Hardening Of Metals Research Articles

Clarifying the effect of irradiation and thermal treatment on the austenitic microstructure and austenitic hardening in austenitic stainless steel weld metal

The weld cladding on the inner surface of nuclear pressure vessels due to irradiation damage and thermal effect presents to a safety issue. Unfortunately, the effect of irradiation and long-term thermal treatment on the austenitic microstructure and austenitic hardening in austenitic stainless steel weld metals (ASSWMs) remains poorly understood. In this study, the effects of irradiation and thermal treatment on the austenitic microstructures and austenitic hardening of 308L ASSWMs were investigated using nanoindentation, atom probe tomography and transmission electron microscopy. The results suggested that irradiation resulted in the formation of Ni/Si-rich clusters, voids, and Frank loops in austenite, thereby inducing austenitic hardening. Subsequently, thermal treatment decreased the size and the number of Frank loops in irradiated austenite, which had a minor effect on austenitic hardening. However, thermal treatment promoted the growth of Ni/Si-rich clusters and void formation, which have a primary effect on austenitic hardening, thereby enhancing the hardening of irradiated austenite. Furthermore, thermal treatment has little effect on the microstructure and hardening of austenite. Then, irradiation promoted the formation of Ni/Si-rich clusters, voids, and Frank loops in thermally treated austenite, resulting in austenitic hardening. The interaction of irradiation and thermal treatment can promote the formation of voids. The austenitic hardening was mainly due to the contribution of Frank loops, voids, and Ni/Si-rich clusters, which acted as short-range barriers by pin-moving dislocations.

An informatics method for inferring the hardening exponent of plasticity in polycrystalline metals from surface strain measurements

The investigation of strain hardening in metals is complex, with the outcome depending on experimental conditions, that may involve microstructural history, temperature and loading rate. Hardening is commonly measured, after mechanical processing, through controlled mechanical testing, in ways that either distinguish elastic (stress) from total deformation measurements, or by identifying plastic slip contributions. In this paper, we conjecture that hardening effects can be unraveled through statistical analysis of total strain fluctuations during the evolution sequence of profiles, measured in-situ, through digital image correlation. In particular, we hypothesize that the work hardening exponent is related, through a power-law relationship, to a particular exponent arising from principal component analysis. We demonstrate a scaling analysis for synthetic data produced by widely applicable crystal plasticity models for polycrystalline solids.Graphical

Solute Hardening and Softening of BCC Metals: Excerpts from an Early Life with Terry Mitchell

Solute Hardening and Softening of BCC Metals: Excerpts from an Early Life with Terry Mitchell

Lode-dependent anisotropic-asymmetric yield function for isotropic and anisotropic hardening of pressure-insensitive materials. Part II: Stress invariant-based coupled quadratic and non-quadratic function

This research couples a Lode-dependent anisotropic-asymmetric (LAA) frame (Lou and Yoon, 2023. International Journal of Plasticity, 166, 103,647) with a stress-invariant-based coupled quadratic-non-quadratic (CQN) anisotropic hardening function to analytically characterize the anisotropic-asymmetric hardening of sheet metals under uniaxial tension and uniaxial compression. Experiments are conducted for AA2A12-O under uniaxial tension, uniaxial compression, equibiaxial tension, plane strain tension and shear. Anisotropy is investigated by conducting the experiments along different loading directions from the rolling. The flow curves are obtained from these experimental data at distinct stress states and loading directions. The plastic hardening is represented by the CQN-coupled LAA function to verify its accuracy. The CQN-coupled LAA model is also utilized to represent the plastic flow of DP980 under uniaxial tension, uniaxial compression, shear and plane strain tension along different loading directions as well as equibiaxial tension. The application to AA2A12-O and DP980 demonstrates that the CQN-coupled LAA function is capable of modeling plastic hardening behaviors under uniaxial tension, uniaxial compression, equibiaxial tension and equibiaxial compression and dramatically improving the prediction accuracy of flow curves under plane strain tension.

