Effect of grain boundary sliding on the toughness of ultrafine grain structure steel: A molecular dynamics simulation study

Molecular dynamics simulations are carried out to investigate the mechanisms of low-temperature impact toughness of the ultrafine grain structure steel. The simulation results suggest that the sliding of the {001 }/{ 110} type and { 110}/{ 111 } type grain boundary can improve the impact toughness. Then, the mechanism of grain boundary sliding is studied and it is found that the motion of dislocations along the grain boundary is the underlying cause of the grain boundary sliding. Finally, the sliding of the grain boundary is analyzed from the standpoint of the energy. We conclude that the measures which can increase the quantity of the {001}/{110} type and {110}/{ 111} type grain boundary and elongate the free gliding distance of dislocations along these grain boundaries will improve the low-temperature impact toughness of the ultrafine grain structure steel.

Microstructure and mechanical properties of cryo-rolled AA6061 Al alloy

The microstructure and mechanical properties of the age hardening AA6061 Al alloy subjected to cryo-rolling（CR） and room temperature rolling（RTR） treatments were investigated. The rolled and aged alloys were analyzed by using DSC, EBSD, TEM, Vickers hardness analysis and tensile test. The results show that the cryo-rolled treatment has an effect on the precipitation sequence of AA6061 Al alloy. The ultrafine grain structures are formed to promote the fine second phase particles to disperse in the aluminum matrix after the peak aging, which is attributed to lots of dislocations tangled in the rolling process. Therefore, the strength and ductility of AA6061 Al alloy are simultaneously modified after the cryo-rolling and aging treatment compared with room temperature rolled one.

Underwater friction stir welding of ultrafine grained 2017 aluminum alloy

2017 aluminum alloy plates with an ultrafine grained （UFG） structure were produced by equal channel angular processing （ECAP） and then were joined by underwater friction stir welding （underwater FSW）. X-ray diffractometer （XRD）, transmission electron microscope （TEM）, scanning electron microscope （SEM） and microhardness tester were adopted to investigate the microstructural and mechanical characteristics of the FSW joint. The results indicate that an ultrafine grained microstructure with the mean grain size of-0.7 Ixm is obtained in the weld nugget by using water cooling. However, The FSW joint exhibits softening compared with the ultrafine grained based material and the heat affected zone （HAZ） has the lowest hardness owing to the coarsening of the strengthening precipitates.

New forming method of manufacturing cylindrical parts with nano/ultrafine grained structures by power spinning based on small plastic strains

A new spinning method to manufacture the cylindrical parts with nano/ultrafine grained structures is proposed, which consists of quenching, power spinning and recrystallization annealing. The microstructural evolution during the different process stages and macroforming quality of the spun parts made of ASTM 1020 steel are investigated. The results show that the microstructures of the ferrites and pearlites in the ASTM 1020 steel are transformed to the lath martensites after quenching. The martensite laths obtained by quenching are refined to 87 nm and a small amount of nanoscale deformation twins with an average thickness of 20 nm is generated after performing a 3-pass stagger spinning with 55% thinning ratio of wall thickness, where the equivalent strain required is only 0.92. The equiaxial ferritic grains with an average size of 160 nm and nano-carbides are generated by subsequent recrystallization annealing at 480°C for 30 min. The spun parts with high dimensional precision and low surface roughness are obtained by the forming method developed in this work, combining quenching with 3-pass stagger spinning and recrystallization annealing.

Intercritical Rolling Induced Ultrafine Lamellar Structure and Enhanced Mechanical Properties of Medium-Mn Steel

The medium-Mn steel with ferrite and austenite structure was rolled in the intercritical region down to dif- ferent rolling reduction. The microstructure and mechanical properties of the rolled steels were investigated by scan- ning electron microscopy, transmission electron microscopy, X-ray diffraction and tensile tests. It was found that the ferrite and austenite structure gradually evolved into an ultrafine structure from the random directional lath structure to lamellar structure with lath longitudinal direction parallel to the rolling direction with increasing rolling strain. It was found that the thickness of the laths was gradually refined with increasing rolling strain. The lath thickness is about 0. 15 9m stored with high density dislocations and the austenite volume fraction of the steel is about 24% after 80% rolling reduction. Furthermore, it was interesting to find that yield strength, tensile strength and total elongation of the 80% rolled medium-Mn steel are about 1000 MPa, 1250 MPa and 24%, respectively, demonstrating an excellent combination of the strength and ductility. Based on the microstructure examination, it was proposed that the grain refinement of the medium-Mn steels could be attributed to the duplex structure and the low rolling temperature. Analysis of the relationship between the microstructure and the mechanical properties indicated that the high yield strength mainly resulted from the ultrafine grain size and the high density dislocation, but the improved ductili- ty may be attributed to the large fractions of austenite retained after intercritical rolling.

