Austenite Phase Transformation Research Articles

Phase transformation behavior and shape memory effect of nickel-titanium shape memory alloy using the magnetic field assisted wire electrical discharge machining

Nickel-titanium shape memory alloy (NiTi-SMA) can restore its original shape after deformation, making it show great application in the fields of biomedicine, aerospace, etc. The wire electrical discharge machining (WEDM) has the advantage of machining micro and complex structures, which can greatly promote the application of NiTi-SMA. However, there are issues such as heat affected layer (HAL) on the surface of WEDM, which can affect the functionality of NiTi-SMA. Therefore, when using WEDM to NiTi-SMA, more attention should be paid to the functional changes after machining. This article primarily examine is the impact of peak current and pulse on time on the phase transformation behavior, shape memory effect, and mechanical properties of NiTi-SMA during WEDM, and introduces into the magnetic field assisted (MFA) process to improve the machining performance of NiTi-SMA. The results indicate that the HAL formed on the surface of NiTi-SMA is primarily composed of the irreversible phase transformation of B2 austenite. This layer induces a hardening effect, resulting in an increase in the phase transition temperature and a decrease in the phase transition energy (∆HM-A), as a result, the martensitic phase transformation behavior is suppressed. When the peak current and pulse on time increase, the surface quality of the NiTi-SMA processed decreases. The thickness of the HAL and microhardness increase, causing an increase in the phase transition temperature of the NiTi-SMA processed samples. Additionally, the transformation energy ∆HM-A, shape recovery rate, and mechanical properties decrease, and after recovery, more surface cracks appear and show larger extensions. MFA-WEDM can improve the surface quality of NiTi-SMA processing, reduce the thickness of the HAL, and effectively enhance the phase transformation temperature and energy ∆HM-A of the NiTi-SMA processed samples. With a magnetic field strength of 0.45 T, the tensile shape recovery rates of 98%, 94.4%, and 91.0% were achieved at 2%, 5%, and 8% strain, respectively. This represents an improvement of 8.9% compared to the maximum shape recovery rate with conventional WEDM. At the same time, it also inhibits the generation and propagation of surface cracks after deformation recovery. Additionally, MFA-WEDM improves the mechanical properties of NiTi-SMA, and reduces the thickness of the brittle fracture region by 49.7% compared to conventional WEDM. Therefore, MFA-WEDM can effectively enhance the shape memory effect of NiTi-SMA. However, it is important to avoid excessive peak currents and pulse on time to achieve optimal shape memory functionality.

Composition and treatment of wear resistant steel for application in mining industry

The service life of machines in the mining industry is in the most cases determined by wear rate of parts in contact with the rock and soil. This work demonstrates the results of investigation on wear resistance of high-carbon low-alloy steel under abrasive wear. Steel samples containing 1.2 wt.% of carbon, 3 wt.% of manganese and 2 wt.% of silicon were hardened from 900 °C and 1000 °C and subjected to two-body and three-body abrasion tests. It has been established that after heat treatment this steel has increased abrasive wear resistance due to the phase transformation of unstable austenite into deformation-induced martensite. The results of study of near-surface region microstructure of samples after wear are presented. X-ray diffraction analysis and measurement of the microhardness of the worn surface were also carried out. It has been established that during abrasive wear a continuous layer of deformation martensite with a microhardness of 1300-1400 HV 0.05 is formed at a depth of up to 10 μm. Such a microhardness significantly exceeds the microhardness of quenching martensite. The results of the work can be used to increase the service life of wear parts of mining equipment.

Multi-field coupling numerical simulation and experimental research of submerged arc surfacing process on the descaling rolls with different wall thickness

Iron and steel industry descale rolls long-term high-pressure water erosion, high temperature, easy to wear, corrosion failure and other problems, the continuous production of steel rolling impact, descale rolls base wall thickness is the main cause of cracking of the surfacing gradient lap organization. This article combines numerical simulation and experimental testing, the use of CALPHAD method to calculate the surfacing material temperature change physical parameters, based on the double ellipsoid heat source model, the establishment of the descale roller submerged arc surfacing process of multi-field coupling three-dimensional numerical model, calculations and reveal the descale roller submerged arc surfacing process of the temperature field, the stress field, the phase change field and the flow field of the transient evolution of the law, and further analyze the descale roller wall thickness of the surfacing process of the influence of the mechanism. Calculation results show that the depth of melting of the surfacing matrix is positively correlated with the width of the double ellipsoid heat source, and the larger the wall thickness, the faster the cooling rate, the greater the formation of residual stress. The austenite and martensite phase transformation of the overlay welding process is greatly influenced by the wall thickness. At the same time, SEM, EDS, XRD material science microcharacterization experiments were carried out to observe the micro-morphology, phase and elemental composition of the cladding layer. This study lays an important theoretical foundation for optimizing the submerged arc surfacing process of descale roller.