Jamming Memory into Acoustically Trained Dense Suspensions under Shear

Systems driven far from equilibrium often retain structural memories of their processing history. This memory has, in some cases, been shown to dramatically alter the material response. For example, work hardening in crystalline metals can alter the hardness, yield strength, and tensile strength to prevent catastrophic failure. Whether memory of processing history can be similarly exploited in flowing systems, where significantly larger changes in structure should be possible, remains poorly understood. Here, we demonstrate a promising route to embedding such useful memories. We build on work showing that exposing a sheared dense suspension to acoustic perturbations of different power allows for dramatically tuning the sheared suspension viscosity and underlying structure. We find that, for sufficiently dense suspensions, upon removing the acoustic perturbations, the suspension shear jams with shear stress contributions from the maximum compressive and maximum extensive axes that reflect or “remember” the acoustic training. Because the contributions from these two orthogonal axes to the total shear stress are antagonistic, it is possible to tune the resulting suspension response in surprising ways. For example, we show that differently trained sheared suspensions exhibit (1) different susceptibility to the same acoustic perturbation, (2) orders of magnitude changes in their instantaneous viscosities upon shear reversal, and (3) even a shear stress that increases in magnitude upon shear cessation. We work through these examples to explain the underlying mechanisms governing each behavior. Then, to illustrate the power of this approach for controlling suspension properties, we demonstrate that flowing states well below the shear jamming threshold can be shear jammed via acoustic training. Collectively, our work paves the way for using acoustically induced memory in dense suspensions to generate rapidly and widely tunable materials. Published by the American Physical Society 2024

A machine learning constitutive model for plasticity and strain hardening of polycrystalline metals based on data from micromechanical simulations

Machine learning (ML) methods have emerged as promising tools for generating constitutive models directly from mechanical data. Constitutive models are fundamental in describing and predicting the mechanical behavior of materials under arbitrary loading conditions. In recent approaches, the yield function, central to constitutive models, has been formulated in a data-oriented manner using ML. Many ML approaches have primarily focused on initial yielding, and the effect of strain hardening has not been widely considered. However, taking strain hardening into account is crucial for accurately describing the deformation behavior of polycrystalline metals. To address this problem, the present study introduces an ML-based yield function formulated as a support vector classification model, which encompasses strain hardening. This function was trained using a 12-dimensional feature vector that includes stress and plastic strain components resulting from crystal plasticity finite element method (CPFEM) simulations on a 3-dimensional RVE with 343 grains with a random crystallographic texture. These simulations were carried out to mimic multi-axial mechanical testing of the polycrystal under proportional loading in 300 different directions, which were selected to ensure proper coverage of the full stress space. The training data were directly taken from the stress–strain results obtained for the 300 multi-axial load cases. It is shown that the ML yield function trained on these data describes not only the initial yield behavior but also the flow stresses in the plastic regime with a very high accuracy and robustness. The workflow introduced in this work to generate synthetic mechanical data based on realistic CPFEM simulations and to train an ML yield function, including strain hardening, will open new possibilities in microstructure-sensitive materials modeling and thus pave the way for obtaining digital material twins.

Surface hardening of MIM porous metals by ultrasonic vibration assisted pressing

Micro MIM is one of the manufacturing methods for medical devices, which has seen a rapid increase in demand in recent years, but due to the porous structure of the product, there are concerns about insufficient strength due to low density. Ultrasonic-assisted surface reinforcement was proposed to supplement the strength by hardening the surface and improving bending stiffness in this study. Ultrasonic-assisted pressing was performed to verify mechanism for the surface hardening by comparing MIM and rolled specimen. The materials used are pure copper MIM. It was found that MIM specimen have a characteristic in which strain is concentrated near the surface compared to rolled specimen. This suggests that the cause of the concentration of strain on the surface may be the absorption of ultrasonic energy by the pores inside the material due to sound and shock absorption properties of the porous structure.

Strain hardening and microstructure evolution in ECAP-processed ultrafine-grained metals: a comparative study of copper, aluminum, and magnesium alloys

Strain hardening and microstructure evolution in ECAP-processed ultrafine-grained metals: a comparative study of copper, aluminum, and magnesium alloys

Shock-induced melting and crystallization in titanium irradiated by ultrashort laser pulse

Modification of titanium microstructure after propagation of a melting shock wave (SW) generated by a femtosecond laser pulse is investigated experimentally and analyzed using hydrodynamic and atomistic simulations. Scanning and transmission electron microscopy with analysis of microdiffraction is used to determine the microstructure of modified subsurface layers of titanium. We found that two layers are modified beneath the surface. A top surface polycrystalline layer of nanoscale grains is formed from shock-molten material via rapid crystallization. In a deeper subsurface layer, where the shock-induced melting changes into plastic deformation due to attenuation of SW, the grain structure of solid is considerably affected, which results in a grain size distribution differing from that in the intact titanium. Molecular dynamics simulation of single-crystal titanium reveals that the SW front continues to melt even after its temperature drops below the melting curve Tm(P). The enormous shear stress of ∼12 GPa generated in a narrow SW front leads to free slip of atomic planes, collapse of the crystal lattice, and formation of a supercooled metastable melt. Such melt crystallizes in an unloading tail of SW. The mechanical melting ceases after drop in the shear stress giving rise to the shock-induced plastic deformation. The last process triggers a long-term rearrangement of atomic structures in solid. The overall depth of modified layers is limited by SW attenuation to the Hugoniot elastic limit and can reach several micrometers. The obtained results reveal the basic physical mechanisms of surface hardening of metals by ultrashort laser pulses.

