Microstructural Investigations Research Articles

The impact of heat treatment process on the drilling characteristics of Al–5Si–1Cu–Mg alloy produced by sand casting

Aluminum–Silicon (Al–Si) based materials are commonly preferred in engineering studies requiring high mechanical performance as an alternative to steel materials. These alloys are especially preferred in the automotive industry, aerospace components and heavy machinery parts. In post-casting processes, machining operations are of great importance for high geometric precision, surface quality and longer fatigue life in mechanical environments. In this research, the microstructural, mechanical and machining characteristics of the Al–5Si–1Cu–Mg material produced by sand casting method in both as-cast (AC) and heat-treated (HTed) (Solid solution, quenching and aging-SQA) states were experimentally investigated. Microstructural examinations were carried out with optical microscope and SEM images. Mechanical properties were determined by tensile and hardness tests. Then, drilling experiments were performed in the CNC vertical machining center with 8 mm diameter uncoated HSS (High Speed Steel) cutting tools at constant cutting conditions (i.e., cutting speed-V: 125 m/min, feed rate-f: 0.05 mm/rev and depth of cut-DoC: 15 mm). In microstructural investigations, it was determined that the microstructure of the Al–5Si–1Cu–Mg material in AC state consists of α-Al, eutectic Si, β-Fe (β-Al5FeSi) and π-Fe (π-Al8Mg3FeSi6) intermetallics. After the SQA, the existing phases generally exhibited a spherical structure, and it was seen that the β phase in the microstructure transformed into the Ɵ (Al7FeCu2) phase. SQA improved the hardness, yield and tensile durability of the material, whereas decreasing the elongation to fracture. SQA process improved the machining characteristics of the material by decreasing thrust force, moment, surface roughness and built-up edge (BUE) formation. The machined subsurface structure of HTed alloy under constant cutting conditions was determined to be more stable and smooth and the machined surface microhardness of HTed alloy was higher compared to as-cast alloy due to the effect of solid precipitation hardening. In addition, shorter and more broken chips occurred in the machining of HTed alloys due to the effect of low elongation to fracture.

Systematic Investigation of Microstructures and Gas Separation Performance of Thermally Rearranged Polybenzoxazole Membranes Derived from 9,9-Bis(3,4-dicarboxyphenyl)fluorene Dianhydride

Systematic Investigation of Microstructures and Gas Separation Performance of Thermally Rearranged Polybenzoxazole Membranes Derived from 9,9-Bis(3,4-dicarboxyphenyl)fluorene Dianhydride

Mechanical and microstructural investigation of multi-layered Inconel 825 wall fabricated using CMT-based WAAM

In this research, a multi-layered wall was produced using the Wire-Arc Additive Manufacturing (WAAM) technique, specifically employing the Cold Metal Transfer (CMT) method with Inconel 825 wire. The optimized CMT-WAAM parameters were identified using multivariate regression analysis. The mechanical and microstructural properties of the wall were assessed in its lower, middle, and upper sections. The tensile properties showed that the ultimate tensile strength (UTS) ranged from 505 MPa to 514 MPa, closely matching that of conventionally wrought Inconel 825 (505–514 MPa). The yield strength (YS) varied from 199 MPa to 207 MPa, while elongation values ranged from 49.7 % to 57.5 %, depending on the section of the wall. A gradual decrease in hardness was observed from the bottom (246.16 Hv) to the top (221.75 Hv) of the wall. Microscopy identified continuous and discontinuous cellular-dendritic microstructures across the sections. Tensile and impact test fractographs revealed a fibrous ductile fracture mode, with SEM images highlighting the presence of Laves phases and micro-voids, particularly in the upper sections. Despite the formation of Laves phases, which can act as crack initiation sites, the mechanical properties of the WAAM-fabricated wall were comparable to those of wrought Inconel 825.

