Microstructural Refinement Research Articles

Anomalous ductility-strength synergy occurred after cold rolling in a high Gd content Mg alloy

During cold rolling in magnesium (Mg) alloys, there will always be serious ductility deterioration, which greatly restricts the application of this processing method. In this work, ductility-strength synergy occurred after cold rolling was carried out on a 〈0001〉 fiber textured Mg-17.5Gd-Zr alloy (wt.%). The ductility-strength synergy can be mainly attributed to texture randomization, microstructure refinement, and easier basal slip transfer condition in the twinned grains. Firstly, the activation of multiple twinning behaviors, i.e., {10−12}, {10−11}, and {11−21} twinning, and related twin-twin interaction restrict the formation of strong basal texture and facilitate the texture randomization. Secondly, profuse high-angle grain boundaries form through dislocation-twin interaction, twin-twin interaction, and dislocation interaction, which promote microstructure refinement. Thirdly, the basal slip transfer becomes easier in the twinned grains owing to the activation of multiple twinning behaviors and their impingements. Activation of {11−21} twinning and 〈c + a〉 dislocations and the 〈0001〉 fiber texture which is favorable for multiple twinning activating during cold rolling are considered to be the key roles for microstructure and texture optimization.

Insights into room- and elevated-temperature micro-mechanisms of laser shock peened M50 steel with superior tribological performance

M50 steel, commonly utilized in aircraft engine bearings, is susceptible to friction-induced failures, particularly in high-temperature service conditions. To address this issue, various strategies have been proposed, with laser shock peening (LSP) garnering significant attention due to its deeper residual stress penetration and excellent surface integrity, whereas the underlying strengthening mechanisms have not yet been fully elucidated. In this study, we systematically investigate the impact of LSP treatment on the tribological properties of M50 steel at temperatures of 25 and 300 °C, alongside elucidating the relevant micro-mechanisms. Microstructural analysis reveals that laser impact strengthening primarily arises from dislocation proliferation, resulting in a surface hardness increase of approximately 14 % and the formation of a substantial compressive stress layer reaching a maximum value of about 1200 MPa, with a depth of around 2 mm. Friction test results demonstrate reduced coefficients of friction and wear rates following LSP treatment at both temperatures. Notably, a more pronounced reduction is observed at 300 °C, with values decreasing by 41.4 % and 55.8 %, respectively. The enhanced performance is attributed to the synergistic interplay of compressive residual stresses, work-hardening layers, increased density of dislocations, and substantial microstructure refinement.

Friction and Wear of Hard Yet Tough TiN Coatings Deposited Using High-Power Impulse Magnetron Sputtering

The friction and wear response of hard coatings is complex, which largely depends on a good combination of hardness and toughness, and their service life is difficult to predict. Hence, in this work, hard yet tough TiN coatings were deposited using high-power impulse magnetron sputtering at 5–10 kW. With increasing sputtering power, the coatings showed a transition in crystal texture from (200) to (111), along with a refinement in microstructure, leading to an improvement in hardness (H) of 29.8–31.2 GPa and an effective Young’s modulus (E*) of 310–365 GPa. The hard yet tough TiN coatings deposited at 6.5 kW exhibited the highest H/E* and H3/E*2 ratios of 0.097 and 0.29, respectively, as well as the highest fracture toughness of 2.1 MPa·m1/2 and elastic recovery of 42.5%. Accordingly, the coatings possessed an enhanced adhesion and cohesion, in terms of micro-scratch critical load (LC3 = 19.67 N) and HF Rockwell HF1 level. The friction and wear response of hard yet tough TiN coatings under the normal load of 1–10 N were investigated to explore their durability and predict their critical load up to failure. Wear mechanisms changed from oxidative to severe abrasive wear, with load increasing from 1 to 10 N. At 2–5 N, a combination of oxidative and abrasive wear was observed. The coatings maintained their integrity up to the critical load of 9.4 N before failure event, with a maximum wear track depth of 1.8 μm, indicating their durability under the loading conditions.

