Formation Of Martensite Phase Research Articles

Unveiling the mechanism of the carbon ordering and martensite tetragonality in Fe–C alloys studied via deep-potential molecular dynamics simulations

The martensitic transformation plays a pivotal role in the strengthening and hardening of steels, yet an accurate interatomic potential for a comprehensive description of the martensitic phase formation in Fe–C alloys is lacking. Herein, we develop a deep learning-based interatomic potential to perform molecular dynamics (MD) simulations to study the martensitic phase transformation across a range of carbon (C) concentrations. The results reveal that an increased C concentration leads to a suppression of phase boundary movement and a deceleration of the phase transformation rate. To overcome the timescale limitations inherent in MD simulations, metadynamics sampling was employed to accelerate the simulations of C diffusion. We find that C atoms tend to cluster at distances equivalent to the lattice parameter of Fe with the same sublattice occupation, leading to local lattice tetragonality. Such C-ordered structures effectively inhibit dislocation movement and enhance strength. The stress field induced by dislocations facilitates a higher degree of ordering. The formation of C-ordered structures is identified as a potentially crucial strengthening mechanism for martensitic steels. The consistency between our simulation results and reported experimental observations underscores the effectiveness of the developed DP model in simulating martensitic phase transformation in Fe–C alloys, providing detailed insights into the mechanisms underlying this process.

Hydrogen penetration into the NiTi superelastic alloy investigated in-situ by synchrotron diffraction experiments

Microstructural changes induced by a hydrogen permeation into the NiTi superelastic alloy were investigated in-situ using the X-ray synchrotron diffraction. A new design of an electrochemical cell enabled to uncover time and position dependent processes under a flat alloy surface exposed to the cathodic hydrogen. The diffraction data supported by thermo-elastic FEM calculations helped to quantify an evolution of compressive stresses in the B2 austenitic phase hosting hydrogen atoms. The compressive stress state initiates a formation of martensitic phases starting from the exposed surface layer and advancing into the alloy volume with increasing time of hydrogen charging. We have performed the ab-initio DFT study in order to rationalize volumetric changes associated with variations in the B2 austenite and B19′ martensite lattice parameters. The numerical results also contributed to the identification of a new hydride phase with orthorhombic crystal structure and lattice parameters a = 0.8505 nm, b = 0.7366 nm and c = 0.4722 nm.

Challenge of fabricating difficult-to-nitride materials: Formation of martensitic phase in (Fe0.8Co0.2)8N foil based on industrially suitable gas-nitriding process

Nitrogen-martensitic phase (Fe0.8Co0.2)8N was successfully synthesized by using an industrially suitable gas-nitriding process of a bulk foil. So far, the nitrogen-martensitic phase of the bulk material has not been synthesized at such a high Co content. We found that this is because the nitride was easily denitrided by elevating temperature. In this work, by exploiting a NH3 gas-nitriding process combined with quenching at a rapid cooling rate (>800 °C/s) in NH3 atmosphere, we found that nitrogen stayed at the surface layer of the foil. By using cross-sectional laser microscopy, the nitride region was observed as a 7-μm-thick layered shape at the surface of the 100-μm-thick foil. An x-ray diffraction technique revealed that the nitride layer was a martensitic phase that was characterized as a body-centered tetragonal structure with c/a = 1.04. These findings can be applied to nitriding and surface treatments for alloy systems in which a nitrogen solid solution is hardly formed. Our developed method is promising because the martensitic phase is expected to be formed in whole bulk by further optimizing parameters clarified in this work.

Effect of Segregation Band on the Microstructure and Properties of a Wind Power Steel before and after Simulated Welding