Evaluation of cement-treated recycled concrete aggregates for sustainable pavement base/subbase construction

Rapid urbanization and population growth have caused a substantial rise in the generation of all kinds of waste globally. The use of recycled waste not only can replace the enormous volume of virgin materials used in construction but also alleviate environmental concerns by lowering landfilling loads. This study aims to evaluate the feasibility of using non-treated and cement-treated recycled concrete aggregates (RCA) in pavement base and subbase layers. A detailed study focused on the strength, durability and microstructural aspects of treated RCA. To fulfill this, experimental test program and analysis covered particle-size distribution, Los Angeles abrasion, aggregate impact value, modified Proctor compaction, unconfined compressive strength (UCS), weight loss under wet-dry cycles, and scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) analysis. The recycled aggregates are found to show excellent physical and mechanical characteristics in terms of abrasion and aggregate impact values. Cement treatment has shown to significantly enhance the strength and durability characteristics of RCA, rendering it suitable for pavement base/subbase layer application. RCA samples treated with a cement content of 3% and above (by weight) are found to satisfy the minimum specified unconfined compressive strength (UCS) values of 2MPa at 7 days of curing, according to the Texas Department of Transportation (TxDOT) and Austroads guidelines for pavement base and subbase layer applications. However, a relatively higher cement contents (5% and 7%) is required to meet the minimum 7-days UCS criterion of 3 and 4.5MPa for base layer applications of low- and high- volume roads, respectively, according to Indian road standards (IRC SP 72 (2015) and IRC 37(2018)). Cement-treated samples with 2% cement or above showed minimal weight loss after 12 alternate wet-dry cycles, which was well within the maximum permissible limit (14%) specified in various standards for stabilized materials. Additionally, microstructural investigations have clearly showed formation of hydration products corroborating the mechanism behind strength gain with time. Findings from this study provide references to promote the use of RCA in construction of pavement base and subbase layer, contributing towards the goal of net zero set by the United Nations.

Synthesis, Microstructural Investigation, and Mechanical Behavior of AA5083/Boron Nitride Nanocomposite

The as‐received aluminum alloy (AA5083) powders (44 μm) and hexagonal boron nitride (BN) nanoparticles (65–75 nm) are used to fabricate AA5083‐BN nanocomposites with varying BN content (0, 3, 6, 9, and 12 wt%). A powder metallurgy solid‐state method is employed, involving ball milling for 20 h at 100 rpm with a ball‐to‐powder mass ratio of 10:1. The processed powders are then consolidated through forging–sintering at 550 °C for 30 min, followed by hot forging at 225 MPa. The microstructure is examined using X‐ray diffraction, scanning electron microscopy, energy‐dispersive X‐ray, transmission electron microscopy, and electron backscatter diffraction to understand the effect of processing variables. The compaction behavior is investigated both experimentally and empirically, revealing a relative green density exceeding 88% at 500 MPa. The empirical models display coefficients of determination (R2 values) exceeding 99% for predicting the compaction behavior. Compression tests on bulk samples show that BN reinforcement significantly enhances ultimate compressive strength, with values reaching 473.256 ± 5.54, 536.374 ± 2.87, 567.694 ± 4.22, and 601.911 ± 6.54 MPa for TRB‐3, TRB‐6, TRB‐9, and TRB‐12, respectively. The developed composites achieve relative densities greater than 97%, indicating promising applications in the aerospace, automotive, and marine industries.

A Comprehensive investigation of microstructure and magnetic properties of novel equiatomic (MnFeCoNi)3O4 cubic spinel nanoparticles

A Comprehensive investigation of microstructure and magnetic properties of novel equiatomic (MnFeCoNi)3O4 cubic spinel nanoparticles

Efficient Optimization of Friction Stir Welding Parameters for AA7075 and AA6082 Alloys: A Hungarian Method Approach

