Martensitic Phase Research Articles

Use of repeated indenter sliding coupled with finite element computations to investigate the tribological response of nano-twinned 316L stainless steel

The wear response of dynamically deformed 316 L austenitic stainless steels with varying percentages of compressive strain has been investigated. A relatively high strain rate of 80 s−1 has been used while compressing 316 L at engineering strains of 20 % and 80 %. Nano-indentation coupled with inverse algorithm approaches were used to extract the yield strength and strain hardening exponent of all the specimens. The strain hardening exponent decreased with deformation percentage while the strength enhanced, and these properties have been used in the finite element model (FEM) to find out the stresses and strains during repeated sliding to make scratches. A depth-sensing instrumented nano-indenter with a conical tip was used to make multiple scratches on all the specimens, and the damage in the form of scratch groove depth and width was characterized using a combination of profilometry and scanning electron microscopy. The results showed that the 80 %deformed sample exhibited the least wear damage evident in the form of thinnest scratch profiles, shallowest scratch depths and hence the least specific wear rate. However, after multiple cycles of sliding, undeformed and 20 % deformed samples showed similar wear rates. The friction coefficient decreased with an increase in the percentage deformation, but the dynamic friction coefficient for all the specimens saturated after about 30 passes of sliding. Cross-sectional scanning electron microscopic (SEM) imaging of the scratches revealed refinement and alignment of the microstructure along the shape of the bottom of the groove after repeated sliding. We discovered through the utilization of transmission electron microscopy imaging and selected area diffraction pattern that the region affected by scratching exhibited partial transformation of the initial austenitic phase into martensitic phase.

Uncovering the isothermal transformation dynamics and elemental diffusion behavior of reversed austenite in PH13-8Mo steel

The isothermal transformation kinetics and microstructural evolution of the reversed austenite significantly influence the properties of PH13-8Mo steel. However, the microscopic phase transformation behavior related to elemental diffusion during isothermal processes for PH13-8Mo steel needs further examination. Herein, we systematically investigate the isothermal transformation and elemental diffusion behavior of reversed austenite in PH13-8Mo steel. Phase transformation kinetic measurements unravel that the complete elimination of residual austenite can be achieved. Only hierarchical martensitic microstructure without austenite is detected after the solution-cooling treatment at 927 °C with a cooling rate of 10 °C/s, determining the critical martensite transformation start and finish temperature to be 176.6 °C, 11.6 °C. The isothermal kinetics of reverted austenite satisfy the Johnson-Mehl-Avrami relationship. The fraction of reversed austenite increases from 0.14 % at 510 °C to 8.73 % at 593 °C. Lath martensite boundaries with large kernel average misorientation values promote elemental diffusion at the interface. Further, the underlying behavior of elemental diffusion is elucidated during the isothermal aging process. The interfacial model indicates the growing width of the reversed austenite up to 100 nm and the compositional gradient formed by the elemental diffusion during aging for 0–5 h at 593 °C. The interdiffusion coefficient calculations interpret the diffusion direction of alloying element Ni from martensite to reversed austenite; Cr and Mo may form alloying carbides at the austenite-martensite interface; Al possesses the fastest diffusivity in martensite and austenite phases. The present work unveils the phase transformation kinetics of PH13-8Mo steel, achieving quantitative regulation for maraging stainless steel.

Simulation of SMA-based engineering applications considering large displacement and rotation, thermomechanical coupling and partial phase transformation

Shape memory alloys (SMAs) undergo large recoverable deformation due to their inherent diffusionless martensitic phase transformation. Two factors need to be taken into account to incisively emulate the behaviour of SMA based structures: (i) consideration of large displacement and rotation (LDR) effect, and (ii) capturing the consequence of transformation induced material level coupling. In this study, both these effects are apprehended by extending the infinitesimal strain-based constitutive model of SMA as proposed by Lagoudas et al. (2012), assuming small elastic strain but with finite inelastic strain. To preclude any spurious stress generation out of large rotation, the Jaumann stress rate is used, and incremental objectivity for finite rotation between two successive time instants is conserved by rotating the strain increment and spin tensor to the rotation-independent mid-point configuration following Hughes–Winget (Hughes and Winget, 1980) algorithm. In addition, the contribution of thermoelastic and latent heat evolved during transformation is taken into consideration in the thermal equilibrium equation. Moreover, the effect of partial phase transformation yielding minor hysteresis loop has also been captured. The equilibrium equation is expressed in the current configuration following the Updated Lagrangian formulation. The mechanical and thermal equilibrium equations are solved concurrently in block matrix form using the Newton–Raphson (NR) iterative technique, considering the coupling terms resulting from the phase transformation. The efficacy and robustness of the developed finite element model are corroborated through various practical applications of SMA-based members, e.g., SMA ring, SMA-actuated beam, morphing of corrugated airfoil, orthodontic palatal expander, compliant gripper, undergoing LDR while subjected to different thermomechanical loading conditions. The combined effects of LDR along with thermomechanical coupling yield a stiffening behaviour during loading and sluggish response at the time of thermal recovery.