Electropulsing of low-density duplex steel for strengthening and ductilization by microstructural refinement

The present investigation finds the effect of electropulsing treatment on the mechanisms of refining and reprecipitation of ordered L20 structure (B2) and its impact on tensile properties of low-density duplex steel of Fe-18Mn-10.5Al-1 C-6Ni composition. The selected composition without manganese is melted at 1600°C by induction heating in a vacuum but Mn is added into the melt at 1560°C, in the argon atmosphere to reduce the loss. The melt is cast into a plate form in a copper mold. The cast steel is homogenized, hot rolled, and annealed (PD1-A sample) to get elongated bands, spheroids, and platelets of B2 phase with an austenitic matrix which results in a yield strength of 1015 MPa, and tensile strength of 1285 MPa with a plastic elongation of 16.3%. Electropulsing partially dissolves the B2 phase at a temperature much lower than the equilibrium solvus by reducing barrier energy because of electron wind. As a result, the sizes of bands and spheroids are reduced, and platelets are disintegrated. Electron wind force deforms coarse precipitates, and fragments as well as spheroidized it. Electropulsing of annealed steel (PD1-AE) mobilizes dislocations, and recovers it in B2, but induces localized recrystallization of austenite. At a later time of the signal, the temperature rapidly falls and the matrix becomes supersaturated but electropulsing accelerates low-temperature precipitation of the B2 phase. Therefore, electropulsing can be adopted as a fast manufacturing process to dissolve, deform, fragment, spheroidize and reprecipitate high temperature phase at much lower temperature by dominant athermal effect of electron wind energy over thermal effect. Electropulsing also induces recovery and recrystallization at a relatively low temperature and reduces defect density. Reduction in the overall size of B2 and its distribution towards better uniformity provided an increased yield strength of 1077 MPa, tensile strength of 1355 MPa, and a plastic elongation of 21.6%. The selected duplex low-density steel follows two-stage work hardening of Ludwigson model with an additional stage of dynamic recovery. Fractography analysis confirms that both samples show mixed modes of ductile and brittle fracture, with an increase in size and area percentage of dimples.

Influence of Severe Plastic Deformation on the Magnetic Properties of Sm–Co Permanent Magnets

High pressure torsion (HPT) is presented as a new fabrication route to produce bulk Sm–Co magnets with a strongly refined microstructure down to the nanometer regime. The initial powders, based on the compositions SmCo5, Sm2Co7 and Sm2Co17, are compacted and subsequently deformed by HPT. The microstructural evolution in dependence on the applied deformation parameters is characterized by electron microscopy and the effect of HPT on the phase stability is monitored by synchrotron X‐ray diffraction. An increasing amount of applied strain leads to a stronger reduction in grain size while strain localization counteracts a homogeneous microstructural refinement. The positive effect of elevated deformation temperatures is demonstrated for Sm2Co17, which promotes homogeneous grain refinement, but causes strain‐induced phase transformations at the same time, strongly affecting the magnetic behavior. Superconducting quantum interference device magnetometry is used to characterize the magnetic properties after HPT deformation, which indicates the formation of a magnetic texture depending on the respective phase.

Refinement of Microstructure and Enhancement of Wear Resistance of Ni–Cr Cast Iron by NbC

This article investigates the influence of niobium addition on the microstructure and properties of Ni–Cr cast iron. In situ observations and analysis of phase precipitation during solidification are conducted through a high‐temperature laser scanning confocal microscope and Thermo‐Calc software. The refining mechanism of the microstructure and the enhancement of wear resistance by NbC are explored. The results indicate that with an increase in Nb content, the precipitation temperature of NbC particles gradually rises. When the niobium content exceeds 0.2%, NbC becomes the first precipitated phase during the solidification of the liquid phase. NbC particles can serve as heterogeneous nucleus for austenite, refining the grain structure of Ni–Cr cast iron. The existence of NbC particles within the grain impedes the elongation of martensite and bainite bundles while providing additional nucleation sites to accelerate the phase transformation. The grain refinement and the obstructive influence of NbC particles collectively induce a more disordered bainitic structure. Moreover, the exceptional hardness of NbC particles effectively safeguards the matrix from abrasion, consequently enhancing the wear resistance of Ni–Cr cast iron.