Friction stir welding (FSW) is a key technique for welding high-strength aluminium alloys without melting, making it indispensable in industries where maintaining material integrity is of utmost importance. To determine the optimal FSW parameters for AA7075 and AA6082 alloys, this study presents a novel application of the Hungarian Method, a combinatorial optimization technique. The weldments were produced experimentally by applying different feed rates (40, 50, and 60mm/min) and rotational speeds (1000, 1100, and 1200rpm) in accordance with a L9 orthogonal array. The optimal combination was, a feed rate of 40mm/min and a rotational speed of 1200rpm, yielding an UTS of 162.26MPa, a % elongation of 2.56 and an impact energy of 2.5J/mm². The microstructural investigation showed substantial improvement in the grain refinement of the weld nugget, which significantly contributed to the improved mechanical properties. However, it exhibited significant negative corrosion potential compared to the base metals under optimal welding conditions, indicating a higher susceptibility to corrosion. These results show that the Hungarian Method provides a systematic approach to optimize FSW parameters, but it might be necessary to adapt post-weld treatments to lower the chances of corrosion. This study illustrates not only the application of the Hungarian Method in the optimization of operating parameters of FSW but also establishes a base for further studies dealing with combinatorial optimization problems in welding and other manufacturing processes.

The effect of trace vanadium addition on mechanical properties, microstructure and austenite phase characteristics in a 3.9% Al-Delta TRIP steel

This study aimed to examine the effect of adding 0.12 wt% vanadium on the properties of delta TRIP steel through microstructural and mechanical investigations. The delta ferrite phase was observed in all annealing temperatures and samples. Carrying out bainitic transformation at 350 °C for 6, 10, and 15 min resulted in the stability of the austenite phase and the formation of ferritic bainite phase after heat treatment. The presence of vanadium led to the formation of MC precipitate phases and reduced the remaining austenite percentage from 14% to 6% by weight in the heat-treated sample. The low percentage of austenite in vanadium-containing steel has weakened the TRIP effect in strengthening. On the other hand, the presence of MC precipitates has compensated for the reduction in strength from the TRIP strengthening process through precipitation hardening and reducing the grain size of the delta ferrite phase. The resulting properties (UTS: 866 MPa and El: 41.3%) have led to the creation of a formability index of 35 GPa% in vanadium-containing steel, which is better than the mechanical properties (UTS: 857 MPa and El: 37.4%) and the formability index of vanadium-free steel (32 GPa%). These factors make this steel a suitable option for automotive industry applications. Based on these features make this steel a suitable candidate for automotive industries. Microstructural investigations revealed that the addition of vanadium reduces the grain size of the delta ferrite phase. Typically, the austenite phase forms around the ferritic bainite phase and delta ferrite grain boundaries. During tensile testing, it transforms into martensite through the TRIP process.

Simultaneously Enhancing the Dynamic Damping and Static Strength Capabilities of Structures: A Case Study Conducted on Fly Ash-based Geopolymer Concrete Specimens using Modal, Mechanical and Microstructural Investigations

Enhancing both the dynamic damping and static strength of ordinary Portland cement (OPC) concrete simultaneously is a significant challenge. Geopolymer concrete (GPC), particularly fly ash-based GPC, offers a promising alternative. This study explores the relationship between damping and strength in heat-cured, low-calcium fly ash-based GPC using sodium silicate (SS) and sodium hydroxide (SH) activators. The findings reveal that SS activators demonstrate stronger positive correlations between damping and strength compared to SH activators. Microstructural analysis indicated that increasing SS dosage from 55 kg/m³ to 98 kg/m³ resulted in a 17% increase in dynamic damping ratios and a 39% increase in static compressive strength. These results highlight the potential of GPC to surpass OPC concrete in applications requiring both enhanced damping and strength, offering a dual benefit not typically achievable with OPC. The study contributes to a deeper understanding of GPC's capabilities, paving the way for its broader adoption in construction projects.

