Hardness Evolution Research Articles

Comparative study of microstructural evolution in W-3Re alloy under high-temperature conditions: High heat flux loading versus furnace heating

In tokamak devices, tungsten (W), as the main candidate for plasma facing material, will suffer from the simultaneous challenges of high heat flux (HHF) and neutron irradiation. This study involved annealing experiments on W-3Re alloy using two distinct heating methods: furnace heating and electron beam heating. The evolution of hardness and microstrure of tungsten-rhenium (W-3Re) alloy after isochronous annealing and isothermal annealing has been analyzed and compared with pure tungsten (PW). The recrystallization kinetics of W-3Re alloy have been obtained employing the Johnson Mehl-Avrami Kolmogorov (JMAK) equation. Experimental results showed that recrystallization kinetics during electron beam annealing were faster than those in furnace annealing. This phenomenon can be attributed to the rapid heating process of electron beam annealing, which preserved more deformation energy within the material, thus providing more driving force for recrystallization at high temperatures. Additionally, Re appeared to inhibit grain boundary migration in W-3Re alloy, causing the recrystallization temperature to be approximately 100 °C higher than in pure tungsten, and also resulted in a smaller average grain size compared to pure tungsten. This research provides experimental data and reference for predicting the microstructural evolution of tungsten-rhenium alloy in future fusion reactor environments.

INTEGRAL view of GRB 221009A

The gamma-ray burst GRB 221009A is among the most luminous of its kind and its proximity to Earth has made it an exceptionally rare observational event. The International Gamma-ray Astrophysics Laboratory (INTEGRAL) was in an optimal aspect position to use its all-sky instruments for recording the prompt emission and early gamma-ray afterglow in unprecedented detail. Following the initial detection, a swiftly scheduled follow-up observation allowed for the hard X-ray afterglow time and spectral evolution to be observed for up to almost a week. The INTEGRAL hard X-ray and soft gamma-ray observations have started to bridge the energy gap between the traditionally well-studied soft X-ray afterglow and the high-energy afterglow observed by Fermi/LAT. We discuss the possible implications of these observations for follow-ups of multi-messenger transients with hard X-ray and gamma-ray telescopes.

Hardness and fracture toughness enhancement in transition metal diboride multilayer films with structural variations

The simultaneous enhancement of hardness (H) and fracture toughness (KIC) through the formation of superlattice structures challenges the conventional belief that these quantities are mutually exclusive. Here, this approach has been applied to the transition metal diborides, whose inherent brittleness severely restricts their application potential. The mechanical properties of TiB2/TaB2 systems as a function of bi-layer period Λ are investigated, combining theoretical and experimental approaches. Density Functional Theory is used to investigate the structural stability and mechanical properties of stoichiometric hexagonal TiB2/TaB2 superlattices for Λ = 3.9 – 11.9 nm. The calculations predict the highest H = 38 GPa and KIC (100) of 3.3 MPa.m1/2 at the value of Λ = 5.2 nm. Motivated by the theoretical results, multilayer films with Λ = 4–40 nm were prepared by direct current magnetron sputtering. Due to the sputtering effects, the deposited diboride films differ significantly from the view of stoichiometry and structure. A detailed structure investigation reveals TiB2/TaB2 in form of superlattices exhibiting coherent interfaces for Λ = 4 nm. For higher Λ, parts of TaB2 layers transform from the crystalline to the disordered phase. These transformations are reflected in the mechanical properties as measured by nanoindentation and micromechanical bending tests. The evolution of hardness follows Hall-Petch behavior, reaching a maximum of 42 GPa at Λ = 6 nm. Enhancing fracture toughness involves more complex mechanisms resulting in two KIC maxima: 3.8 MPa.m1/2 at Λ = 6 nm and 3.7 MPa.m1/2 at Λ = 40 nm.

Investigating the Dynamic Mechanical Properties and Strengthening Mechanisms of Ti-6Al-4V Alloy by Using the Ultrasonic Surface Rolling Process.