This article uses scanning electron microscopy (SEM) and electron back-scattering diffraction (EBSD) to study the effect of C and Mn segregation on the microstructure and mechanical properties of high-strength steel with 20 mm thickness used for wind power before and after simulated welding. A Gleeble-3500 (GTC, Dynamic Systems Inc., Poestenkill, NY, USA) was used to study the microstructure evolution of the simulated coarse-grained heat-affected zone (CGHAZ) of experimental steel under different welding heat inputs (10, 14, 20, 30 and 50 kJ/cm) and its relationship with low-temperature impact toughness (−60 °C). The results indicate that alloy element segregation, especially Mn segregation, significantly affects the impact toughness scatter of the steel matrix, as it induces the formation of low-temperature martensite or hard phase, such as M/A (martensite/austenite) constituent. In addition, segregation also reduces the low-temperature impact toughness of the simulated welding samples and increases the fluctuation range. For high-strength steel with yield strength higher than 460 MPa used for wind power generation, there is an optimal welding heat input (~20 kJ/cm), which enables the simulated coarse-grained heat-affected zone (CGHAZ) to obtain the highest impact toughness due to the formation of lath bainite (LB) and the finest crystallographic block units. Excessive or insufficient heat input can induce the formation of coarse granular bainite (GB) or lath martensite (LM), leading to a larger size of crystallographic block units, reducing the hindering effect of brittle crack propagation and deteriorating low-temperature impact toughness.

Effect of α″-Ti Martensitic Phase Formation on Plasticity in Ti-Fe-Sn Ultrafine Eutectic Composites.

Extensive research has been conducted on Ti-Fe-Sn ultrafine eutectic composites due to their high yield strength, compared to conventional microcrystalline alloys. The unique microstructure of ultrafine eutectic composites, which consists of the ultrafine-grained lamella matrix with the formation of primary dendrites, leads to high strength and desirable plasticity. A lamellar structure is known for its high strength with limited plasticity, owing to its interface-strengthening effect. Thus, extensive efforts have been conducted to induce the lamellar structure and control the volume fraction of primary dendrites to enhance plasticity by tailoring the compositions. In this study, however, it was found that not only the volume fraction of primary dendrites but also the morphology of dendrites constitute key factors in inducing excellent ductility. We selected three compositions of Ti-Fe-Sn ultrafine eutectic composites, considering the distinct volume fractions and morphologies of β-Ti dendrites based on the Ti-Fe-Sn ternary phase diagram. As these compositions approach quasi-peritectic reaction points, the α″-Ti martensitic phase forms within the primary β-Ti dendrites due to under-cooling effects. This pre-formation of the α″-Ti martensitic phase effectively governs the growth direction of β-Ti dendrites, resulting in the development of round-shaped primary dendrites during the quenching process. These microstructural evolutions of β-Ti dendrites, in turn, lead to an improvement in ductility without a significant compromise in strength. Hence, we propose that fine-tuning the composition to control the primary dendrite morphology can be a highly effective alloy design strategy, enabling the attainment of greater macroscopic plasticity without the typical ductility and strength trade-off.

High-temperature dry sliding wear behavior of additively manufactured austenitic stainless steel (316L)

Austenitic steels are commonly used in aero-engine components owing to their higher thermal resistance to wear and corrosion. This study investigates the high-temperature dry sliding wear behavior of direct metal laser-sintered (DMLS) austenitic stainless steel (SS 316L) up to 400°C. A dry sliding reciprocating wear test conducted on a high-frequency tribometer with a ball-on-flat setup. High-hardness chrome steel (900 HV0.5) and alumina (1165 HV0.5) balls of 6 mm diameter were used as the steel and ceramic counterbodies against 316L specimen (320 ± 5 HV0.5). When tested against the chrome steel, the initial average coefficient of friction (CoF) decreased to a lower value (0.45) during the initial running-in period. However, with rising temperatures, the CoF also increased, stabilizing at 0.6 under steady-state conditions. A similar CoF pattern was observed in experiments with the alumina counterbody. Wear resistance declined at 100°C, followed by an increase at 200°C due to the development of an oxide glaze on the surface. Subsequently, wear resistance decreased as the temperature continued to rise. Up to 200°C , the wear mechanism was characterized by a combination of abrasive and oxide debris wear, transitioning to adhesive wear with plastic deformation at higher temperatures (400°C). Diffraction (XRD) analysis revealed the formation of oxide layers and martensite phase at higher temperatures, contributing to the observed improved wear resistance around 200°C . However, these oxide layers were progressively removed from the surface at 400°C, leading to an increased wear. The measured wear rates are two times lower, up to 200°C, compared to the reported values for conventional 316L due to the fine microstructure and higher hardness of DMLS SS 316L. This results in potentially rendering it suitable for applications where high-temperature (up to 200°C) wear-resistant SS is needed. This study provides further insights into the importance of post-treatment of DMLS parts for temperature beyond 400°C.