Cost-Effective Thermomechanical Processing of Nanostructured Ferritic Alloys: Microstructure and Mechanical Properties Investigation

Nanostructured ferritic alloys (NFAs), such as oxide-dispersion strengthened (ODS) alloys, play a vital role in advanced fission and fusion reactors, offering superior properties when incorporating nanoparticles under irradiation. Despite their importance, the high cost of mass-producing NFAs through mechanical milling presents a challenge. This study delves into the microstructure-mechanical property correlations of three NFAs produced using a novel, cost-effective approach combining severe plastic deformation (SPD) with the continuous thermomechanical processing (CTMP) method. Analysis using scanning electron microscopy (SEM)-electron backscatter diffraction (EBSD) revealed nano-grain structures and phases, while scanning transmission electron microscopy (STEM)-energy dispersive X-ray spectroscopy (EDS) quantified the size and density of Ti-N, Y-O, and Cr-O fine particles. Atom probe tomography (APT) further confirmed the absence of finer Y-O particles and characterized the chemical composition of the particles, suggesting possible nitride dispersion strengthening. Correlation of microstructure and mechanical testing results revealed that CTMP alloys, despite having lower nanoparticle densities, exhibit strength and ductility comparable to mechanically milled ODS alloys, likely due to their fine grain structure. However, higher nanoparticle densities may be necessary to prevent cavity swelling under high-temperature irradiation and helium gas production. Further enhancements in uniform nanoparticle distribution and increased sink strength are recommended to mitigate cavity swelling, advancing their suitability for nuclear applications.

Microstructural and properties investigations of tantalum-doped tungsten diboride ceramic coatings via HiPIMS and RF magnetron sputtering

In this work, tantalum-doped tungsten boride ceramic coatings were deposited from a single sputtering target with the radio frequency (RF) and high-power impulse magnetron sputtering (HiPIMS) methods. Two-inch torus targets were synthesised from pure elements with the spark plasma sintering (SPS) method with a stoichiometric composition of W1-xTaxB2.5 (x = 0, 0.08, 0.16, 0.24). Films were deposited with RF and HiPIMS power suppliers at process temperatures from RT to 600 °C. The substrate heating and the energy of the ionised material impacting the substrate increase the surface diffusivity of adatoms and are crucial in the deposition process. The results of SEM and XRD investigations clearly show that the addition of tantalum also changes the microstructure of the deposited films. The coatings without tantalum possess a finer microstructure than those with 24% of tantalum. The structure of films is homogeneous along the film thickness and composed mainly of columns with a (0001) preferred orientation. Deposited coatings are composed mainly of P6/mmm α-WB2 structures. The analysis of nanoindentation results allowed us to determine that ceramic coatings obtained with the HiPIMS method possess hardness above 41 GPa and a ratio of hardness to reduced Young modulus above 0.1. The thickness of HiPIMS-deposited films is relatively small: only around 60% of the RF magnetron sputtered coatings even when the average power input was two times higher. However, it has been shown that the RF coatings require heating the substrate above 400 °C to obtain a crystalline structure, while the HiPIMS method allows for a reduction of the substrate temperature to 300 °C.

Investigation of microstructure and microtexture development and its correlation with mechanical and formability behavior of dissimilar friction stir welded AA6061 and AA5754 sheets