Effect of Ball to Powder Ratios on the Phase Transformation of Austenite and Ferrite During Ball Milling of SAF-2507 Super Duplex Stainless Steel Powders

Effect of Ball to Powder Ratios on the Phase Transformation of Austenite and Ferrite During Ball Milling of SAF-2507 Super Duplex Stainless Steel Powders

Lüders and Portevin–Le Chatelier processes in austenitic-martensitic TRIP steel

The authors studied the nature of mobile fronts of localized deformation that generate and propagate during deformation of metastable austenitic-martensitic TRIP steel VNS9-Sh along the entire length of the loading curve from the yield point to fracture. A joint research of the nature of the deformation fronts movement and kinetics of the magnetic phase accumulation made it possible to establish that the fronts under consideration are the fronts of the thermoelastic phase transformation of metastable austenite into martensite. This transformation is realized firstly by formation of the Chernov–Lüders bands and then the Portevin–Le Chatelier bands. Both processes are consistent with staging of the deformation curve, which contains a pseudo-plateau, a section with an increasing hardening coefficient, and a section with a decreasing hardening coefficient. It is shown that the deformation-induced phase transformation corresponds to the fronts propagating on the pseudo-plateau and on the section of loading curve with an increasing hardening coefficient. The Portevin–Le Chatelier bands, which are formed in the section of the loading diagram with a decreasing hardening coefficient, are not associated with “austenite-martensite” transformation and have a twin nature. The kinetics of thermoelastic transformation fronts, as well as deformation fronts in materials with a shear mechanism of shaping, can be described in terms of the autowave concept. On the yield plateaus, the phase transformation occurs through generation and propagation of localized plasticity switching autowaves. In the section with an increasing hardening coefficient, it continues through generation and movement of excitation autowaves. The propagation regions of excitation autowaves are limited in the sample space. They are set by the zones of origin and annihilation of primary switching autowaves which were formed on the yield plateau.

Effect of martensitic transformation on ratchetting in medium-manganese steel: Experiment and homogenized constitutive model

Transformation-induced plasticity (TRIP) behavior and related cyclic plasticity model of medium-manganese steel subjected to uniaxial cyclic loading were studied. Cyclic loading tests were conducted at different mean stresses and stress amplitudes. The results show that medium-manganese steel exhibits two different ratchetting evolution characteristics. Moreover, microscopic characterizations reveal that martensitic transformation occurs in medium-manganese steel during asymmetrical cyclic stressing, and the higher the applied stress amplitude, the greater the ratchetting strain and the more likely the martensitic transformation will occur. The interaction between TRIP and ratchetting leads to accelerated accumulation of plastic strain. Based on the existing meso-mechanics cyclic elastoplastic constitutive model, a kinetic equation for the strain-induced martensitic transformation and a modified nonlinear kinematic hardening equation are introduced. Subsequently, the two-leveled homogenization method was used to construct a homogenized cyclic plasticity model containing a three-phase microstructure of austenite, martensite, and ferrite. By comparing the predicted results with the experimental results, the proposed model can reasonably describe the evolutionary features of each phase and the effect of the phase transformation on the ratchetting of medium-manganese steel. Finally, ratchetting is further predicted and discussed for different austenite contents and phase transformation rates using the proposed model.