Effect of martensitic phase formation on the nano-mechanical attributes during the electron beam melting process of Ti-6Al-4V

Electron Beam Melting based Ti-6Al-4V parts plays a predominant role in the field of bio medical and aerospace applications, especially in the aero due to its sustainability. However, there were not enough studies on nano-mechanical properties that looked at how phase changes during the heat and cooling process affected the printing process. With the assistance of OM and FESEM with EDX, the EBM Ti64 coupons were analysed for the grain orientation and phase transformation studies. The FESEM with EDX supported the existence of columnar grains and orthogonally oriented martensitic needles. Additionally, both XRD and TEM confirmed the presence of vanadium-rich precipitates and the HCP α-Ti and BCC β-Ti phases. EBSD analysis was adopted to further investigate the orientation of grains. The nano-hardness and elastic reduced modulus mapping confirmed the phase transformation to martensitic phase. The strong “martensitic” phase had a maximum reduced modulus of 183.5 GPa and a hardness value of 7.31 GPa. However, the EBM based Ti64 possesses an average nano-hardness and reduced modulus of 4.26 GPa and 118.3 GPa under the applied load of 2500μN. In addition, topographic wear and scratch tracks confirmed the soft nature of α colonies by forming more piles of materials, while nano-wear and scratch analysis predicted the functions caused by the presence of hard martensitic phases. The SR sensitivity of the Ti64 specimens was investigated using three different quasi-static strain rates. It was determined that the EBM Ti64 parts have a considerable effect over the strain rate, whereas, a quasi-static strain rate allowed for the achievement of the highest possible Ultimate Tensile Strength (UTS) and ductility.

Highly oriented crystallization, and tunable structural transformation and magnetic transition in Ni-Co-Mn-Al shape memory alloys

Alloy ribbons of Ni50-xCoxMn50-yAly with x = 5–10 and y = 16–17 were prepared by rapidly solidified technique. Highly oriented column-shaped crystalline grains are observed in the alloy ribbons. The formation of martensitic (L10) and austenitic (B2) phases and their structural transformation are strongly dependent on the concentration correlation of Co and Al. Co tends to enhance the fraction and the ferromagnetic interaction of the austenitic phase leading to a greatly increase of the saturation magnetization and Curie temperature of the alloy. The martensitic-austenitic (M−A) structural transformation process can be tuned to occur entirely above room temperature by appropriate concentrations of Co and Al. An irregular variation of the M−A transformation temperature happens in a narrow range, 8–8.5 at%, of Co concentration, while the ferromagnetic–paramagnetic (FM-PM) phase transition temperature monotonically increases. The external magnetic field favors the appearance of the austenitic phase and induces shifts to low and high temperatures for the M−A transformation and the FM-PM transition, respectively.

Enhancement of the Microstructure and Mechanical Properties of 1020 Carbon Steel and AISI 304 Stainless Steel Dissimilar Weld Using Different Post-Weld Heat Treatments

In order to improve the microstructure and mechanical properties of gas metal arc dissimilar weldment of AISI 304 and 1020 carbon steel, different post-weld heat treatment (PWHT) processes including annealing, tempering and normalizing were performed. The post-tempered weldment exhibited improved grain refinement over the as-welded. The as-welded joint is characterized with the formation of hard martensitic phase and CrC precipitates while the post-weld heat treated (PWHTed) joints consist more of softer ferritic phase. The PWHTs resulted in the weldment hardness reduction with post-annealed demonstrating the least hardness. Only the post-tempered weldment demonstrated improved tensile strength (~5.2%) over the as-welded (421 MPa). All the PWHT processes resulted in improved elongation (i.e., ductility) and impact energies over the as-welded. While the entire PWHTed weldments demonstrated ductile fracture mode, the as-welded sample exhibited a combination of ductile and brittle fracture mode after the tensile test.