Revealing the microstructure, texture evolution and mechanical properties along the whole wall of Mg-Gd-Y-Zn-Zr cylinders by tailoring rotary backward extrusion parameters

In this article, industrial-specification Mg-Gd-Y-Zn-Zr alloy cylinder parts were successfully prepared by rotary backward extrusion process. The microstructure distribution and mechanical properties evolution of the whole wall of different cylinder parts were investigated, and the effects of different rotation speeds on the microstructure and properties were also revealed. The results showed that the strain of the cylinder wall increased significantly with rotation speed increased, thereby generating a higher driving force for the microstructure refinement. The transformation of the inner wall progressed from the deformed zone (bulk-shaped long period stacking ordered (LPSO) phases, coarse deformed grains, and fine recrystallized grains) to the strongly affected zone (particle-shaped LPSO phases and fine recrystallized grains), with the latter expanding as rotation speed increased. Furthermore, there was a gradient distribution of strain in the cylinder walls, with the strain gradient intensifying alongside rotation speed. The transformation of the strongly affected zone into the deformed region from the inner to the outer wall suggested a diminishing effect of rotation speed on LPSO phases fragmentation and grain refinement. Notably, increasing rotation speed rendered improvements in the strength and ductility of the cylinder wall. And mechanical properties also displayed a gradient decrease from the inner wall to the outer wall. This variation in mechanical properties of different regions was closely associated with the contributions of grain boundary strengthening, texture strengthening and second phase strengthening.

Influence of synchronized pulse bias on the Microstructure and Properties of CrSiN nano-composite ceramic films deposited by MIS-HiPIMS

In this study, CrSiN nano-composite ceramic films with optimized structure were prepared at low temperature using MIS-HiPIMS technique, owing to improved utilization of the high ionization characteristic of HiPIMS under specially designed synchronized pulsed-bias working mode. The correlations of the synchronized pulsed-bias width, the chemical composition, microstructure evolution, mechanical properties and tribological behaviors were studied systematically. According to the results, the waveforms of the pulsed bias experienced obvious fluctuating periods, indicating fluctuating intensities of positive ions arrived at the substrates. Meanwhile, the fluctuating duration increased with increasing synchronized pulse-width, providing an effective way of adjusting the energetic-ion flux for improved bombardment of the growing film. Therefore, obvious microstructure refinement of the as-deposited CrSiN films had been achieved. Due to optimized ion bombardment, the CrSiN films evolved from a coarse and transgranular columnar structure to a more densified columnar structure with the mixture phases of hcp-Cr2N, fcc-CrN as well as a-Si3N4. Moreover, the hardness and elastic modulus of the CrSiN films were also elevated significantly, to the maximum values of 20.2 GPa and 348.3 GPa, respectively. Furthermore, with the best mechanical properties as well as the highest density, the optimized CrSiN film deposited at 400 μs also displayed the most excellent wear resistance, with the wear rate as low as 9.1 × 10−16 m3/N.m. This study not only proposes a method of optimizing the structure and tribological properties of CrSiN nano-composite ceramic films at relatively low temperature, and also enriches the property database of CrSiN films which could expand their application in fields involving thermal-sensitivity materials.