EFFECT OF COLD ROLLING PROCESS ON TEXTURE EVOLUTION IN AUSTENITIC STEEL SHEETS

The investigation presents the evolution of texture and microstructure of cold-rolled austenitic AISI 301 stainless steel sheets. The austenite steel was deformed by cold-rolling to 30%, 50% and 80% of rolling reduction. It was found, that during cold plastic deformation, phase transformation of austenite into martensite takes place. The transformation progress was confirmed by the microscopic investigation, as well as by X-ray diffraction methods. Texture analysis was performed on the basis of the orientation distribution functions (ODFs) calculated from the experimental pole figures. The texture of the deformed austenite is dominated by fiber orientations α =110║KN, mainly Goss orientation {110}001 and alloy type orientation {110}112. On the other hand, plastic deformation-induced martensitic transformation of the type (γ') significantly affects the development of martensite texture ' in the cold rolling process. After rolling with a 30% of deformation degree in the texture of  martensite, the strongest orientation is {001}110, while after maximum deformation (about 80%), the {111}112 and {112}110 type orientations dominate. It was found that in AISI 301 steel structure after deformation range from 30% to 80% proportion of α’ martensite phase changes from about 26.7% to about 67.8%.

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.

White etching area damage induced by shear localization in rolling contact fatigue of bearing steel

AbstractWhite etching area (WEA) is a primary damage in rolling contact fatigue (RCF) of bearing steels. In spite of extensive investigations, there is a large discrepancy in the existing mechanisms of WEA formation. We attempt to unify the mechanisms from the perspective of shear localization and plastic damage accumulation based on ductile damage. RCF tests were conducted to generate WEAs with various microstructures and compositions. A thermodynamically consistent model of the ductile damage evolution from an inclusion was established by developing the phase field damage coupled with the crystal elastic‐viscoplastic constitutive relationship under RCF. The model was implemented into the FE framework through a user materials subroutine. The results indicated that the WEA is the shear band resulting from shear localization. The large inhomogeneity and scatter in WEA's microstructure are due to the influence of the crystal orientation. The development and orientation of SBs predicted by the model and the experimental observation of the WEA are in good agreement. The large micro‐shear strain in the WEA provides the driving force for mechanically controlled austenite phase transformation. The shear band center, which has the largest strain and the least stress, is where cracks initiate. This demonstrates that contrary to earlier reports that WEA is induced by previously formed crack faces friction, cracks actually initiate from the interior of the WEA.

The effects of heat-treatment parameters on the mechanical properties and microstructures of a low-carbon dual-phase steel

The austenite decomposition phase transformation in a low-carbon dual-phase (DP) steel is studied as a function of inter-critical annealing parameters: annealing time, annealing temperature, and cooling rate. The austenization kinetics are obtained by dilatometric analyses, water-quenched metallography, and thermodynamic calculations, and the austenite formed at different inter-critical annealing temperatures influences the subsequent formation of secondary phases during continuous cooling. Results indicate that the austenite volume fraction increases with both annealing temperature and time; although their effect on the final microstructure and mechanical properties lessens when the cooling rate decreases to the slow cooling regime below air cooling. Interestingly, the yield strength (YS) values of samples cooled at ∼1 °C/s with sand cooling, are 75∼100 MPa greater than the corresponding values at a rate of ∼6 °C/s with air-cooling. The results indicate that the yield strength is positively correlated with yield-point elongation but negatively correlated with the ultimate tensile strength. The effects of the cooling rate on the mechanical properties can be explained by the formation of Cottrell atmospheres of carbon together with the increase of the pearlite volume fraction. Based on the secondary phase components and strength evolutions, there are four possible outcomes for the different cooling rates: (I) martensite (M); (II) ferrite-martensite (FM); (III) coexistence of martensite and pearlite (MP); and (IV) ferrite-pearlite (FP). Our results reveal that the microstructure-property correlation can be illustrated schematically as a function of cooling rates and can thereby direct the heat-treatment parameters required to obtain desirable mechanical properties.

Variable stiffness mechanism of flexible composite laminates implanted with modular SMAHC

In this study, a new flexible hybrid shape memory alloy composites (SMAHCs) with a polydimethylsiloxane (PDMS) matrix containing a shape memory alloy (SMA) of a hybrid Basalt/Kevlar fiber 3D woven composite is developed. The SMAHC is implanted into a glass fiber/PDMS composite laminate in a modular manner to study the stiffness of composite plates in response to temperature. The variable stiffness theory of the SMAHC is derived, and the influence of SMA parameters on the stiffness of the SMAHC is analyzed. A three-point bending test is performed to determine the stiffness of the composite plates at multiple temperature points in the form of Joule heat driving the composite plates. The stress and temperature distributions of the composite plate are analyzed in Abaqus under a temperature-displacement coupling setting. The results shows that the SMAHC stiffness is negatively correlated with SMA fiber diameter and positively correlated with SMA fiber volume fraction. With an increase in driving temperature, the stiffness of the composite plates tends to decrease. When the end temperature of the austenitic phase transformation is reached, the high-modulus austenitic SMA slightly increase the stiffness of the composite plate. The stiffness improvement is more significant when the SMAHC is placed in the middle area of the laminate and the SMA is located in the tension area. This study may provide a reference for the design of variable stiffness SMAHC for plate composite structures.