Ultrafine Grained Duplex Structure Developed by ART-annealing in Cold Rolled Medium-Mn Steels

The microstructural evolutions of the cold rolled Fe-0.1C-5Mn steel during intercritical annealing were ex- amined using combined advanced techniques. It was demonstrated that intercritical annealing results in an ultrafine granular ferrite and austenite duplex structure in cold rolled 0.1C-5Mn steel. The strong partitioning of manganese and carbon elements from ferrite to austenite was found during intercritical annealing by scanning transmission elec- tron microscopy （STEM） and X-ray diffraction （XRD）. Strong effects of boundary characters on the austenite for- mation were indicated by austenite fast nucleation and growth in the high angle boundaries but sluggish nucleation and growth in the low angle boundaries. The ultrafine grained duplex structure in 0.1C-5Mn was resulted from the the sluggish Mn-diffusion and the extra high Gibbs free energy of ferrite phase. Based on the analysis of the micro- structure evolution, it was pointed out that the intercritical annealing of the medium Mn steels could be applied to fabricate an ultrafine duplex grained microstructure, which would be a promising approach to develop the 3rd genera- tion austomobile steels with excellent combination of strength and ductility.

Study on grain refinement of interstitial-free steel during continuous frictional angular extrusion

To explore the application of severe plastic deformation for grain refinement in steel production, a new method called continuous frictional angular extrusion （CFAE） was applied to refine the grain of interstitial-free steel. The deformation was carried out at room temperature and individual sheet specimens were processed in different number of passes. An overall grain size of 200nm was achieved after 8 passes and the proportion of high-angle boundaries to the total boundaries was more than 60%. Through the characterization of high resolution EBSD, X-ray diffraction （XRD） and hardness testing,this paper discussed the evolution of microstructures and textures during deformation and explored the development direction of the method.

Effect of mixing structure on the hygroscopic behavior of ultrafine ammonium sulfate particles mixed with succinic acid and levoglucosan

Understanding the interactions between water and atmospheric aerosols is critical for estimating their impact on the radiation budget and cloud formation. The hygroscopic behavior of ultrafine （〈100nm） ammonium sulfate particles internally mixed with either succinic acid （slightly soluble） or levoglucosan （soluble） in different mixing structures （core-shell vs. well-mixed} were measured using a hygroscopicity tandem differential mobility analyzer （HTDMA）. During the hydration process （6-92% relative humidity （RH））, the size of core-shell particles （ammonium sulfate and succinic acid） remained unchanged until a slow increase in particle size occurred at 79Y~ RH; however, an abrupt increase in size （i.e., a clear deliquescence） was observed at ~72% RH for well-mixed particles with a similar volume fraction to the core-shell particles （80：20 by volume）. This increase might occur because the shell hindered the complete dissolution of the core-shell particles below 92% RH. The onset RH value was lower for the ammonium sulfate/levoglucosan core-shell particles than the ammonium sulfate/succinic acid core-shell particles due to levoglucosan＇s higher solubility relative to succinic acid. The growth factor （GF） of the core-shell particles was lower than that of the well-mixed particles, while the GF of the ammonium sulfate/levoglucosan particles was higher than that of ammonium sulfate/succinic acid particles with the same volume fractions. As the volume fraction of the organic species increased, the GF decreased. The data suggest that the mixing structure is also important when determining hygroscopic behavior of the mixed particles.

General synthesis of sponge-like ultrafine nanoporous metals by dealloying in citric acid

A general method is proposed to synthesize ultrafine nanoporous Cu, Ag, and Ni with novel sponge-like morphologies, high porosities, and large surface areas. The materials are produced by dealloying Mgc~/IzsY10 （M = Cu, Ag, and Ni） metallic glasses in citric acid. Citric acid played a key role due to its capping effect, which reduced the surface diffusion of metals. A structural model consistent with the sponge-like morphology was constructed to calculate the porosity and the surface area. The mechanism of the dealloying process in citric acid, involving ligament formation and coarsening, was illustrated. The mechanism was capable of explaining the experimental trends of dealloying, especially the morphology. A glucose sensor, which can be further developed into a high-precision real-time glucose monitor for medical use, was constructed using sponge-like nanoporous copper. Our findings are not only relevant to understanding the dealloying mechanism of metallic glasses, but also provide promising materials for multiple applications.

Formation of Nano/Ultrafine Grains in AISI 321 Stainless Steel Using Advanced Thermo-Mechanical Process

Production of nano/ultrafine grains through deformation-induced martensite formation and its reversion to austenite in an AISI 321 stainless steel was studied. The repetitive cold rolling and subsequent annealing were conducted to obtain nanocrystalline structure. Heavy cold rolling （90% reduction） at ＋20 and -20 ℃ was carded out to induce the formation of α′-martensite from metastable austenitic material. The process was followed by annealing treatment at 700-900 ℃ for 0.5-30 min. Effects of process parameters, i.e., ＂reduction percentage,＂ ＂rolling temperature,＂ ＂annealing temperature＂ and ＂annealing time＂, on the microstructural development were considered. Microstructural evolutions were conducted using feritscope, X-ray diffractometer and scanning electron microscope. Hardness of the specimens was measured by Vickers method. Results revealed that the higher thickness reduction and lower rolling temperature provided more martensite volume fraction and further hardness. X-ray diffraction patterns and feritoscopic results indicated that saturated strain （εs） was reduced from 2.3 to 0.9 when temperature declined from ＋20 to -20 ℃. The smallest grain size （about 70 nm） was achieved in the condition of cold rolling at -20℃followed by annealing at 750 ℃for 5 min.