The Microstructures and Deformation Mechanism of Hetero-Structured Pure Ti under High Strain Rates.

This study investigates the microstructures and deformation mechanism of hetero-structured pure Ti under different high strain rates (500 s-1, 1000 s-1, 2000 s-1). It has been observed that, in samples subjected to deformation, the changes in texture are minimal and the rise in temperature is relatively low. Therefore, the influence of these two factors on the deformation mechanism can be disregarded. As the strain rate increases, the dominance of dislocation slip decreases while deformation twinning becomes more prominent. Notably, at a strain rate of 2000 s-1, nanoscale twin lamellae are activated within the grain with a size of 500 nm, which is a rarely observed phenomenon in pure Ti. Additionally, martensitic phase transformation has also been identified. In order to establish a correlation between the stress required for twinning and the grain size, a modified Hall-Petch model is proposed, with the obtained value of Ktwin serving as an effective metric for this relationship. These findings greatly enhance our understanding of the mechanical responses of Ti and broaden the potential applications of Ti in dynamic deformation scenarios.

Phase field study of heat treatment and strengthening in maraging steels

Martensite decomposition during heat treatment of maraging steels is a complex process, in which different precipitates as well as austenite phase can grow concurrently. Here, we formulate a phase field model to study the martensite decomposition by taking into account the nucleation and growth of phases as well as phase transformation-induced elastic and plastic deformations. To model 18Ni M300 maraging steel, a pseudo ternary Fe-Ni-Ti model is parameterized by using the thermodynamic and kinetic data of the alloy. The effect of transformation induced elastic and plastic strains is incorporated using an elastoplastic model. Using this model, we investigate the decomposition of the martensite phase during continuous heating as well isothermal ageing, focusing on the growth of precipitates as well that of the reverted austenite phase. The data generated from the simulations, in conjunction with an empirical model, are used to estimate the variation of yield strength as a function of temperature and ageing time. The temperature dependence of strength obtained is qualitatively consistent with the experimental data of maraging steel, and thus sheds light on how underlying strengthening mechanisms depend on the ageing temperature and time. In particular, the simulations reveal the important role played by reversed austenite on the temperature dependence of the strength.

Investigation of ductile damage in dual phase steel during tensile deformation by in situ X-ray computed tomography

In situ synchrotron radiation X-ray computed tomography (CT) is used to understand the void nucleation and growth behaviour of dual phase (DP) steel, DP800, during uniaxial tensile deformation. The voids were identified post mortem as having nucleated primarily in the (41 vol%) martensite phase. The behaviour of individual voids was found to be broadly in line with a Rice and Tracey style growth model (their volume increasing by a factor of 10), while the mean void size was broadly in line with a model that included the continuous nucleation of new voids, as well as the growth of existing voids, throughout straining. Voids tended to elongate during uniaxial (low stress triaxiality) straining, but then to dilate more isotropically as necking led to increased triaxiality. One large pre-existing void was found to elongate quicky at first, but then for growth to slow, presumably because it was not located in the highest triaxial stress (necked) region. Perhaps surprisingly the average size of the void population grows only slowly (by a factor of 2) during straining; this is because while the size of each void grows, new small voids are always being nucleated. Finally, the changes of stress triaxiality and shear stress state during tensile deformation of the square cross-section smooth tensile specimen are investigated via finite element modelling, to qualitatively assess the impact of sample geometry on void nucleation behaviour and fracture strain in the current study.