Effects of Building Directions on Microstructure, Impurity Elements and Mechanical Properties of NiTi Alloys Fabricated by Laser Powder Bed Fusion.

For NiTi alloys prepared by the Laser Powder Bed Fusion (LPBF), changes in the building directions will directly change the preferred orientation and thus directly affect the smart properties, such as superelasticity, as well as change the distribution state of defects and impurity elements to affect the phase transformation behaviour, which in turn affects the smart properties at different temperatures. In this study, the relationship between impurity elements, the building directions, and functional properties; the effects of building directions on the crystallographic anisotropy; phase composition; superelastic properties; microhardness; geometrically necessary dislocation (GND) density; and impurity element content of NiTi SMAs fabricated by LPBF were systematically studied. Three building directions measured from the substrate, namely, 0°, 45° and 90°, were selected, and three sets of cylindrical samples were fabricated with the same process parameters. Along the building direction, a strong <100>//vertical direction (VD) texture was formed for all the samples. Because of the difference in transformation temperature, when tested at 15 °C, the sample with the 45° orientation possessed the highest strain recovery of 3.2%. When tested at the austenite phase transformation finish temperature (Af)+10 °C, the 90° sample had the highest strain recovery of 5.83% and a strain recovery rate of 83.3%. The sample with the 90° orientation presented the highest microhardness, which was attributed to its high dislocation density. Meanwhile, different building directions had an effect on the contents of O, C, and N impurity elements, which affected the transformation temperature by changing the Ni/Ti ratio. This study innovatively studied the impurity element content and GND densities of compressive samples with three building directions, providing theoretical guidance for LPBFed NiTi SMA structural parts.

To flat or not to flat? Exploring the impact of flat-side design on rotary instruments using a comprehensive multimethod investigation.

To assess the influence of a flat-side design on the geometry, metallurgy, mechanical performance and shaping ability of a novel nickel-titanium rotary instrument. Sixty-five new 25-mm flat-side rotary instruments (size 25, taper 0.04) and their nonflat-side prototypes (n = 65) were assessed for major deformations and examined regarding macroscopic and microscopic design, determination of nickel and titanium elements ratio, measurement of phase transformation temperature and evaluation of mechanical performance parameters including time/cycles to fracture, maximum torque, angle of rotation, maximum bending and buckling strengths and cutting ability. Additionally, unprepared canal areas, volume of hard tissue debris and percentage reduction of dentine thickness were calculated for each tested instrument after preparing mesial canals of mandibular molars (n = 12), using micro-CT imaging. Statistical analyses were performed using the U-Mann-Whitney test and independent Student t-test (α = 5%). The number of spirals (n = 8) and blade direction (clockwise) were similar between both flat and nonflat instruments, whilst the helical angles were equivalent (⁓25°). Flat-instruments showed inconsistencies in the homogeneity of the gold colour on the flat-side surface, blade discontinuity, and incomplete and variable S-shaped cross-sections. The titanium-to-nickel ratios were equivalent, but significant differences in the R-phase finish and austenitic start phase transformation temperatures were observed between the flat and nonflat-side instruments. The flat-side instruments demonstrated superior cutting ability compared to the nonflat instruments, as well as, significantly lower values for time to fracture, rotation to fracture and maximum torque to fracture (p < .001). No statistical difference was observed between tested instruments regarding angle of rotation (p = .437), maximum bending (p = .152) and buckling load (p = .411). Preparation protocols using flat and nonflat instruments did not show any statistically significant differences (p > .05). All flat-side instruments exhibited deformation after shaping procedures. The flat-side instrument showcased enhanced cutting ability compared to its nonflat counterpart. However, it exhibited inferior performance in terms of time, rotation and maximum torque to fracture, along with distinct phase transformation temperatures. No differences were observed in the titanium-to-nickel ratios, angle of rotation, maximum bending, buckling load, preparation time, percentage of untouched canal walls, volume of hard tissue debris and percentage reduction of dentine thickness.