The demand for titanium alloy has been increasing in various industries, including aerospace, marine, and biomedical fields, as they fulfilled the need for lightweight, high-strength, and corrosion-resistant material for modern manufacturing. However, titanium alloy has relatively low hardness, poor wear performance, and fatigue properties, which limits its popularization and application. These disadvantages could be efficiently overcome by surface strengthening technology, such as the ultrasonic surface rolling process (USRP). In this study, the true thermo-mechanical deformation behavior of Ti-6Al-4V was obtained by dynamic mechanical experiment using a Hopkinson pressure bar. Moreover, USRP was applied on the Ti-6Al-4V workpiece with different parameters of static forces to investigate the evolution in surface morphology, surface roughness, microstructure, hardness, residual stress, and fatigue performance. The strain rate and temperature during the USRP of Ti-6Al-4V under the corresponding conditions were about 3000 s-1 and 200 °C, respectively, which were derived from the numerical simulation. The correlation between the true thermo-mechanical behavior of Ti-6Al-4V alloy and the USRP parameters of the Ti-6Al-4V workpiece was established, which could provide a theoretical contribution to the optimization of the USRP parameters. After USRP, the cross-sectional hardness distribution of the workpiece was shown to initially rise, followed by a subsequent decrease, ultimately to matrix hardness. The cross-sectional residual compressive stress distribution of the workpiece showed a tendency to initially reduce, then increase, and finally decrease to zero. The fatigue performance of the workpiece was greatly enhanced after USRP due to the effect of grain refinement, work hardening, and beneficial residual compressive stress, thereby inhibiting the propagation of the fatigue crack. However, it could be noted that the excessive static force parameter of USRP could induce the decline in surface finish and compressive residual stress of the workpiece, which eliminated the beneficial effect of the USRP treatment. This indicated that the choice of the optimal USRP parameters was highly crucial. This work would be conducive to achieving high-efficiency and low-damage USRP machining, which could be used to effectively guide the development of high-end equipment manufacturing.

Role of high-speed nanoindentation mapping to assess the structure-performance correlation of HVAF-sprayed Cr3C2-25NiCr coating

Cr3C2-NiCr coatings deposited by high-velocity air-fuel (HVAF) spray can potentially exhibit superior wear performance. However, the microstructural variations caused by altering HVAF process parameters and secondary heat treatment considerably impact coating characteristics and wear performance. The effective characterization of such microstructural variations at the micrometer length scale using conventional techniques is limited. In this study, we employ a high-speed nanoindentation mapping technique to spatially map the properties of HVAF sprayed Cr3C2-NiCr coating deposited by varying particle size-combustion mode combination, which was in the as-deposited as well as heat-treated state. Despite the significant microstructural heterogeneity at a small scale, the hardness maps show excellent correlation with the microstructure. The data extracted from the nanoindentation maps estimate the area fraction and average hardness of individual phases (carbide, binder, interacted region) in the coatings. Insights on the role of individual constituents on the coating microstructure and mechanical property evolution are presented. The hardness evolution of the individual constituents with processing conditions and secondary heat treatment is correlated with the solid particle erosion performance of the coatings. Overall, Cr3C2-NiCr coatings deposited through the correct choice of particle size-combustion mode combination and heat treatment improved the desired phase distribution, resulting in superior erosion resistance.