The major attention of the automotive, aerospace, and marine industries currently is towards light-weight structural parts, and this can be achieved by the use of the tailor-welded blanks (TWBs). Using Al alloys as a member of the TWBs lessens the structural weight further, but conventional fusion welding of these alloys is often associated with defects such as porosity, manifestation of segregation and under-fill formation. Friction stir welding (FSW) is a feasible approach not only for producing welds with minimal defects but also for their enhanced formability, which finds extensive applications across industries. Due to its capability of completely negating the rolling directional properties of the as-received sheet, FSW increases the uniformity in the strain distribution during the course of deformation. In the present study, the objective was to perform FSW for dissimilar materials (with advancing side AA6061 and retreating side AA5754 sheets) with a similar thickness of 1.5 mm and to investigate the formability behavior of the TWBs. The effect of different FSW process parameters like rotational speed (950 rpm, 1100 rpm, 1250 rpm, and 1400 rpm) and traverse speed (50 mm/min, 75 mm/min, and 100 mm/min) which were selected based on visual inspection and macrostructure studies of the welds, was investigated on the formability and mechanical behavior of welded AA6061 and AA5754 sheets. Tensile tests were performed for base materials along with the specimens welded at different process parameters according to ASTM E8 standard to evaluate the mechanical properties. Micro-hardness tests were performed at several locations of the cross-section in the transverse direction to the weld in either side. The assessment of formability was executed by deforming the as-received and welded specimens in uniaxial strain paths, plane strain paths, and biaxial strain paths using Limit Dome Height (LDH) tests. Tensile test evaluation of 1250 rpm–50 mm/min, 1250 rpm–75 mm/min, 1400 rpm–50 mm/min, and 1400 rpm–75 mm/min parameter combinations exhibited cracks away from the weld center towards the advancing side (AA6061 side) of the specimens, indicating good quality of the welds, which was in agreement with the stir zone macrostructures. The micro-hardness distribution plots also showed a drop in the hardness in the TMAZ of the advancing side, making it the weakest region of the weld. It has been observed from the LDH test that the formability of all the FSWed specimens was intermediate to the formability of AA5754 (superior formability) and AA6061 (inferior formability), irrespective of the strain paths that might be correlated to their corresponding strain hardening exponents. In the case of uniaxial and plane strain paths in the welded specimens, the failure was towards the advancing side, which was in agreement with the tensile properties, but in the case of biaxial stretching, it was failing at the weld center due to their corresponding microstructural characteristics. An abnormal behavior was witnessed in the equibiaxial stretching, i.e., the minor strain attained for all the welded specimens was significantly less. Comprehensive studies on microstructure and microtexture were performed on the un-deformed and deformed specimens to investigate the peculiar behavior, and it was inferred that the initial local misorientations play a pivotal role in determining the grain orientation spread (GOS) post-deformation. It was further observed that the average GOS values were analogous across all the deformed welded samples in their respective strain paths. It's noteworthy that the presence of the Cube texture component in the biaxial strain path significantly mitigated the impact of the Goss texture, resulting in the peculiar behavior i.e. a notable decrease in the minor strain observed in the deformed welded specimens in the biaxial strain path.

Investigation of microstructure and thermal behavior of CuAlTaGd alloy samples quenched in liquid nitrogen environment after increasing thermal aging times

Investigation of microstructure and thermal behavior of CuAlTaGd alloy samples quenched in liquid nitrogen environment after increasing thermal aging times

Investigating the Tribological Behavior of Bioinspired Surfaces in Agro-waste and Alumina Reinforced AA6063 Matrix Composites

The tribological behavior of three bioinspired (BI) agro-waste ashes—bean pod ash (BPA), cassava peel ash (CPA), and coconut husk ash (CHA)—integrated as reinforcements in an alumina-reinforced AA6063 matrix hybrid and monolithic composite was investigated. Studies utilizing agro-wastes for bioinspired surfaces are limited despite these wastes posing significant environmental challenges. Consequently, there has been a growing effort to valorize these residues for sustainable engineering applications. This study uses a two-step stir-casting methodology to engineer bioinspired materials with lightweight properties, exceptional hardness, and wear-resistant characteristics. Microstructural investigations revealed a homogeneous distribution of natural and synthetic reinforcements within the AA6063 matrix. Lightweight AMCs were successfully fabricated, exhibiting densities ranging from 2.57 to 2.74 g/cm³ and hardness values between 81.28 and 107.47 BHN. The average surface roughness of these composites varied from 2.4 to 3.4 μm, with ANOVA tests confirming a statistically significant difference (p < 0.005). Wear rates and the coefficient of friction were observed to range from 2.52 x 10-3 to 5.50 x 10-3 mm³/m and 0.2476 to 0.5403, respectively. Reinforcing the AA6063 alloy with bioinspired agro-wastes significantly enhanced the hardness and tribological performance of the as-cast BI-AMCs. Notably, the wear rates and friction coefficients were significantly reduced, with the monolithic BI-AMCs demonstrating superior results. Furthermore, adding bioinspired agro-wastes, such as BPA, CPA, and CHA, improved the surface texture of AA6063 more effectively than Al2O3, leading to slightly smoother surfaces. These nuanced bioinspired tribological behaviors present opportunities for tailored load-bearing and aesthetic applications derived from agro-waste-based composites. This study advances our understanding of bioinspired tribological phenomena and paves the way for the development of innovative materials with diverse applications and enhanced performance.