Numerical Simulation and Surface Properties of 42CrMo Steel Treated by Plasma Nitriding and Laser Quenching

In this study, we investigated the nitriding and laser quenching composite modified layers of 42CrMo steel. MATLAB was used to fit the nitrogen concentration distribution during nitriding, and the laser temperature field was fitted using ABAQUS finite element simulation software. Two groups of simulation results were integrated to fit the modified layer depth under different processes, and the nitriding and laser quenching experimental results were compared with the simulation results, which indicated that the simulation results agreed well with the experimental results. The depth of the nitriding–laser quenching composite layer greatly improved compared with the nitriding or laser hardening layers. The austenitizing temperature of the 42CrMo steel was reduced to 577 °C by nitriding. Therefore, the depth of the austenitized layer of the 42CrMo steel heated with the same laser power significantly increased. Under the same laser process conditions, more austenitic phase transformation was observed in the nitriding layer than in the non-nitriding layer, so martensitic phase transformation was more likely to occur in the subsequent cooling process. After plasma nitriding at 460 °C for 16 h and laser quenching, the modified layer depth of the 42CrMo steel reached 990 μm, and the average surface hardness of the 42CrMo steel reached 625 HV0.1. The friction coefficient of the modified layer was the lowest, with a value of 0.433, and the minimum wear value was 1.024 mm3. Double hardness and thickness of the modified layer could be obtained by nitriding and laser quenching composite processes.

Revealing the kinetics of non-metallic inclusion reactions in steel using in-situ high temperature environmental scanning electron microscopy

The kinetics of solid-state reactions involving non-metallic inclusions (NMIs) in steel is a topic of growing interest due to the need to understand the effects of heat treatments on NMI composition and stability. In this study, a High Temperature Environmental Scanning Electron Microscope (HT-ESEM) is utilized to conduct in-situ observations of various types of NMIs, including calcium aluminate complex, CaS, MgAl2O4-CaS, MnS, and TiN, in steels at temperatures of 800 °C and 900 °C for different holding times. The observed morphological variations in the NMIs indicate a decrease in the fractions of CaS and MnS, promoting void formation and ferrite–austenite phase transformation. To estimate the NMI reactions with the residual atmosphere in the sample chamber during experiments, HT-ESEM-driven thermodynamic calculations have been performed using FactSage 8.2 software. These calculations provide valuable insights into the interpretation of the experimental data, facilitating a better understanding of the complex NMI reactions in steel.

Failure analysis of ball screw in the progressive induction hardening and finishing machining

Some failure modes, such as wear, quenching and finishing cracks, are easy to produce on the surface of the ball screw during the progressive induction hardening, and can shorten the fatigue life of ball screw. The hardened layer, microstructure, tensile strength and cracks of a 55CrMo steel ball screw after induction hardening are tested or observed by experiment. Based on the results of experiment and numerical simulation, the effect of heating temperature on the austenitization, microstructure, tensile strength, grain size of austenite and stress state of the ball screw are analyzed and discussed. In the induction hardening process, high heating temperature can result in some problems such as coarse grains and melting of the eutectic Fe-FeS-MnS produced on the grain boundary, which will impact the quenching quality and service performance of ball screw. Before the phase transformation of austenite transformed to martensite, the channel top of ball screw is in the state of three-dimensional tensile stress, which leads to brittle fracture during the cooling process. In the cutting process, the mechanical and thermal stress successively generate the tensile stress in the inner and outer layers of the ball screw channel, resulting in the internal cracks formed during quenching gradually extending to the surface. The essential cause of cracks in the finishing of the ball screw is the cracks formed during the induction hardening.

Significance of adiabatic heating on phase transformation in titanium-based alloys during severe plastic deformation

Experiments were conducted on Ti-based alloys including a commercially pure titanium (α phase), Ti-6Al-4 V (α + β phase) and TiNi (B19’-martensitic phase) alloys in order to investigate the phase transformation during high-pressure torsion (HPT) at room temperature. An allotropic α to ω phase transformation in the commercially pure titanium occurred during HPT processing but increasing the rotation speed led to a decrease in the volume fraction of the ω-phase. This procedure facilitates the loss of the β phase in the Ti-6Al-4 V alloy and promotes a B19’ to B2 (austenite) phase transformation in the TiNi alloy. The results show that the bulk temperature increment in the processed specimens was not sufficiently high to affect phase transformations. However, the calculations by assuming adiabatic conditions revealed that the local temperature increment is responsible to encourage or suppress the phase transformation in the Ti-based alloys. This fundamental understanding of phase transformations during severe plastic deformation is useful for conducting microstructure engineering of Ti-based alloys by tailoring the microstructures containing different phases and thereby producing different mechanical and functional behaviors.