Precipitate-domain wall topologies in hardened Li-doped NaNbO3

Ferroelectric hardening is an absolute requirement for potential applications of piezoceramics in high-power devices. A very recent work demonstrates that ferroelectric hardening by pinning domain walls with precipitates introduced during processing is feasible, akin to precipitation hardening in metals. With the precipitation of plate-like LiNbO3 in Li-doped NaNbO3 piezoceramics, ferroelectric hardening was achieved, exhibiting an enhanced mechanical quality factor (Qm). The specific morphology and corresponding orientation of the LiNbO3 platelets, exhibiting {110}PC habit planes, were characterized by transmission electron microscopy (TEM). The experimentally revealed crystallographic structures and strain incompatibility were found to correlate well with the thermodynamic simulation of the minimum-energy aspect ratio. In particular, the topology of the precipitate-domain wall assembly was clarified, revealing a full pinning on the domain walls by the incorporated plate-like precipitates. The revealed topology provides a fundamental basis for future optimization strategies for platelet hardening in new lead-free piezoceramics for high-power applications.

State of art: Review on laser surface hardening of alloy metals

Surface hardening treatments are performed to increase the hardness, wear resistance, durability and their micro structural properties. After the development of High-Powered Diode Lasers (HPDL), the laser surface hardening process is widely applied due to their non-contact treatment process, rapid cooling without any medium, controlled method of heat treatment and depth with automation and precision with relatively minimum energy input. Laser surface hardening of steels, carbon alloys, iron and other metals in manufacturing of components, stamping and forging dies, tools and other components in the field of mechanical, automotive and metallurgical have shown a positive progression in hardness and wear resistance with controlled phase changes. The factors of laser power density and travel speed plays key roles in phase changes and the surface transformation hardening. The surface hardening of heavily loaded components under motion should possess good surface roughness, tribological characteristics of wear resistance, hardness and temperature coefficients. Thus, laser surface hardening is optimum, controlled and economic way of surface treatment processes in manufacturing of mechanical components.

Recent Progress in Phase Stability and Elastic Anomalies of Group VB Transition Metals

Recently discovered phase transition and elastic anomaly of compression-induced softening and heating-induced hardening (CISHIH) in group VB transition metals at high-pressure and high-temperature (HPHT) conditions are unique and interesting among typical metals. This article reviews recent progress in the understanding of the structural and elastic properties of these important metals under HPHT conditions. Previous investigations unveiled the close connection of the remarkable structural stability and elastic anomalies to the Fermi surface nesting (FSN), Jahn–Teller effect, and electronic topological transition (ETT) in vanadium, niobium, and tantalum. We elaborate that two competing scenarios are emerging from these advancements. The first one focuses on phase transition and phase diagram, in which a soft-mode driven structural transformation of BCC→RH1→RH2→BCC under compression and an RH→BCC reverse transition under heating in vanadium were established by experiments and theories. Similar phase transitions in niobium and tantalum were also proposed. The concomitant elastic anomalies were considered to be due to the phase transition. However, we also showed that there exist some experimental and theoretical facts that are incompatible with this scenario. A second scenario is required to accomplish a physically consistent interpretation. In this alternative scenario, the electronic structure and associated elastic anomaly are fundamental, whereas phase transition is just an outcome of the mechanical instability. We note that this second scenario is promising to reconcile all known discrepancies but caution that the phase transition in group VB metals is elusive and is still an open question. A general consensus on the relationship between the possible phase transitions and the mechanical elasticity (especially the resultant CISHIH dual anomaly, which has a much wider impact), is still unreached.