Co-strengthening and deformation behaviors of hybrid additive manufactured U75V/15-5PH bimetal via optimized heat treatment

Bimetals composed of U75V pearlitic steel and 15–5 precipitation hardening (PH) steel were successfully processed by a hybrid additive manufacturing method. However, the U75V/15-5PH bimetal in the as-deposited condition showed inferior mechanical properties owing to the inhomogeneous microstructures. Herein, optimized heat treatment for the co-strengthening of U75V/15-5PH bimetal was designed by investigating the microstructural evolution and mechanical properties thoroughly. Results showed that the mechanical properties of the U75V steel region were recovered after solution treatment, reflected in the formation of lamellar pearlite. For the additive 15-5PH steel region, the formation of martensitic phase and NbC carbides contributed to the strengthening in the solution condition. Furthermore, this region was further hardened by the massive precipitation of Cu-riched precipitates induced by aging treatment. Due to the difference in yield strength of U75V/15-5PH bimetals with various locations, the deformation transfer phenomenon from U75V steel to 15-5PH stainless steel was detected during the uniaxial tensile tests. After heat-treated through the solution and aging treatment, the optimized yield strength, ultimate tensile strength, and elongation of the U75V/15-5PH bimetal achieved 878 MPa, 1201 MPa, and 11.2%, respectively. This study demonstrated experimentally that the mechanical performance of the U75V/15-5PH bimetal with various microstructures could be improved by designing a suitable solution and aging heat treatment.

Development of a cold spot joining method that enables sound joining of carbon steel below the A1 temperature

A novel solid-state joining method, which we call cold spot joining (CSJ), was successfully developed. In this joining concept, the material in the vicinity of the interface is plastically deformed using high pressure to form the joining interface, and impurities at the interface are expelled to the outside. Medium-carbon steel sheets were joined using the CSJ method under various process conditions. The joining temperature can be modified by the pressure applied during CSJ. Microstructural observations and hardness distributions revealed that proper pressurisation led to a joining temperature below the A1 point and prevented the formation of brittle martensitic phase. A sound S45C joint showing plug rupture at the base metal under both tensile shear and cross-tension tests was successfully fabricated by providing appropriate applied pressure and energising conditions.

Comprehending the correlation between microstructural features and wear performance in multiphase lightweight steels

This work investigates the wear behavior of three different Fe-Mn-Al-C steels using ball-on-disc reciprocating wear test and 3D optical surface profilometer. Further, the correlation between microstructural features and wear performance of these steels has been established. The experimental results indicate that the wear resistance of the studied steels is in the order: austenite and martensite (AM) > austenite and ferrite (AF) > ferrite and pearlite (FP) steels. In addition to higher hardness, the combined effects of finer width of martensite, larger proportion of high-angle grain boundaries, and higher dislocation density render excellent wear resistance in AM steel. Importantly, the formation of martensite phase during wear test significantly enhances the wear resistance of AM steel. In contrast, the absence of austenite phase reduces the wear resistance of FP steel.

Synthesis and characterization of novel NiTi–Ni3Ti/SiC nanocomposites prepared by mechanical alloying and microwave-assisted sintering process

Equiatomic Ni–Ti alloy reinforced with 0, 2.5, 5, and 10 vol % SiC nanoparticles was synthesized through mechanical alloying and microwave sintering process. In this method, a mixture of Ti and Ni powders and SiC nanoparticles (0–10 vol%) were mechanically milled for 30 h, compressed under 1 GPa pressure, and then sintered at 900–1100 °C by microwave heating. Microstructural analysis of the synthesized samples was performed using X-ray diffraction and scanning/transmission electron microscopy, and their hardness was determined using a Vickers microhardness tester. Structural studies revealed the formation of a complex phase structure consisting of the austenite (B2–NiTi), the martensite (B19′-NiTi), and the nickel-rich (Ni3Ti) and titanium-rich (NiTi2) phases. HR-TEM investigations confirmed an increased Ni3Ti phase in the austenitic NiTi matrix nearby the martensitic NiTi phase. It was found that adding SiC nanoparticles to Ni–Ti increased the amount of hard Ni3Ti phase, promoted the stability of the martensite phase at room temperature, and retarded the matrix grain growth during the sintering process. Microhardness tests showed that when the content of SiC nanoparticles increased up to 5 vol %, the microhardness increased considerably to 9 GPa and then declined. The high microhardness of synthesized nanocomposites is due to the formation of Ni3Ti and NiTi martensitic phases and their nanocrystalline structure.