The effect of nanopores on the mechanism of martensitic transformation in pure iron during the heating-cooling cycle: A molecular dynamics study

Numerous molecular dynamics (MD) simulations have been performed to investigate the effect of the nanopores on the martensitic transformation (MT) of pure iron (Fe) during the heating-cooling cycle. As per the simulation results, nanopores can obviously affect the difficulty on MT during the cooling process in two ways: On the one hand, the volume ratio of nanopores to simulation box model (Rp/b) can influence the distribution configuration of stacking fault (SF) to change the type of austenite microstructure during the heating process owing to the obstruction of the nanopores on the dislocation slip. The effect of nanopores is not obvious when Rp/b is less than 2.22 %. The main type of austenitic changes from defect-free/simple-SF types to the intersecting-SF type and, finally, to the dislocation-cell type with an increase in Rp/b from 2.22 % to 17.9 %. The dislocations in intersecting-SF type and dislocation-cell type austenitic microstructures can provide the nucleation site of MT, thus increasing the MT temperature. On the other hand, the high surface energy of the nanopores caused by the incomplete atomic lattice can provide the necessary energy for the nucleation of martensite, which then promotes the occurrence of MT during the cooling process. When the nanopores intersect with the dislocation wall in dislocation-cell type austenitic microstructure, martensite nucleates at the nanopores surface and then grows along the dislocation wall. In the previous investigation of selective laser melting martensitic steel, the nanopore in the microstructure was generally believed harmful to the mechanical property of specimen; however, in this study, it has been observed to promote the MT during solution-quenching heat treatment, which improves the mechanical property by improving the content of martensitic phase. This provides a new perspective in determining the significance of nanopores in microstructures.

Revealing the cleavage mechanism of the crack propagation process in martensitic steels

A phase field model is presented to investigate the influence of cleavage planes in martensite and austenite phases and the loading rate on the crack propagation process with considering the Kurdjumov-Sachs (K-S) orientation relationship in martensitic steels by the finite element method. The transgranular (through the martensite and austenite phases) and intergranular (along the martensite/austenite interface) fracture processes in martensitic steels can be simulated by this model. This model can simulate the crack propagation in martensitic steels with displaying the crack propagation path in austenite and martensite phases and stress distribution directly. Moreover, this model reveals the energy difference required by transgranular and intergranular fracture in martensitic steels and the hindering effect of martensite on the crack propagation process. The simulation results further indicate that the preferential cleavage planes of crack propagation in martensite and austenite phases are (100)α’ and (111)γ planes, respectively, which is consistent with the experimental results in martensitic steels. An interesting finding is that the influence of the loading rate on the maximum loading for material failure only acts on the crack propagation stage rather than the crack initiation stage. Therefore, this model can be a powerful tool to investigate the cleavage mechanism of the crack propagation process during the water quenching process and to avoid water quenching cracking in martensitic steels.

Phase Stability of Ni–(Co)–Mn–<i>Z</i> (<i>Z</i> = Ga, In, Sb, Sn) Heusler Alloys

Abstract—This work is devoted to the first-principle studies of phase stability and segregation stability of austenitic and martensitic phases of Heusler alloys Ni2 – xCoxMn1 + yZ1 – y (x = 0, 0.25, 0.5 and y = 0, 0.25, 0.5, 0.75; Z = Ga, In, Sb, Sn) with different types of magnetic ordering. Among all the considered compounds, only alloys Ni1.5Co0.5MnGa and Ni2MnGa in cubic and tetragonal structures with ferromagnetic ordering, respectively, as well as Ni2Mn2 in tetragonal structure with staggered and layer-by-layer antiferromagnetic ordering demonstrate stability. For these compositions, the presence of zero convex hull energy and the absence of reactions with positive decomposition energy are shown. The remaining compounds appear to be metastable both due to the presence of stable reactions with negative decomposition energy and decay reactions with positive decomposition energy. The number of decay reactions increases with increasing chemical disorder, i.e., deviation from stoichiometry.

Fine-grained dual-phase steels fabricated via cold-rolling ferrite-martensite structure and subsequent intercritical annealing

A proposed alternative method for processing dual phase (DP) steels is the utilization of a cold-rolled duplex ferrite-martensite structure, followed by intercritical annealing. The obtained results demonstrate the feasibility of this approach in producing a desirable microstructure. The microstructure consists of ferrite grains with an average size of less than 6 μm, along with a uniformly distributed fine martensitic phase. Treatment of the steels using this alternative route results in improved strength-ductility properties and a greater work-hardening exponent. These steels exhibit an excellent combination of mechanical properties with tensile strength of 866.69 MPa, total elongation of 24.40%, and the product of tensile strength and total elongation of exceed 20 GPa%.