Atomic structure of clusters and GP-zones in an Al-Mg-Si alloy

The structures of atomic clusters and GP-zones preceding precipitation in an Al-0.70Mg-0.85Si-0.15Fe-0.25Mn (wt. %) alloy have been investigated by annular dark field scanning transmission electron microscopy imaging and density functional theory calculations. One analysed condition consisted of one-month natural aging, and another was in a pre-baked state after 24 h aging at 90 °C. To quantify the effect of clusters and GP-zones on microstructure development during a subsequent isothermal artificial aging at 185 °C, hardness evolution was measured, and precipitate statistics was performed at the peak hardness corresponding to 5 h aging. Several types of clusters and GP-zones have been observed and based on structure and visual appearance named 'Disordered Frank-Kasper', 'Square', 'Binocular', and 1β", 2β", 3β", 4β". The first four are most common in the natural aged condition and are associated with a well-known influence on the initial hardness evolution. The last three are fractions of the hardening β" phase and refer to the number of structural units they contain. These GP-zones have an increased density in the pre-baked condition, leading to a rapid increase in hardness during artificial aging and to a finer microstructure of high-density small needle precipitates at peak hardness.

Laser ablation behavior and mechanism of Cf/C–SiC composites under different laser energy densities

Ceramic matrix composite (CMC) is typical difficult-to-machine material, laser assisted machining (LAM) is an effective method to solve the problem of poor machinability, and a deep understanding of the interaction between laser and material can lay a foundation for LAM. Aiming at the unclear mechanism of microstructure formation, crack propagation and material hardness evolution of laser-irradiated CMC, laser ablation behavior and mechanism of Cf/C–SiC composites under different laser energy densities were studied in this paper. The results show that Cf/C–SiC presents two different states of modification and ablation under irradiating. In modified state, fiber interface in central area is oxidized, and crystal transformation occurs. Amorphous Si–O–C, Si and spherical SiO2 exist in edge deposition area, and continuous phase disappears in heat affected zone. In ablative state, carbon nanosheets and clusters are formed in ablation groove. Surface of convex and deposition area are composed of sedimentary basement and clusters, and there are recast layer and heat affected zone in cross section. When thermal stress exceeds interfacial bond strength and critical matrix stress, the fiber interface debonding and matrix cracking occur, and both transgranular and intergranular fracture exist in the matrix. When laser scanning direction is perpendicular to fiber extension, crack propagation can be inhibited. The propagation and intersection of cracks and coarsening of SiC crystal phase in heat affected zone lead to the decrease of matrix microhardness. The results of this study can be used to provide references for parameters selection and theoretical support for LAM of CMC.

Enhanced Microhardness and Corrosion Performance of Additively Manufactured Inconel 718 Specimens through Nanostructuring by Severe Plastic Deformation

Severe plastic deformation (SPD) processes, particularly high-pressure torsion (HPT) have been increasingly applied to metallic specimens fabricated by laser powder bed fusion (L-PBF) additive manufacturing (AM) for enhancing their mechanical and functional properties through nanoscale grain refinement (≤ 100 nm). In this study. L-PBF AM-fabricated Inconel 718 (IN 718) specimens are initially subjected to 10 HPT revolutions to produce nanosized grains. Subsequently, microstructural characterisation, as well as hardness and electrochemical tests are conducted to evaluate the evolution of microstructures, hardness, and corrosion performance of the as-received and HPT-processed specimens by using various microscopy, Vickers microhardness (HV) measurements, and corrosion performance, respectively. The results reveal an average grain size of ~ 46 nm, dense dislocation networks, and nanotwins after 10 HPT processing, which contribute to the two-fold hardness increase compared to the as-received condition. Such microstructures also contributed to the overall improved corrosion performance after 10 HPT processing, as quantified by the 83% and 73% reduction in corrosion rate and pitting potential, respectively.

Dislocation Hardening in a New Manufacturing Route of Ferritic Oxide Dispersion-Strengthened Fe-14Cr Cladding Tube.