Body-thorough ultrahigh strength and ductility of austenitic 316L stainless steel by industry-applicable 3-axis stress-released impact

Austenitic stainless steel (SS) has been seeking a decent strength to match its other exceptional advantages such as remarkable corrosion resistance, antioxidant activity and good formability. High-strain-rate deformation techniques that are effective in strengthening SS surfaces, disappointingly, bring high risk of fracturing when applied to strengthen a bulk SS part. We demonstrate that a 3-axis room-temperature impact with intermediate annealing treatment can consistently promote the strength of bulk 316L SS to an unprecedented high level while posing no fracturing danger to the bulk SS. The impacts from multiple directions enable the smallest nanograins and a high volume of the thinnest twins in the sample, and in the meantime they inhibit the formation of martensite phase. The annealing treatment not only induces element segregation in the SS, which adds extra strengthening to the already significant grain/twin boundary strengthening, but also recovers the capability of the SS in storing dislocations upon deformation. Consequently, the 316L SS achieves an excellent ductility on the basis of the extraordinarily high strength. The steel also has a surprisingly low degree of hardness anisotropy, enhancing its applicability in complicated loading environments. Moreover, the novel process is realized with industry instruments and applicable for industries.

Improving mechanical properties of Ni electrode through alloying and heat treatment

Due to the ferromagnetic nature of iron (Fe) and nickel (Ni) and most of their alloys, it is likely that their inherent magnetism could cause a defect called residual magnetism in welds. Although seldom explored, this property may contribute to the formation of brittle martensite and carbide phases by promoting carbon diffusion from the base material to the surface. This study focusses on solid-solution-strengthening (sss) of Ni electrode by alloying it with chromium (Cr) and copper (Cu), since the introduction of these metals is known to suppress magnetism while increasing strength.

Additive friction stir deposition of SS316: Effect of process parameters on microstructure evolution

Solid state nature of additive friction stir deposition (AFSD) additive manufacturing process is very advantageous in terms of defect formation and microstructural refinement in the material. Current study presents the process optimization, microstructural evolution and kinetics of recrystallization for AFSD deposited low stacking fault energy material - SS316. As deposited microstructure shows equiaxed ultra fine grains with an average grain size of ~5.0 ± 0.5 μm. Shear deformation at high temperature during processing leads to the operation of restoration mechanisms. Observation of necklace type microstructure in the as deposited SS316 is attributed to discontinuous dynamic recrystallization during processing. Recrystallization kinetics of the AFSD SS316 is characterized using Johnson-Mehl-Avarami-Kolmogorov (JMAK) model. Deformation – thermal cycling during AFSD process resulted in inconsistent recrystallization kinetics. Variation in strain, strain rate and temperature during processing with processing parameters result in varying microstructure and tool wear. High strength-ductility combination and sustained work hardening in as deposited SS316 appear to arise from transformation and twinning during deformation, leading to the formation of hierarchical twins and martensitic phase after deformation. • A high strength and work hardening alloy – SS316 is additively manufactured using a novel additive friction stir deposition. • Porosity free builds with fine equiaxed grain structure is formed in as deposited material as a result of discontinues dynamic recrystallization during processing. • Microstructure of the deposited material and the tool wear during processing varied with variation in the process parameters. • Activation of twining and transformation during deformation resulted in excellent tensile properties with yield strength of 415 ± 10 MPa and ductility 75 ± 5% in as deposited material.

Microstructure and Mechanical Behavior of Cu–Al–Ag Shape Memory Alloys Processed by Accumulative Roll Bonding and Subsequent Annealing

Ultrafine-grained Cu/Al/Ag composites were processed by an accumulative roll bonding (ARB) technique from pure copper and aluminum sheets and a silver powder. The Al content was fixed to 11 wt.% while the silver concentration was 1, 2, or 3 in wt.%. The ARB-processed samples were heat treated at different temperatures between 750 and 1050 °C for 60 min and then quenched to room temperature (RT) for producing Cu–Al–Ag alloys. The effect of the addition of different Ag contents and various heat treatment temperatures on the structural evolution was investigated. The ARB-processed samples were composed of Cu and Al layers with high dislocation density and fine grain size (a few microns). During heat treatment of the ARB-processed samples, new intermetallic phases formed. For the lowest Ag content (1 wt.%), the main phase was a brittle simple cubic Al4Cu9, while for higher Ag concentrations (2 and 3 wt.%), the quenched samples contain mainly an orthorhombic β1-AlCu3 martensite phase. The martensite phase consisted of very fine lamellas with a thickness of one micron or less. The heat treatment increased the microhardness and the strength of the samples at RT due to the formation of a fine-grained hard martensite phase. For 2 and 3% Ag, the highest martensite phase content was achieved at 850 and 950 °C, respectively. The annealed and quenched samples exhibited good shape memory behavior at RT.

