and heterogeneous nano/ultrafine grains

The objective of the present study is to obtain austenitic stainless steel with super-high yield strength and good ductility through a heterogeneous nano/ultrafine-grained design, fabricated by the reverse transformation of strain-induced martensite and the recrystallization of deformed austenite using two cold-rolling and annealing processes. The second cold-rolling and annealing process was performed on a bimodal microstructure obtained by the first cold-rolling and annealing process on the original microstructure. The bimodal microstructure consists of nanometer grains and a certain quantity of micrometer grains distributed in the lamella. The results of this study demonstrate that after the second process, both the coarse-grain and fine-grain areas of the bimodal microstructure formed their own finer bimodal microstructure and the lamella of the grain distribution became narrower. The yield strength (1221 MPa) and tensile strength (1376 MPa) of the heterogeneous nano/ultrafine-grained steel were greatly improved in the situation where the total elongation still remained at the high level of 45.3%. The fine-grained strengthening, back-stress strengthening, twinning induced plasticity (TWIP) effect and transformation induced plasticity (TRIP) effect were produced by the heterogeneous nano/ultrafine-grained microstructure during tensile possess synthetically contributed to the high strength and ductility.

Heterogeneous nano/ultrafine-grained medium Mn austenitic stainless steel with high strength and ductility

Superior fracture toughness in a high-strength austenitic steel with heterogeneous lamellar microstructure

In the present work, we reported the microstructure and mechanical properties of CrxMnFeNi high-entropy alloy (HEA). The microstructures of the HEAs were characterized by scanning electron microscopy (SEM) with electron back-scattered diffraction (EBSD) and energy dispersive spectrometer (EDS) system, X-ray diffraction (XRD), and transmission electron microscopy (TEM). Mechanical properties of the HEAs were measured by nano-indentation and tensile test. The results showed that the single face-centered cubic (FCC) structure was transformed into the FCC and body-centered cubic (BCC) dual-phase when the Cr content was higher than the threshold value. Also, in the dual-phase HEAs, the fraction of the BCC phase increased proportionally with the increasing Cr content. There was an excellent correlation between the phase composition and the corresponding concentration of the valence electron concentration (VEC). The single FCC phase structure alloy demonstrated lower yield strength and higher tensile ductility. In the dual-phase alloys, the presence of the BCC phase notably strengthened the alloy but deteriorated its ductility. The close relationship between the mechanical properties of the alloy and each phase and the effect of the FCC/BCC dual-phase were discussed. It was confirmed that the grain refinement and higher Hall–Petch strengthening coefficient caused by the BCC phase formation showed a good strength-ductility matching mechanism in the dual-phase alloys.

Huibin Wu

Papers

Effect of Nb addition on the microstructure and mechanical properties of an 1800 MPa ultrahigh strength steel

Strong and ductile steel via high dislocation density and heterogeneous nano/ultrafine grains

Heterogeneous nano/ultrafine-grained medium Mn austenitic stainless steel with high strength and ductility

Superior fracture toughness in a high-strength austenitic steel with heterogeneous lamellar microstructure

Microstructural evolution and strengthening mechanisms in CrxMnFeNi high-entropy alloy