The microstructure evolution associated with the cold forming sequence of an Fe-14Cr-1W-0.3Ti-0.3Y2O3 grade ferritic stainless steel strengthened by dispersion of nano oxides (ODS) was investigated. The material, initially hot extruded at 1100 °C and then shaped into cladding tube geometry via HPTR cold pilgering, shows a high microstructure stability that affects stress release heat treatment efficiency. Each step of the process was analyzed to better understand the microstructure stability of the material. Despite high levels of stored energy, heat treatments, up to 1350 °C, do not allow for recrystallization of the material. The Vickers hardness shows significant variations along the manufacturing steps. Thanks to a combination of EBSD and X-ray diffraction measurements, this study gives a new insight into the contribution of statistically stored dislocation (SSD) recovery on the hardness evolution during an ODS steel cold forming sequence. SSD density, close to 4.1015 m-2 after cold rolling, drops by only an order of magnitude during heat treatment, while geometrically necessary dislocation (GND) density, close to 1.1015 m-2, remains stable. Hardness decrease during heat treatments appears to be controlled only by the evolution of SSD.

Scaling-up laser cladding of rails

With rail travel getting more popular more demands are being put on rail infrastructure. The wheel/rail contact is the one of the main parameters that is key to maintaining a high performing network. With increasing demands for higher loads and higher speeds, wear and rolling contact fatigue (RCF) are the main two problems.New maintenance techniques can overcome some problems, but new solutions that reduce the damage are needed. Laser cladding is one of the best methods for improving wear and RCF of rail steels by adding layers of more durable materials to the rail.In this article full-scale tests of clad layers have been carried out at realistic contact conditions to verify outcomes seen in twin-disc testing to see if the results can be scaled to predict field performance of clad layers. Martensitic Stainless Steel (MSS) and Stellite 6 were used for the clad layers and R260 was used as the base material. Un-clad R260 was used as reference material.Results of full-scale tests in terms of wear rates, sub-surface deformation, hardness and roughness evolution were comparable. This showed that the clad layers deposited on actual rail performed very well compared to un-clad rail and that the tests run at the small-scale were appropriate for predicting full-scale behaviour.

Evolution of high-temperature hardness of multimodal γ′ nickel-based superalloy

Hardness reflects the comprehensive mechanical properties of materials, and its evolution during a wide temperature range reflects high-temperature performance. This study investigates the evolution of the high-temperature hardness of a nickel-based superalloy Ni16Cr13Co4Mo. Various thermal cycles featuring distinct peak heating temperatures were utilized to examine the variation of Vickers hardness with temperature. The effect of precipitated phases (specifically the γ′ phase) in the superalloy on high-temperature hardness and the deformation mechanism of the alloy was analyzed. The results show that the high-temperature hardness of the nickel-based superalloy decreases as temperature increases. However, the hardness after thermal cycles demonstrates an increase which is determined by the peak heating temperature. The increased hardness following thermal cycles is attributed to the reduction in size and the increase in the volume fraction of secondary γ′. Detailed TEM observation revealed that as the peak temperature increases, the deformation mechanism transforms from dislocation pile-ups in the γ matrix, dislocation and stacking fault shearing, microtwining in γ and γ′ phases to stacking fault shearing and piled-up dislocations in γ phase. As the result, hardness decreases as temperature increases.

An Industrial-Scale Study of the Hardness and Microstructural Effects of Isothermal Heat Treatment Parameters on EN 100CrMo7 Bearing Steel

The 100CrMo7, commonly employed for bearings in rotating machinery, relies on precise heat treatment parameters to ensure an optimal microstructure and, in turn, mechanical properties. Typically, an austenitizing treatment, followed by rapid cooling in a salt bath for martempering or austempering, is crucial in achieving the desired microstructure and hardness. The present industrial-scale study involved a comparative analysis between martempering and austempering routes regarding the hardness and microstructure evolution of EN 100CrMo7 large-size rings. The investigation delves into the effects of austempering temperatures, holding times, and austenitizing temperature. Furthermore, the role of tempering in reducing the amount of retained austenite was also considered. The results indicate that martempering yielded the highest hardness values while austempering exhibited a decrease in hardness at the center of the rings, though a lower amount of retained austenite (in the range of 3.0–4.9 vol.%) was detected in comparison with martempering. In addition, a 850 °C austenitizing temperature reduced the hardness by 16% in the center of the rings and promoted a high content of upper bainite, thus suggesting its inefficacy for the involved large-size rings. In contrast, a 880 °C austenitizing temperature maintained consistently high HRC values across the ring’s height. Lastly, the analysis highlighted that the cooling rate played a more crucial role than the austempering holding time. Such industrial-scale investigations performed on real components improve the knowledge and control of heat treatment parameters in comparison with the nominal guidelines provided by steel suppliers. These outcomes offer insights for optimizing industrial heat treatment parameters, with practical implications for enhancing steel bearings’ microstructural and mechanical performance and lifespan.