Numerical simulation of a magnetic induction coil for heat treatment of an AISI 4340 gear

In manufacturing industry, heat treatment is a fundamental requirement for improving the material quality of readily manufactured products. Induction heating technology is repeatable and easily controlled by the advantage of having an electronical control unit. Nowadays, numerical methods have gained so much importance that it become as a reference for the induction heating industry. Experimental methods are costly and time demanding procedures. However, making use of finite element method software, induction heating simulations of a steel gear can be performed relatively cost effective and in a short time. In this paper, induction heating simulation of an AISI 4340 steel gear using FEA software is performed. The effect of variation of inductor frequency and gear workpiece-inductor coil distance on the hardening depth of the gear surface is investigated. The temperature profile of the workpiece is obtained. From the temperature distribution on the steel gear workpiece, the regions of the gear at which the austenitizing temperature (Ac3) - responsible for martensite phase formation- are observed. From the numerical results, hardening profile and hardening depth of the gear is interpreted.

Dynamic Plastic Deformation Induced by Repetitive Hammering on Cr-Mn Austenitic Stainless Steel

Austenitic stainless steels have advantages, such as high ductility and good corrosion resistance. The cold working process can increase the hardness and strength of the material. However, because a metastable austenite phase occurs in that material, there is a phase change of γ austenite to α’-martensite and ε-martensite, which will reduce the ductility and its corrosion resistance. The strengthening process with dynamic plastic deformation (DPD) can prevent the formation of martensitic phases through repeated impact at high strain rates. This study analyzed microstructures and hardness evaluation on Cr-Mn austenitic stainless steel due to dynamic plastic deformation through the repetitive hammering method. Repetitive hammering with a strain rate of 6,2 s-1 on Cr-Mn austenitic stainless steels was carried out on five specimens with variations in the impact of 50, 100, 150, 250, and 350 times with impact energy of 486 J/cm2, 2.207 J/cm2, 2.569 J/cm2, 6.070 J/cm2, and 11.330 J/cm2 respectively. Microstructure, hardness, and XRD (X-ray diffraction) analyses were carried out on specimens before and after repetitive hammering. Metallography was carried out to observe the microstructure using an optical microscope. The hardness was tested through the Rockwell A hardness test. XRD examination was used to identify the phases formed and indications of nano-twins. The repetitive hammering process up to 350 times has succeeded in increasing hardness from 53.5 HRA to 71.6 HRA. Plastic deformation introduced by repetitive hammering produced slip bands, cross bands, wavy bands, and indication of nano-twins formation and increased the hardness.

Structure and residual stress distribution in TiNi substrate after fabrication of surface alloy using electron-beam treatments

In this work, an origin of the residual stresses formed in the sublayer of TiNi substrate adjacent to the [Ti-Ni-Ta-Si] surface alloy (SA) was proposed. The as-cast SA was fabricated through an additive thin-film electron-beam synthesis using a low-energy high current electron beam (LEHCEB) treatment of the [Ti60Ta30Si10 (at. %)/TiNi] system. An additional post-LEHCEB treatment was employed for the SA synthesis (denoted as EB-treated SA) in order to reduce the value of residual stresses. Based on the XRD, TEM/SEM/EDS data, it has been found that the transition zone between the outer SA zone and the TiNi substrate exhibits several sublayers: eutectic B2+Ti3Ni4, martensitic and heat-affected zone (HAZ). However, the martensitic interlayers formed beneath the as-cast and EB-treated SAs possess different types of martensitic structures. Indeed, in the as-cast SA the sublayer of TiNi substrate is a Ni-rich (Ti<50Ni>50) one, so the martensitic transformation В2→R takes place in this sublayer. In turn, in the EB-treated SA the TiNi sublayer shows the composition close to equiatomic (Ti∼50Ni∼50) one, therefore the В2→B19′ martensitic transformation has occurred. The formation of martensitic phases led to a partial decrease (by a factor of ∼1.5–2) of the residual stresses in the underlying HAZ of TiNi substrates. The mechanism of B2 phase stabilization in the HAZ is closely related with a substructural hardening accompanied by formation of dense dislocation structures and residual elastic stresses that could be relaxed through short-term annealings induced by electron beam treatments.