Prediction of Mechanical Properties in the Sub-Critical Heat Affected Zone of AHSS Spot Welds Using Gleeble Thermal Simulator and Hollomon-Jaffe Model

Measuring the mechanical properties of weld Heat Affected Zone (HAZ) remains one of the main challenges in the failure analysis of spot-welded components. Due to the small size of the HAZ and variation in the temperature history, different peak temperatures and cooling rates impose a range of phase transformations across the resistance spot weld. Among the HAZ sub-regions, the sub-critical HAZ (SCHAZ), which experiences temperatures below AC1 (350–650 °C), usually shows a reduction in the hardness in most of the modern AHSS grades due to the martensite tempering phenomenon. SCHAZ softening may lead to strain localization during loading. Therefore, it is important to characterize the local properties of the SCHAZ region to accurately predict RSW failure. However, it is not feasible to extract standard mechanical test specimens out of the SCHAZ of the spot-welded structure due to its small size. In this work, the SCHAZ of the spot weld for two AHSS, 3G-980 and PHS-1500, was simulated using a Gleeble® (Dynamic Systems Inc., 323 NY-355, Poestenkill, NY 12140, USA) 3500 thermo-mechanical simulator. An in-situ high-speed IR thermal camera was used to measure the entire temperature field during the Gleeble heat-treatment process, which allowed for the visualization of the temperature distribution in the gauge area. The temperature and hardness data were fit to a Hollomon-Jaffe (HJ) model, which enables hardness prediction in the SCHAZ at any given temperature and time. Using the HJ model, a heat treatment schedule for each material was chosen to produce samples with hardness and microstructure matching the SCHAZ within actual spot weld coupons. Tensile specimens were machined from the coupons heat treated using simulated heat treatment schedules, and mechanical testing was performed. The results showed that the 3G-980 SCHAZ has a slight increase in yield strength and tensile strength, compared to the base metal, due to the formation of fine carbides within the microstructure. In contrast, the SCHAZ of PHS-1500 showed a significant reduction in the yield and tensile strength with yield point elongation behavior due to the reduction of the martensite phase and an increase in carbide formation due to the tempering process.

ANALISA PENGARUH HOLDING TIME DAN TEMPERATUR PADA PACK CARBURIZING MENGGUNAKAN LIMBAH KULIT SINGKONG TERHADAP SIFAT FISIK DAN MEKANIK BAJA 1020

The purpose of this study to determine the effect holding time on increasing the surface hardness of AISI 1020 steel material and analyzing the physical and mechanical properties of the results pack carburizing cassava peel waste on AISI 1020 steel. Process pack carburizing in this study have variations temperature 875oC and 900oC withholding time 60 minutes, 80 minutes, 100 minutes and 120 minutes. The catalyst used in this study was a carbonate compound, namely 20% BaCo3 to accelerate the reaction of adding carbon. The research results show that pack carburizing from cassava peel waste carbon can increase the surface hardness of AISI 1020 steel or part of the mechanical properties of the material. Testing at a temperature of 875oC withholding time 60 minutes which is 10.7 HRC and is the lowest value in the study while the highest hardness value occurs at a temperature of 900OC withholding time 120 minutes is 48.2 HRC. Microstructure observation results (metallography) on the edges indicates the martensite phase. The more martensite phase, the higher the value of the surface hardness of the steel material in the same direction as the effect of the length of holding time(holding time)

Toughening the grain boundaries by introducing a small amount of the second phase: Ni-Cu-Mn-Ga shape memory alloys as an example

The grain boundary brittleness widely exists in structural and functional materials that possess covalent bonds. Particularly, toughening the grain boundaries and simultaneously retaining the functionalities are of critical importance for functional materials. In this work, we focus on the Heulser-type Ni-Mn-X (X=Ga, In, Sn, Sb, Al, Ti) shape memory alloys, the severe grain boundary brittleness of which have hindered their developments over the past more than two decades. By performing two-step heat treatment for a Cu alloyed Ni30Cu20Mn42.4Ga7.6 alloy, a unique dual-phase configuration with a small amount of thin layer (width less than 20 µm) of single crystalline γ phase just distributing along boundaries of the martensitic grains was formed at room temperature. As a result, the bulk polycrystalline samples could be compressed with the engineering strain up to 81 % without any cracks. More importantly, the tension deformation, which was previously rather difficult for bulk single-phase Ni-Mn-X alloys, was realized in the dual-phase Ni30Cu20Mn42.4Ga7.6 alloy with a tensile strain up to 13 %, exhibiting high plasticity. Simultaneously, remarkable shape memory effect of 3.2 % under the tension condition was achieved via the martensitic transformation. Microstructure and fracture surface analysis revealed the “glue effect” of the γ phase. The high plasticity of the Ni30Cu20Mn42.4Ga7.6 alloy originated from the cooperative deformation of the martensite phase and the γ phase. This work supplies an attractive strategy of grain boundary engineering to toughen brittle functional materials.