Quantification of micro-damage evolution process in ceramics through extensive analysis of multi-source heterogeneous data

Due to the randomness of hard and brittle material damage evolution, assessing damage and interpreting mechanisms is challenging. This study focuses on analyzing material damage mechanisms by employing extensive multi-source heterogeneous data to extract quantitative information. A machine vision-based method for detecting and assessing defects in ceramics was proposed, achieving 98 % accuracy. Leveraging this method in scratching experiments, the progression of damage and micro-failure mechanisms in Si3N4 was investigated through the analysis of defect area distribution, force and acoustical signals. Results revealed that the distribution of defect areas conforms to a three-parameter Weibull distribution. At scratch depths of 0.01, 0.02 mm, approximately 90 % of damage accumulated within 1e2–1e4 μm2; conversely, 0.03–0.06 mm led to a decrease to 30 – 40 %. Extensive crack propagation occurs within 0.02–0.03 mm, resulting in the intersecting formation of spalling blocks. Damage evolution occurs in three stages: shallow and small cracks with scattered spalling blocks in the first stage, deep and long cracks with continuous and irregularly shaped spalling blocks in the second stage, and nearly no cracks with continuous and regularly shaped blocks in the third stage. This study not only proposes an efficient method to detect and evaluate ceramic defects, but also deeply understands the evolution process of material damage mechanism. Considering defect distribution and statistical trends, this research not only provide guidance for the optimization of machining parameters, especially in the scene of specific precision and high removal rate. At the same time, the damage evolution and failure mechanism of Si3N4 provide useful experimental basis and data support for deepening the understanding of material behavior during ultra-precision grinding.

Effect of sigma (σ) phase precipitation on the recrystallization kinetics of AISI 904L superaustenitic stainless steel

Microstructural evolutions, σ-phase precipitation, and recrystallization kinetics of cold-rolled AISI 904L stainless steel during annealing were studied in the present work. The microstructure and hardness evolution were different in the temperature ranges of T < 950 °C (with σ-phase precipitation) and T ≥ 950 °C. The former range was associated with the fall of hardness due to recrystallization softening, followed by hardening due to the concurrent σ-phase precipitation; while the latter range was associated with the continuous fall of hardness. Using the Johnson–Mehl–Avrami–Kolmogorov (JMAK) analysis, the recrystallization activation energy in these ranges were respectively determined as 325 and 257 kJ/mol, where the higher activation energy in the former range was correlated to the retardation effect of the σ-phase. Via applying the Kissinger's equation to the differential scanning calorimetry (DSC) results, the activation energy of ∼332 kJ/mol was obtained, which was consistent with the precipitation of the σ-phase.

Geological timescales' aging effects of lunar glasses.

Physical aging is a long-lasting research hot spot in the glass community, yet its long-term effects remain unclear because of the limited experimental time. In this study, we discover the extraordinary aging effects in five typical lunar glassy particles with diameters ranging from about 20 to 53 micrometers selected from Chang'e-5 lunar regolith. It is found that geological time scales' aging can lead to unusually huge modulus enhancements larger than 73.5% while much weaker effects on hardness (i.e., varies decoupling evolutions of Young's modulus and hardness during aging) in these lunar glassy samples. Such extraordinary aging effects are primarily attributed to the natural selected complex glassy compositions and structures, consistent with high entropy and minor element doping criteria, prevailing under the special lunar conditions and the extensive aging time for the lunar glasses. This study offers valuable insights for developing high-performance and stable glassy materials for radiation protection and advanced space explorations.