Porosity suppression and mechanical property improvement of wire-arc directed energy deposited TiCp/Al-Cu alloys joint by oscillating pulsed wave laser beam welding

Hybrid manufacturing technology combining wire-arc directed energy deposition (WA-DED) and laser beam welding (LBW) presents a promising avenue for efficiently manufacturing large-size aluminum alloy components. The high tendency of porosity defects, particle decomposition, and fusion zone softening are outstanding challenges in the LBW welding of additively manufactured particle-reinforced aluminum alloys. Herein, a WA-DED-processed Al-6.3Cu alloy embedded with 0.6 wt% TiC particles was employed as weldment. Four different laser beam modes, including continuous wave (CW), pulsed wave (PW), oscillating continuous wave (OCW), and oscillating pulsed wave (OPW), were used to join TiCp/Al-6.3Cu alloy. The effects of different laser modes on the porosity defects, microstructure evolution, and mechanical properties of WA-DED particle-containing aluminum alloy joints were studied comparatively. Multi-scale characterization results showed that the enhanced molten pool flow via laser beam modulation effectively suppressed porosity defects and facilitated the uniform particle distribution. The OPW mode provides an optimal effect on porosity inhibition and weld microstructure refinement compared to the pulsed or oscillating modulation modes. The heat-treated LBW joint via OPW mode achieved excellent comprehensive mechanical properties, whose elongation and ultimate tensile strength reached 9.5% and 449 MPa, respectively. This enhanced performance can be attributed to the decreased porosity, refined weld microstructure, and high-density nano-precipitation. This study provides fundamental guidance for the welding of high-strength Al-Cu alloy fabricated by additive manufacturing.

Effects of two-step austempering processes on the wear resistance and fatigue lifetime of high-carbon nanostructured bainitic steel

The previous research on two-step austempering processes have been mainly focused on the bainitic transformation kinetics, with few details the relationships between the parameters of two-step austempering process and the wear properties and fatigue lifetime. In this paper, the effects of two-step austempering process on wear resistance and fatigue life of high carbon nano-bainite steel were investigated. The results show that the first-step bainite with high volume fraction prolongs the total bainite transformation time. The two-step austempering samples contain finer bainitic ferrite plate and higher volume fraction of retained austenite, as well as resent excellent hardnesses, tensile properties, wear resistance and fatigue lifetime. In particular, the wear resistance of the 50%-290 °C sample is better than that of the 210 °C sample, this is caused by the transformation of the high fraction of retained austenite in the two-step austempering samples into hard martensite and the microstructure refinement zone that forms on the surface of the sample during the wear process. The 50%-290 °C sample have better fatigue properties at low total strain amplitudes (0.7% and 0.8%). When the total strain amplitude is 0.7%, the fatigue life of the 50%-290 °C sample is 57.5% higher than that of the 210 °C sample, which is because of the higher yield strength and finer microstructure of the 50%-290 °C sample. These results support the later study of controlling two-step austempering process for high-carbon nanostructured steels.

Reduction of oxygen content in laser powder bed fusion process atmosphere – Effects on stochastic defect formation and mechanical properties

Stochastic defects in materials manufactured via laser powder bed fusion (LPBF) can severely compromise mechanical performance and are challenging to predict and detect, thus motivating the development of defect mitigation strategies. Particle oxidation is a factor well-known to generate defects by disturbing melt pool dynamics. If the particles are spatters, additional disturbances increase the likelihood of defect formation. In this study, restricting oxygen content in the process atmosphere to 50 ppm is investigated to minimize stochastic spatter-induced defects and improve the mechanical properties of Hastelloy X. Specimens were manufactured under this condition at two nominal layer thicknesses, analyzed for internal defects, and mechanically tested. Contrary to expectations, reducing the oxygen content did not prevent spatter-induced defect formation; rather, it could exacerbate the formation of more numerous and larger defects. Nevertheless, this tighter control of the process atmosphere led to significant microstructural refinement, which, when combined with sparse defects, resulted in improved fatigue performance. Despite the inherent ductility of Hastelloy X, the presence of abundant defects significantly larger than the microstructural characteristic size proved detrimental to fatigue performance. Notably, the occurrence of defects exhibited considerable variation across the build area, contributing to scatter in fatigue data. However, quantitative analysis of in-situ monitoring data enabled prediction of variability in defect content and mechanical performance.