Effect of Heat Treatment Parameters on the Modification of Nano Residual Austenite of Low-Carbon Medium-Chromium Steel

The ball milling lining board operates in a harsh environment, and the current materials fail to meet the requirements for large-sized boards due to the lack of synergistic properties between impact toughness and wear resistance. To address this issue, a low-carbon medium-chromium steel with martensite and nano residual austenite phases have been designed for future use. However, the residual austenite network could decrease the properties. Heat treatment, which includes processes like quenching and tempering, has the potential to alter the morphology and quantity of nano-scale residual austenite in the steel. In this study, the influence of heat treatment parameters on the morphologies and properties of steel has been investigated to address the wide-ranging fluctuations in impact toughness affected by nano residual austenite. Furthermore, the effect of cooling transformation on the microstructure has also been examined. The research findings indicate that modifying the quenching temperature of the steel within the range of 950–1100 °C results in a microstructure comprising martensite and nano residual austenite. At all quenching temperatures, the hardness exceeds 45 HRC, and the impact toughness shows a consistent improvement with increasing quenching temperature, indicating a modification of the nano residual austenite phase. The failure mode is primarily dimple fracture, with quasi-dissociation fracture as a secondary mode. The optimal heat treatment parameters are annealing at 930 °C, oil quenching at 1050 °C, and tempering at 250 °C. Under this condition, the steel exhibits a hardness of 51 HRC and impact toughness of 40 J/cm2 and an approximate fourfold increase compared to the untreated sample.

Strong and ductile niobium-based refractory alloy via deformable zirconia nanoparticles

The use of dispersed precipitates is an effective method for strengthening metals, but it typically results in a reduction in ductility. A potential way to overcome this strength-ductility trade-off is through the incorporation of coherent nanoscale deformable precipitates. Here, we present an approach of designing and fabricating a niobium-based (Nb-5W-2Mo-1Zr, Nb521) alloy with copiously and semi-coherently dispersed nanoscale zirconia (ZrO2) precipitates through laser powder bed fusion followed by heat treating. The intergranular ZrO2 precipitates at grain boundaries underwent tetragonal-to-monoclinic martensitic phase transformation and deformation twinning, while the intragranular ZrO2 precipitates within the matrix experienced deformation twinning and substantial plastic deformation during tension. The combination of transformation induced plasticity (TRIP) and twinning induced plasticity (TWIP) from the nanoscale ZrO2 precipitates provides sufficient capacity to accommodate strain and relieve stress concentration. Additionally, the intragranular ZrO2 precipitates effectively piled up incoming dislocations from the Nb matrix to form high-density dislocation networks during tension, which interacted with each other via the Orowan mechanism and finally resulted in its significant plastic deformation. These mechanisms contribute to the exceptional tensile properties of the Nb521 alloy, including an ultimate strength of 628.7 ± 2.3 MPa and a ductility of 17.8 ± 0.6% with a superior work-hardening capacity (∼230 MPa), achieving a record of strength and ductility combination for the Nb521 alloys fabricated by additive manufacturing to date. This strategy represents a promising approach for designing dispersion-strengthened refractory alloys with excellent synergy of strength and ductility.

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.

A Giant Exchange Bias Effect Due to Enhanced Ferromagnetism Using a Mixed Martensitic Phase in Ni50Mn37Ga13 Spun Ribbons.

The structure of a material is an important factor in determining its physical properties. Here, we adjust the structure of the Ni50Mn37Ga13 spun ribbons by changing the wheel speed to regulate the exchange bias effect of the material. The characterization results of micromorphology and structure show that as the wheel speed increases, the martensite lath decreases from 200 nm to 50 nm, the structure changed from the NM to a NM and 10M mixed martensitic structure containing mainly NM, then changed to NM and 10M where 10M and NM are approaching. Meanwhile, HE first increased and then decreased as the wheel speed increased. The optimum exchange bias effect (HE = 7.2 kOe) occurs when the wheel speed is 25 m∙s-1, mainly attributed to the enhanced ferromagnetism caused by part of 10M in NM martensite, which enhanced the exchange coupling of ferromagnetism and antiferromagnetism. This work reveals the structural dependence of exchange bias and provides a way to tune the magnitude of the exchange bias of Heusler alloys.