Optimization, formation, and evolution of the photoinduced curing gradients and in-situ lamellar gaps in additive manufacturing of ZrO2 ceramics: From curing to sintering behaviors

The photoinduced curing gradients and in-situ lamellar gaps were related to the three-dimensional curing mechanism of stereolithography and researched in this work. Based on the photocuring mathematic theories presented by our team, photoinduced curing gradients were influenced by printing parameters such as laser power and frequency, scanning angle, and printing thickness. Among these curing gradients, the inadequate curing area was the main factor affecting the surface quality and physical properties of ceramics. The in-situ lamellar gaps were derived from the inadequate curing area. The lamellar gap size of green bodies was enlarged in brown bodies (sintering at 1170 ℃) and shrunk in sintered bodies (sintering at 1500 ℃). The correlations between the surface quality and printing parameters were clarified by establishing mathematical formulations of the lamellar gap size. Furthermore, anisotropic volume shrinkage, anisotropic surface topography, and density and hardness evolutions were associated with the wrapped particle density differences induced by the photoinduced curing gradients. Finally, the optimal printing parameters (75% of laser power, laser frequency of 150 kHz, scanning angle of 90º, printing thickness of 25 μm, and scanning speed of 2.5 m/s) were obtained by the orthogonal experimental analysis. This study provides a distinct understanding of the high-quality forming and extreme-performance manufacturing of stereolithography.

X-ray line profile analysis study on the evolution of the microstructure in additively manufactured 316L steel during severe plastic deformation

316L stainless steel was manufactured by additive manufacturing (AM), and then, the samples were severely deformed by the high-pressure torsion (HPT) technique. The evolution of the microstructure was monitored by X-ray line profile analysis. This method gives the crystallite size and the density of lattice defects, such as dislocations and twin faults. The AM-processing of the HPT disks was performed in two different modes: the laser beam was parallel or orthogonal to the normal direction of the disks. The subsequent HPT deformation was carried out for ½, 1, 5 and 10 turns. The microstructure and hardness evolution during HPT were similar regardless of the laser beam direction. For both sample series, the minimum achievable crystallite size was about 30 nm, while the dislocation density and the twin fault probability got saturated at the values of 300–350 × 1014 m−2 and 3.5–4%, respectively. The microstructure evolution during HPT of the AM-prepared 316L steel was compared with the HPT-induced changes in an as-cast counterpart. It was found that while the AM-prepared 316L steel remained a single-phase face-centered cubic γ-structure during HPT, in the as-cast samples a body-centered cubic (bcc) martensitic α-phase became the main phase with increasing the imposed strain of HPT due to the lower Ni content. In the saturation state achieved by HPT the initially as-cast 316L steel had a considerably higher hardness (about 6000 MPa) than that for the AM-prepared samples (~ 5000 MPa) due to the large fraction of the hard bcc phase formed during HPT.

Effect of high temperature annealing on the microstructure evolution and hardness behavior of the Inconel 625 superalloy additively manufactured by laser powder bed fusion