SELECTIVE SURFACE HARDENING OF GEAR MECHANISM SHAFT BY ROBOTIC LASER 3D SYSTEM

A robot-based laser 3D hardening method is proposed as a finishing operation to increase the wear resistance of the metal end-products. The laser hardening process of the surface layer of the products by changing its structure is one of the most effective methods of selective surface hardening. Thermal hardening of metals and alloys by laser radiation is based on local heating of a surface area under the influence of radiation and subsequent cooling of this surface area at a supercritical speed due to heat removal into the inner layers of the metal. The used robot-based laser 3D system (FANUC industrial robot and SCANLAB scanning optics) allows processing the surfaces of any complexity and geometry, including the shafts of the gear mechanism of the seed drill. It was found that the development and improvement of the technological process of manufacturing steel shafts of the gearbox of the seed drill is an urgent technological problem due to the rapid retirement of the shaft, which leads to the expenditure of time and money for its replacement. The most heavily loaded sections of the shaft were pre-estimated using the SolidWorks Simulation software package. A high-power TruDisk 8002 solid-state disk laser with a maximum laser power of 8 kW and a wavelength of laser radiation of 1030 nm is used for high-quality laser surface treatment of the AISI 1066 steel shaft of the gear mechanism. The laser surface heat treatment was carried out using a constant power strategy (continuous mode), varying the laser power in the range of 1.35–2.25 kW. Based on the Fe-Mn two-component state diagram, the critical points of the temperature of complete austenization of the studied steel were previously predicted taking into account the chemical composition of the material. Additionally, the energy density values of the laser beam with a diameter of 1 mm on the working surface was estimated. The results showed that the surface hardness of the shaft was about 2.5 times higher compared to the untreated surface. The working ranges of laser heat treatment parameters of the gearbox shaft were established to increase the hardening intensity (100–150%) of the responsible areas.

Dislocation Dynamics Model to Simulate Motion of Dislocation Loops in Metallic Materials

Dislocation dynamics has been an intensive research subject in materials science and engineering due to the significant roles it plays in plastic deformation and the hardening of metals, fracture mechanics, and the fabrication of semiconductor thin films. However, a long-standing problem from the three-dimensional dislocation dynamics is that the motion and interaction of dislocation loops heavily depend on the loop-segment sizes, which substantially reduces the accuracy of simulation. We herein propose a new three-dimensional dislocation dynamics model together with its physical background. The proposed model incorporates the inherent interactions among differential dislocation segments. The simulation results on motion of Frank–Read sources demonstrate that the proposed model can resolve the paradoxical segment-dependent phenomenon in dislocation dynamics.

Dynamic size effects across the scales: Localization in round notched bars

The strengthening and hardening of metals increase with diminishing size due to plastic strain gradient accompanied by Geometrically Necessary Dislocations (GNDs). In contrast, the inertia forces under severe dynamic loading conditions decrease. To shed the light on the interaction between the two mechanisms, the present work focuses on the localization in notched bars subject to high strain-rate tensile loading and first lays out a new computational model where the Fleck and Willis strain gradient plasticity theory is implemented into a dynamic finite element framework. The new framework allows accounting for strain gradient strengthening through the non-local plasticity material model and models the “smaller is stronger” mechanism (controlled by a built-in length parameter), while material inertia governs the “larger is more dynamic” effect. The work by Needleman (2018) [Effect of size on necking of dynamically loaded notched bars, Mech. Materials, 116:180-188] is taken as a starting point, and the present study first conveys the message that the scale-dependent dynamic effect observed in visco-plastic tensile bars also exists for rate-independent materials across scales (questioned by Needleman, 2018). Subsequently, the notched bar’s size and corresponding loading are scaled to the micrometer size range where plastic strain gradients influence the material response. Focusing attention on the shift between plastic localization in the notch and away from the notch (in the region subject to uni-axial tension) allows cementing this shift at the micron scale. The study shows that gradient strengthening owing the GNDs stabilizes the early localization in the notch and delays the shift between the localization mechanisms when the size of the notched bar diminishes while keeping the material length parameter constant. Moreover, severe gradient strengthening gives rise to a slow-growing neck-like (micron scale) localization mechanism where a neck-type deformation evolves near the notch — but not in the notch as oppose to larger bar sizes.

Dislocation kinetics explains energy partitioning during strain hardening: Model and experimental validation by infrared thermography and acoustic emission

The energy dissipation and storage during strain hardening of metals have been investigated by means of complementary in situ techniques - infrared thermography (IRT), digital image correlation (DIC) and acoustic emission (AE). Inspired by experimental results obtained in the present work and data available in literature, we proposed the analytically tractable thermodynamic modelling approach, which is conceptually based on a single-variable dislocation evolution approach with a total dislocation density serving as a principal variable governing the strain hardening process. Unified by the kinetic approach involving generation, annihilation and motion of dislocations, the models accounting for the acoustic emission behaviour and heat dissipation under load have been proposed and verified experimentally. For the first time, we were able to demonstrate that the key parameters governing the dislocation storage and annihilation rates can be, in principle, recovered from independent AE and IRT measurements. These results agree favourably with the predictions of the dislocation-based constitutive strain hardening models of the Kocks-Mecking type. The self-consistency, versatility and predictive capacity of the proposed modelling approach are demonstrated in the examples of Cu–Zn alloys with different concentrations of Zn varied from 0 to 30% and correspondingly different stacking fault energies.