High-temperature strength and kink-band formation in mille-feuille-structured Ti/FeTi eutectic alloys

To develop ultrahigh-strength and lightweight structural materials, the temperature dependence of the deformation behavior of Ti/FeTi eutectic alloys was examined using compression tests. An extremely high yield stress of ∼2.3 GPa was obtained at room temperature accompanied by ∼5 % of plastic strain by the alignment of a “flower-like” lamellar microstructure. High stress was maintained up to 400 °C; however, the yield stress considerably decreased to ∼0.8 GPa at 600 °C. Shear deformation was observed at room temperature. However, at high temperatures, at 600 °C and above, the formation of a deformation kink band was observed for the first time as the deformation mechanism, and this resulted in a rapid decrease in yield stress. The size of the kink bands was governed by the diameter of the aligned lamellar grains, which was termed the colony size. Large kink bands were formed in the alloy containing coarse colonies, resulting in the buckling of the specimen. The introduction of primary β-Ti grains in the eutectic microstructure improved the ductility at low temperatures, which was only accompanied by the marginal decrease in yield stress to ∼1.9 GPa. Furthermore, the high-temperature strength of the hypoeutectic alloy at 600 °C showed a higher value of ∼1.0 GPa compared to that in eutectic alloys. The refinement of the colony microstructure of the hypoeutectic alloy suppressed the formation of kink bands. The experimental results suggest that microstructural control is extremely important for improving the high-temperature strength of Ti/FeTi eutectic alloys by increasing the formation stress of the kink bands.

Effects of hot extrusion on compressive strength and densification of aluminium-based metal matrix composites

ABSTRACT Metal matrix composites (MMC) made from powder metallurgy (PM) require post-processing to improve the mechanical properties or achieve the final shape. In this experimental study, Aluminium metal matrix composite (Al-MMC) was first fabricated through PM, followed by a hot extrusion process to study the effects of extrusion on compressive strength and density. Hot extrusion of Al-MMC results in microstructure refinement, enhanced dispersion of reinforcement particles and improved mechanical properties. The experimental findings revealed that hot extrusion of Al-MMC resulted in densification, reflected as an average improvement of 5% in the density. An average improvement of up to 22% was observed in the compressive strength of the Al-MMC. Therefore, hot extrusion can be considered an effective post-processing method for enhancing the Al-MMC fabricated through PM. It was also found experimentally that the extrusion ratio also affects the mechanical properties and performance of Al-MMC. It was revealed that the compressive strength is enhanced with an increase in extrusion ratio. It is also important to note that the PM process is one of the processes which offers reasonable control over the utilisation of starting material. Hence, reinforcement retention is higher than other methods like stir casting.

Sustainable utilization of low-value lithium-ion battery wastes in cement and concrete

Spent LiFePO4 batteries will surge soon due to the global trend towards adopting electric vehicles. The recycling of the batteries leaves low value FePO4 dominated waste (FPW), which is currently not efficiently repurposed. This study aims to reveal the characteristics of FPW, investigate its effects on cement hydration, and explore potential applications in the concrete industry. FPW was used to replace cement at increased ratios up to 10%. The results showed that a suitable dosage of the battery waste (e.g. 5%) enabled the blended cement binder to achieve refined pore structure and enhanced 7- and 28-day compressive strength. FePO4 reacted in cement binder which consumes crystal CH and generates amorphous hydroxyapatite, potentially leading to microstructure refinement. Additionally, FPW exhibited the potential to be developed as a concrete retarder. Continuous retarding on cement hydration was observed, primarily for C3S and C3A at an early age of 1 day, and turned to C2S and C4AF thereafter.