Additive manufacturing of Inconel 625 components attracts great interest due to its ability to produce parts with complex geometries that are needed for high-temperature applications in the aerospace, energy, automotive and chemical industries. To take full advantage of the potential of additive manufacturing, an in-depth understanding of the effects of prolonged high-temperature annealing on microstructure and hardness evolution is needed. Previous research in this field has mainly focused on a limited range of temperature and time. This study aims to determine the effect of prolonged high-temperature annealing on the evolution of intermetallic phases and carbides, as well as changes in the dislocation substructure of Inconel 625 superalloy additively manufactured by laser powder bed fusion subjected to stress relief annealing and subsequent isothermal annealing at a temperature up to 800 °C for 5–500 h. The microstructure development is correlated with hardness behaviour. It is determined that the microstructure evolution proceeds in four stages with temperature and time increase. In the initial stress-relieved condition, a cellular microstructure with nano-sized precipitates of the Laves phase and NbC carbides at the cell walls occurs, and hardness is equal to 300 HV10. In the 1st stage of the microstructure evolution, the γ'' phase particles precipitate on the cell walls, which results in hardening up to 383 HV10 in the specimen annealed at 700 °C for 5 h. The 2nd stage involves the precipitation of the γ'' phase both on the cell walls and inside the cells, as well as the formation of dislocation networks, which contribute to the softening effect and hardness drop to 319 HV10. In the 3rd stage, at temperature 700 and 800 °C, the δ phase, M23C6 carbides, and the Laves phase precipitate and grow, and the subgrain boundaries are formed. The hardness is in the range of 340–350 HV10 and is higher than in the 2nd stage. In the 4th stage, as the annealing time is increased at a temperature of 800 °C, the δ phase and M23C6 carbides coagulate, and the Laves phase particles spheroidize or partially dissolve. Very intense precipitation and growth of the hard δ phase particles provide an increase in hardness to 402 HV10. As a result of systematic studies, the various strengthening and softening mechanisms acting during high-temperature annealing are determined.Graphical abstract

Effect of Mechanical Alloying on the Dissolution of the Elemental Mn and Al-Mn Compound in Aluminum

The grain boundary, solid solution, and precipitation strengthening mechanisms are important for controlling the mechanical properties of Al-based alloys. Due to severe plastic deformation, mechanical alloying refines grain structure to a nanoscale level which leads to a strong increase in solute content and the related strengthening effect of solute atoms and secondary-phase precipitates. This study analyzed the elemental Mn and Al6Mn phase dissolution in Al during high-energy ball milling. For this purpose, XRD data, microstructure, and hardness evolutions were compared for two Al—5.2 at% Mn alloys prepared by mechanical alloying using elemental Al and Mn powders and a pre-melted master alloy. In the two-phase master alloy, containing the Al solid solution and the Al6Mn phase, the strain accumulation, grain refinement, solid solution supersaturation, and milling-induced hardening effects were facilitated. Both elemental Mn and intermetallic compound were dissolved during mechanical alloying, and the maximum solute content was near 3.1 at% Mn. A fine crystalline size of ~25 nm and the maximum Mn solute content were observed after milling of elemental powders and the master alloy for 60 h and 20 h, respectively. The microhardness of ~3 GPa corresponded to a ~3.1% solute Mn content, and the microhardness increased to ~5 GPa after long–term milling due to precipitation strengthening effect of the secondary Al6Mn phase in the master alloy.

Hardening mechanism of high manganese steel during impact abrasive wear

To investigate the hardening mechanism of high manganese steel (HMS) during impact-abrasive wear (IAW), the microstructures and hardness evolution behaviours of the worn sub-surfaces of Mn13 (13.32% Mn and 0.22% Cr), Mn13-2 (13.23% Mn and 2.13% Cr), and Mn18-2 (18.35% Mn and 2.04% Cr) were investigated. The matrix hardness values of Mn13, Mn13-2, and Mn18-2 were 240.2, 256.6, and 266.5 HV, while their worn sub-surface hardness values were 670.1, 638.2, and 713.1 HV, respectively. The increase in the hardness values was attributed to the formation of ε-martensite and nano-sized mechanical twins in the worn sub-surfaces. The ε-martensite observed in the worn sub-surface of each of the three materials was similar. Therefore, the differences between the degrees of increase in the hardness values of the materials were attributed to the presence of the mechanical twins. The formation of the mechanical twins was affected by the stacking-fault energy (SFE), which increases with Mn content but decreases with Cr content. The degree of increase in the hardness increased in the following order: Mn13-2 (381.6 HV), Mn13 (429.9 HV), and Mn18-2 (446.6 HV). In summary, the degree of increase in the hardness of HMS during IAW can be improved by engineering a higher SFE for a higher number of mechanical twins.