Giant magnetoelectric effect of Ni–Mn-Ga/piezopolymer composites tailored by a martensitic transformation

Polymer-based magnetoelectric (ME) composites, composed of magnetostrictive and piezoelectric phases, in addition to exhibiting an effective coupling between the magnetic and electric orders of the matter, also offer important advantages for the Internet-of-Things, digitalization, and 4.0 revolution environments: light weight, flexibility, wearability, environmental friendliness, printability, and biocompatibility. Nevertheless, their successful implementation on applications such as sensors, actuators, energy harvesters, biomedical devices and spintronics strongly depends on the increase of its ME voltage response.This work explores, both experimentally and theoretically, the magnetostrictive Ni–Mn-Ga's martensite ‒ austenite phase transformation to shift/tailor/design the ME resonance peak exhibited by Ni–Mn-Ga/P(VDF-TrFE) piezoelectric composites. In the austenite phase, the determined peak value of the ME coefficient of 18.1 V cm−1 Oe−1 was much higher than the value of 6.05 V cm−1 Oe−1 obtained for the martensite phase, whereas the estimated theoretically value of magnetoelastic constant in the austenite is much smaller. The ways to further increase the magnetically induced ME response are also outlined.

Engineering austenite/martensite mesostructured materials by controlled localised laser treatments in a Fe–Ni–C alloy

Localised laser treatments enable the creation of sophisticated austenite/martensite mesostructures in Fe–Ni–C steel with the potential of achieving enhanced mechanical performance. The control of phase topology is essential to modify the properties of these structures on demand and requires a profound understanding of the effect of the processing parameters on the development of the different phases upon the application of laser treatment. In this work, the microstructure evolution under exceptional gradients in temperature and heating rates is thoroughly investigated. The extent of the laser-affected zone and the heat input were tailored by varying laser parameters and specimen thickness, based on a model that considers transient material properties and the coupling between temperature and microstructure. The predicted temperature fields resulted in a complex interplay between martensite to austenite phase transformation and martensite tempering. Considering the high heating rates of up to 25000 K/s and the observed microstructures, it is suggested that austenite was formed by a pseudo-displacive mechanism and subsequently fully recrystallised in the zones most directly affected by the laser heat source. A smooth strength transition from austenite to martensite, affected by the laser parameters, could be exploited for more effective deformation mechanisms and improved material mechanical properties.

A Detailed Analysis of the Microstructural Changes in the Vicinity of a Crack-Initiating Defect in Additively Manufactured AISI 316L

The fatigue life of metals manufactured via laser-based powder bed fusion (L-PBF) highly depends on process-induced defects. In this context, not only the size and geometry of the defect, but also the properties and the microstructure of the surrounding material volume must be considered. In the presented work, the microstructural changes in the vicinity of a crack-initiating defect in a fatigue specimen produced via L-PBF and made of AISI 316L were analyzed in detail. Xenon plasma focused ion beam (Xe-FIB) technique, scanning electron microscopy (SEM), and electron backscatter diffraction (EBSD) were used to investigate the phase distribution, local misorientations, and grain structure, including the crystallographic orientations. These analyses revealed a fine grain structure in the vicinity of the defect, which is arranged in accordance with the melt pool geometry. Besides pronounced cyclic plastic deformation, a deformation-induced transformation of the initial austenitic phase into α’-martensite was observed. The plastic deformation as well as the phase transformation were more pronounced near the border between the defect and the surrounding material volume. However, the extent of the plastic deformation and the deformation-induced phase transformation varies locally in this border region. Although a beneficial effect of certain grain orientations on the phase transformation and plastic deformability was observed, the microstructural changes found cannot solely be explained by the respective crystallographic orientation. These changes are assumed to further depend on the inhomogeneous distribution of the multiaxial stresses beneath the defect as well as the grain morphology.