Workability, mechanical, durability, and microstructural investigation of sustainable concrete utilizing secondary treated wastewater, fly ash, and sodium nitrite

ABSTRACT The present study focused on utilizing secondary treated wastewater (STW) from three secondary-level wastewater treatment plants, fly ash (FA), and sodium nitrite (SN), to manufacture sustainable concrete and compared the results with those of concrete manufactured using tap water. The workability (slump cone), strength (compressive, split tensile, and flexural strength), and durability (rapid chloride permeability and efflorescence test) properties of the materials were examined. Furthermore, scanning electron microscopy (SEM) analysis was conducted on the concrete samples to analyse the microstructure. The workability, mechanical, and durability properties of the concrete mixtures were significantly affected by the FA and SN content, while the water type used in the preparation of the concrete mixtures had no significant effect. Based on durability tests, the penetrability of concrete mixtures is low to moderate, indicating the better quality of the mixtures, and the efflorescence in concrete mixtures was found to be nil. Finally, it can be concluded that STW from three secondary-level wastewater treatment plants with 10% FA and 2% SN had comparable results to those of concrete manufactured using tap water.

Microstructural investigation and performance of LC3 cementitious materials

Limestone calcined clay cement (LC3) is identified as an auspicious solution to the exhaustion of natural resources intended for use as construction materials. This is seen as a means of reducing both the carbon dioxide emissions and the environmental effects of energy-intensive cement manufacture. Three novel types of LC3 cements and mortar varieties were synthesized, characterized, and their potential use was assessed. Microstructural investigations were performed using experimental techniques such as XRD, XRF, FTIR, SEM, setting time, and thermal analysis. Physical and mechanical properties were also evaluated at 2, 7, and 28 d. The results showed that hydrated phases alongside the original ones were encountered. SEM micrographs exhibited some features belonging to the already detected phases. The mechanical strength of the manufactured LC3 samples (up to 35 MPa) was quite promising compared to that of commercial cements. From the above, it could be stated that the used materials might be a good substitute for clinker in the manufacture of LC3 cements. LC3-50 and LC3-52 may both be used in favor of CPJ35 and CPJ45, respectively.

Investigation of microstructure, mechanical and oxidation properties of WC-Co-Ni-Fe-Al multicomponent hardmetals

In this paper, the influences of Al element originating from AlN additives on the mechanical properties and oxidation behavior of WC-Co-Ni-Fe hardmetals were studied systematically. During the sintering process, Al element diffused into the WC hard grains and led to the formation of (Ni,Co,Fe)3Al particles in the metal binder phase of hardmetals. A small amount of Al element enhanced the hardness of WC-Co-Ni-Fe hardmetal via the solution strengthening and dispersion strengthening. However, the addition of Al element decreased the fracture toughness of hardmetals. The hardness and fracture toughness of multicomponent hardmetals with the addition of 0.5 wt% AlN were 90.4HRA and 10.6 MPa m1/2, respectively. Additionally, the introduction of Al also significantly improved the oxidation resistance of hardmetals due to the existence of Al in WO3 and (Ni,Co,Fe)WO4. With the addition of 2.0 wt% AlN, the weight gain of WC-Co-Ni-Fe-Al multicomponent hardmetal after 5 h of oxidation at 700 °C was 66.82 % of that of the Al-free hardmetal.