Simulation of dislocation evolution in microparticle impacts over a wide range of impact velocities

The dynamic loading environment created by high velocity microparticle impacts is very difficult to model. The extremely high strain rates, large deformations, and high temperatures affect the flow stress of the material in ways that standard flow stress models cannot capture. Plastic deformation and strain hardening in metals is controlled by the motion of dislocations. Dislocations can be nucleated, stored as forest dislocations, or be annihilated as loading progresses. A comprehensive accounting of dislocation density change is needed to accurately describe rate and temperature dependent dislocation glide and evolution across the wide rage of loading conditions present in microparticle impact problems. Therefore, we implement and apply a newly proposed flow stress model (Hunter and Preston, 2015, 2022) that is then coupled with mobile and immobile dislocation density evolution equations. This model is implemented in Los Alamos National Laboratory’s hydrodynamics code, FLAG, to model copper-on-copper microparticle impacts. This new strength model allows for accurate simulation of particle rebound and flattening across a wide range of impact velocities.

Coherent Precipitates with Strong Domain Wall Pinning in Alkaline Niobate Ferroelectrics.

High-power piezoelectric applications are predicted to share approximately one-third of the lead-free piezoelectric ceramic market in 2024 with alkaline niobates as the primary competitor. To suppress self-heating in high-power devices due to mechanical loss when driven by large electric fields, piezoelectric hardening to restrict domain wall motion is required. In the present work, highly effective piezoelectric hardening via coherent plate-like precipitates in a model system of the (Li,Na)NbO3 (LNN) solid solution delivers a reduction in losses, quantified as an electromechanical quality factor, by a factor of ten. Various thermal aging schemes are demonstrated to control the average size, number density, and location of the precipitates. The established properties are correlated with a detailed determination of short- and long-range atomic structure by X-ray diffraction and pair distribution function analysis, respectively, as well as microstructure determined by transmission electron microscopy. The impact of microstructure with precipitates on both small- and large-field properties is also established. These results pave the way to implement precipitate hardening in piezoelectric materials, analogous to precipitate hardening in metals, broadening their use cases in applications.

Stored and dissipated energy of plastic deformation revisited from the viewpoint of dislocation kinetics modelling approach

In the present work, we revisited the classical topic of elastic energy storage during strain hardening of metals from a perspective of the analytically tractable thermodynamic modelling framework inspired by the widely accepted phenomenological single-variable dislocation evolution approach. The model versatility has been extended towards predicting the energy partitioning during plastic flow. With a total dislocation density serving as a principal variable governing strain hardening during constant strain rate tensile tests, we have been able to demonstrate a very good predictive capability of the proposed analytical solutions. Besides the simplicity, the flexibility and predictive power of the obtained analytical solutions suggest that the entire approach can be used for further modelling, where the emphasis should be placed on the integration of various possible mechanisms of heat dissipation into the proposed framework. Although the examples of successful application of the model refer to the low-carbon austenitic stainless 316L steel, their adaptation to other fcc, bcc or hcp metals is rather straightforward.

Revealing the synergistic effect of invisible helium clusters in helium irradiation hardening in tungsten

Irradiation hardening caused by helium (He) ion bombardment is directly related to the pinning effect of He-related defects on dislocation glide in metals. Taking tungsten as an example, we have systematically investigated the pinning effect of different size He-related defects on the 1/2<111> dislocation glide by molecular dynamics simulations. Interestingly, it is found that not only the nanoscale He bubbles, but also the invisible He clusters play a critical role in He-irradiation hardening. Under the multiple invisible He clusters synergy, the critical resolved shear stress (CRSS, t) will linearly increase with the increasing of He cluster number (m), following the equation τCRSS=τdis+m·ΔτHen−Vi. Accordingly, the CRSS of the multiple invisible He clusters is easy to catch up or even surpass that for nanoscale He bubbles. Consequently, the synergistic effect of invisible He clusters must be considered for He-irradiation hardening in metals.