A Phase Field Study of the Influence of External Loading on the Dynamics of Martensitic Phase Transformation

An elastoplastic phase field model was employed for simulations to investigate the influence of external loading on the martensitic phase transformation kinetics in steel. The phase field model incorporates external loading and plastic deformation. During the simulation process, the authenticity of the phase field model is ensured by introducing the relevant physical parameters and comparing them with experimental data. During the calculations, loads of various magnitudes and loading conditions were considered. An analysis and discussion were conducted concerning the volume fraction and phase transition temperature during the phase transformation process. The simulation results prominently illustrate the preferential orientation of variants under different loading conditions. This model can be applied to the qualitative phase transition evolution of Fe-Ni alloys, and the crystallographic parameters adhere to the volume expansion effect. It is concluded that uniaxial loading promotes martensitic phase transformation, while triaxial compressive loading inhibits it. From a dynamic perspective, it is demonstrated that external uniaxial loading accelerates the kinetics of martensitic phase transformation, with uniaxial compression being more effective in accelerating the phase transformation process than uniaxial tension. When compared to experimental data, the simulation results provide evidence that under the influence of external loading, the martensitic phase transformation is significantly influenced by the applied load, with the impact of external loading being more significant than that of plastic effects.

Experimental and numerical investigation of martensite phase transformation in austenitic stainless steel subjected to ultrasonic nanocrystal surface modification at room and cryogenic temperatures

This study aims to investigate the evolution of martensite volume fraction (ξ) following ultrasonic nanocrystal surface modification (UNSM) on metastable austenitic stainless steel 304L specimens at room and cryogenic temperatures using both numerical and experimental methods. A user subroutine (VUMAT) was utilized to implement a constitutive model that describes the phase transformation occurring in the target during the UNSM process. An alternative method was employed to simulate UNSM. The numerical predictions of residual stress (σres) and ξ exhibited good agreement with those measured experimentally. Finite element models were employed to undertake a comprehensive study of the impact of UNSM parameters on σres and ξ. The findings demonstrated that UNSM at cryogenic temperature produced significantly higher levels of compressive σres in comparison with UNSM at room temperature, while the depth of the layer with compressive σres and the extent of plastic deformation decreased. Based on the experimental and numerical findings, a significant value of ξ was observed to form within the subsurface region subsequent to UNSM at both temperature conditions. The value of ξ on the surface of UNSMed specimens at room and cryogenic temperatures was quantified using X-ray diffraction, resulting in respective values of 82 % and 88 %. Moreover, a significant increase in near-surface layer hardness for the UNSMed specimen at cryogenic temperature was observed, with a value of approximately 600 Vickers. This value is notably higher than the hardness of 480 Vickers obtained for the UNSMed specimens at room temperature.

Analysis of Hydrogen Distribution and Diffusion in Pre-Strained SUS316L through Scanning Kelvin Probe Force Microscopy and Thermal Desorption Spectroscopy

Austenitic stainless steels (γ-SS) play an important role in the storage of high-pressure hydrogen. However, hydrogen embrittlement (HE) can significantly degrade the mechanical properties of γ-SS. Measuring the distribution of hydrogen in γ-SS is a vital way to learn about HE. In this paper, scanning Kelvin probe force microscopy (SKPFM) and thermal desorption spectroscopy (TDS) have been utilized to analyze the distribution and diffusion of hydrogen in pre-strained SUS316L. Additionally, the McNabb–Foster model is employed to calculate hydrogen in the lattice and phase boundaries along the sample’s thickness direction. The results demonstrate that the combination of SKPFM and TDS is an effective approach for studying hydrogen distribution and diffusion in metals. It was observed that hydrogen segregation occurs at the boundary between the martensitic (α′) and austenite (γ) phases. The inhibitory effect of the oxide film on hydrogen diffusion is more significant at lower temperatures. However, it should be noted that the McNabb–Foster model exhibits relatively high accuracy in predicting hydrogen desorption at higher temperatures while disregarding the influence of the native oxide film.