Investigation of microstructure, hydrogen storage performance of Re-Mg-based alloy modified by RE2O3 (RE = Dy, Er, Yb)

In order to further study the hydrogen storage mechanism of Mg-based hydrogen storage materials, Mg90Ce5Y5 alloy was used as the basis, and three kinds of heavy rare earth oxides were doped into the alloy by ball milling technology. The microstructure and hydrogen storage properties were characterized by XRD, SEM, TEM and PCI. The results show that the hydrogen absorption rate and discharge rate of modified Mg90Ce5Y5 alloy are significantly increased, and the performance is Er2O3 > Dy2O3 > Yb2O3. The hydrogen storage capacity of Er2O3 catalyzed samples is 5.02 wt%, which is higher than 4.82 wt% and 4.80 wt% of Dy2O3 and Yb2O3 catalyzed samples. The hydrogen absorption saturation rate of the sample catalyzed by Er2O3 for 2 min is ∼90 %, the complete release of hydrogen only takes 50 min at 573 K, and the dehydrogenation activation energy is 76.9 kJ/mol. The hydrogen absorption and emission rate is fast, and the dehydrogenation activation energy is slightly lower than that of the other two catalysts. In the process of hydrogen absorption and desorption, the three catalysts exhibit different phase transitions, namely DyH2↔DyH3, ErH2↔ErH3 and irreversible YbH2. DyH2↔DyH3, ErH2↔ErH3 phase transitions have the "hydrogen pump" effect, which can effectively improve the hydrogen absorption and desorption kinetic rate of Mg90Ce5Y5 alloy. Some nano-rare earth compounds and phase transformation significantly improve the kinetic properties of the alloy. However, the enthalpy change of all three catalytic alloys is around 76 kJ/mol H2, which is considered only a slight improvement in thermodynamic properties.

The Influence of Microstructure and Process Design on the Plastic Stability of 4 wt% Medium‐Manganese Steels

The influence of different microstructures on the plastic stability of an air‐hardened industrially produced medium‐manganese steel is presented. For this matter, heat treatment parameters before and during intercritical annealing (IA) are varied, to achieve different microstructures. The resulting duplex microstructure is consecutively tested by tensile tests, which are monitored by digital image correlation (DIC) to obtain information on the local plastic deformation. The tests are accompanied by microstructure investigations using optical, scanning electron, and transmission electron microscopy. Finally, X‐ray and electron backscatter diffraction experiments are performed before and after deformation, to describe the altering phase fractions. It is demonstrated that the effect of the deformation temperature prior to IA treatment has a significant influence on the duplex microstructure, as it changes the austenite morphology from lamellar to globular and increases the phase fraction. The change in austenite phase fraction and morphology results in a higher yield strength (≈100 MPa), as well as higher uniform and total elongations (+2% and +5%, respectively). The DIC and tensile tests reveal that these differences in the austenite phase lead to a complete change in the strain hardening behavior, from continuous flow to discontinuous serrated flow, with clearly visible deformation bands during plastic deformation.

Investigations of Metallurgical Differences in AISI 347 and Their Influence on Deformation and Transformation Behaviour and Resulting Fatigue Life.

Due to variations in chemical composition and production processes, homonymous austenitic stainless steels can differ significantly regarding their initial microstructure, metastability, and thus, their fatigue behavior. Microstructural investigations and fatigue tests have been performed in order to evaluate this aspect. Three different batches and production forms of nominally one type of steel AISI 347 were investigated under monotonic tensile tests and cyclic loading under total strain and stress control in low and high cycle fatigue regimes, respectively. The deformation induced α'-martensite formation was investigated globally by means of in situ magnetic measurements and locally using optical light microscopy of color etching of micrographs. The investigation showed that the chemical composition and the different production processes influence the material behavior. In fatigue tests, a higher metastability and thus a higher level of deformation induced α'-martensite pronounced cyclic hardening, resulting in significantly greater endurable stresses in total strain-controlled tests and an increase in fatigue life in stress-controlled tests. For applications of non-destructive-testing, detailed knowledge of a component's metastability is required. In less metastable batches and for lower stress levels, α'-martensite primarily formed at the plasticization zone of a crack. Furthermore, the formation and nucleation points of α'-martensite were highly dependent on grain size and the presence of δ-ferrite. This study provides valuable insights into the different material behavior of three different batches with the same designation, i.e., AISI 347, due to different manufacturing processes and differences in the chemical composition, metastability, and microstructure.