Fabrication of tungsten inert gas-welded dissimilar aluminium joints using different friction stir processing tools through response surface method

ABSTRACT AA6061-T6 and AA7075-T6 materials find wide application in manufacturing industries due to their excellent engineering properties. Fusion welding generates solidification cracks, high residual stresses, and brittle intermetallic compounds that degrade joint qualities. To solve these difficulties, friction stir processing (FSP) is frequently employed, which improves joint properties through local microstructural refinement. In this study, the combined effect of FSP parameters, namely tool pin profile (TPF), tool rotation speed (TRS), and travel speed (TS), on tungsten inert gas (TIG) weld properties was investigated. Response surface methodology (RSM)-based mathematical models were developed to predict responses, namely ultimate tensile strength (UTS), impact toughness (IT), microhardness (MH), and residual stress (RS). The optimal FSP parameters were determined through the desirability function technique. Post-FSP increased the UTS, IT, and MH of TIG welds by 71.51%, 95.40%, and 72.46%, respectively, and decreased the RS by 58.46%. ANOVA findings exhibited that TPF was the most influential parameter, followed by TRS and TS. The dissimilar aluminium TIG+FSP joints showed an optimal UTS of 282.08 MPa, an IT of 17.32 J, a MH of 121.48 HV, and an RS of 22.09 MPa when using a TRS of 1108.49 rpm and a TS of 37.74 mm/min with a THF pin.

Intergranular stress and plastic strain formation during laser scanning of additively manufactured stainless steel: An experimentally-driven thermomechanical simulation study

A novel experiment-driven modelling and simulation approach is proposed to study the formation of intergranular stresses and plastic strains during laser scanning of additively manufactured 316L stainless steel. Recently, laser scanning experiments were performed using a unique coupling between a continuous-wave laser and a scanning electron microscope. These experiments form the basis for the development of a thermo-elasto-viscoplastic polycrystal model endowed with the minimum necessary physics of the problem in order to facilitate simulating sufficiently large domains to obtain reasonable stress magnitudes. The case of a single laser line scan performed in vacuum is analyzed in detail. Polar dislocation density magnitudes and distribution predicted from the simulation are compared with those measured experimentally. Results reveal that for 93% of the grains in the lasered zone, a statistical measure of the predicted polar dislocation density (Nye’s) tensor lies within a factor of 2 (higher or lower) of the experimental one; this result sets a benchmark for future experiment-simulation comparisons. Subsequent investigation studies the origin of the local residual stresses and plastic strains. On the lasered surface, the grain surface-averaged normal residual stress component and plastic strain component along the scan direction reaches a value ∼1.7GPa and 0.04, respectively. The contribution of elastic anisotropy, plastic heterogeneity and strengthening due to microstructure refinement after laser scanning on the formation of stresses and plastic strains is studied in detail.

The Role of Rare Earths on Steel and Rare Earth Steel Corrosion Mechanism of Research Progress

Corrosion has always been an important factor affecting the life of steel, which causes huge economic losses every year. How to improve the corrosion resistance of steel has always been a research focus. Adding rare earth elements into steel is an important method to improve the corrosion resistance of steel. In this paper, the effects of rare earth elements on steel are summarized, including the purification of molten steel, modification and modification of inclusions, improvement of grain boundaries by solid solution strengthening, the influence of phase transformation and the refinement of microstructure, and reduction in C and N desolubilization. On this basis, the progress of research on the corrosion resistance mechanisms of rare earth steel is summarized, focusing on rare earth-modified inclusions. This includes the changes in composition and sizes of inclusions by rare earth addition, promoting the transformation of MnS and Al2O3 in rare earth inclusions with regular shapes, smaller sizes and better performance, or composite rare earth inclusions. The corrosion pits that form in the early stages of corrosion are shallow in depth, fewer in number and light in corrosion degree. The effects of rare earth materials on the rust layer include: rare earth promotes the formation of a more stable corrosion product α-FeOOH, and rare earth promotes the formation of a dense rust layer, which covers the surface of the matrix and hinders the transmission of corrosion ions. The protective effect of the rare earth atomic layer on the substrate and the corrosion inhibition effect of rare earth ions are formed by the segregation of rare earth at the interface. In the end, the existing problems in the research into rare earth steel and future research directions are briefly put forward, including improving the addition process of rare earth steel, theoretical guidance on enhancing the corrosion resistance mechanism of rare earth steel, and extending the research from La, Ce, and Y steel to more rare earth steels.

Parts repairing and microstructural refinement of high-pressure die cast aluminum alloys through friction stir processing for bulk production

A key challenge in the production of high-grade automotive aluminum components through the High-Pressure Die Casting (HPDC) process is the imperative to minimize imperfection. In addressing this concern, this study utilizes friction stir processing (FSP), a widely recognized intense plastic deformation technique. FSP is applied to systematically alter the microstructure of HPDC Al-4Mg-2Fe, a prominent alloy extensively used in the die-casting sector. By using the pass strategy to incorporate both one-pass and two-pass approaches, the microstructure is selectively altered to establish a defect-free processed zone. The utilization of FSP demonstrates its efficacy in breaking aluminum dendrites and acicular silicon particles, leading to a uniformly dispersed arrangement of equiaxed silicon particles within the aluminum-based matrix. In addition, FSP eradicates porosity and disintegrates needle-like Fe particles, resulting in a more refined and homogeneously distributed structure. Subsequently, the material's mechanical properties processed by FSP were assessed in the longitudinal direction concerning the processing axis and then compared with those of the original base material.The microstructural refinement and reduction in porosity induced by FSP result in a notable enhancement in hardness, with an increase of 23 % after one pass and 37 % after two passes. The substantial improvement in mechanical properties during the FSP process is predominantly attributed to modifications in the morphology, refinement, and dispersion of intermetallic particles within the matrix. This improvement is further complemented by the ultrafine dispersion of casting defects.This study underscores the efficacy of FSP as a valuable tool for modifying microstructures and improving mechanical properties in HPDC Al-4Mg-2Fe alloys. Such advancements align with the lightweighting objectives pursued by the automotive industry.

Investigation on Mechanism of Microstructure Evolution during Multi-Process Hot Forming of GH4169 Superalloy Forging.

Typically, in the manufacturing of GH4169 superalloy forgings, the multi-process hot forming that consists of pre-deformation, heat treatment and final deformation is required. This study focuses on the microstructural evolution throughout hot working processes. Considering that δ phase can promote nucleation and limit the growth of grains, a process route was designed, including pre-deformation, aging treatment (AT) to precipitate sufficient δ phases, high temperature holding (HTH) to uniformly heat the forging, and final deformation. The results show that the uneven strain distribution after pre-deformation has a significant impact on the subsequent refinement of the grain microstructure due to the complex coupling relationship between the evolution of the δ phase and recrystallization behavior. After the final deformation, the fine-grain microstructure with short rod-like δ phases as boundaries is easy to form in the region with a large strain of the pre-forging. However, necklace-like mixed grain microstructure is formed in the region with a small strain of the pre-forging. In addition, when the microstructure before final deformation consists of mixed grains, dynamic recrystallization (DRX) nucleation behavior preferentially depends on kernel average misorientation (KAM) values. A large KAM can promote the formation of DRX nuclei. When the KAM values are close, a smaller average grain size of mixed-grain microstructure is more conductive to promote the DRX nucleation. Finally, the interaction mechanisms between δ phase and DRX nucleation are revealed.

Assessing the tribo-corrosion resistance of surface nanostructured stainless-steel

In this work, surface mechanical rolling treatment (SMRT) was applied as a post-process nanostructuring technique to improve the tribo-corrosion degradation resistance of coarse grain (CG) 316L stainless steel (SS). The experimental findings reveal that when tribo-corrosion wear took place, the CG specimen experienced severe surface de-passivation, which increased wear-corrosion synergy and the rate of material loss. On the other hand, the SMRT specimen experienced enhanced re-passivation kinetics, which reduced wear-corrosion synergy and the rate of material loss. The wear mechanism for both the CG and SMRT specimens was abrasion-dominant, with the CG specimen having the presence of micro-pits within the wear track. In addition to these findings, the reduced surface roughness, improved hardness, formed compressive stresses, microstructural refinement, and martensite phase transformation induced by the SMRT process contributed to the enhanced tribo-corrosion resistance of the SMRT specimen. In total, SMRT was found to be a feasible technique to improve the tribo-corrosion resistance of CG 316L SS.