This study investigates the relationship between recrystallisation behaviour and mechanical properties of laser beam powder bed fusion (LB-PBF) stainless steel 316L through both uniaxial tensile and shear punch (ShP) testing. Components were built in vertical (90 degrees) and horizontal (0 degrees) orientations and subjected to systematic heat treatments designed to vary temperature (1050-1200 degrees C) and soak time (75-225 min). Microstructural evolution was characterised by electron backscatter diffraction (EBSD), focussing on grain morphology, recrystallised fraction, and crystallographic texture. Results show that recrystallisation is strongly temperature-dependent, with near-complete recrystallisation above 1150 degrees C, accompanied by equiaxed grains and increased Sigma 3 twin boundaries, reducing anisotropy relative to the as-built state. Time played a secondary role, mainly influencing abnormal grain growth. Mechanical testing revealed a clear strength-ductility trade-off, with lower temperatures retaining strength at the expense of elongation, and higher temperatures promoting ductility while reducing yield strength. ShP testing exhibited good repeatability and captured orientation-dependent behaviour consistent with tensile results. Strong correlations between ShP and tensile properties were established, enabling predictive relationships tailored to annealed LB-PBF 316L. These findings confirm ShP as an effective small-scale testing method for AM alloys, providing a practical route for linking local recrystallisation behaviour to bulk mechanical performance.

This study investigates the influence of post-build heat treatments (HTs) on the microstructural evolution, mechanical performance, and corrosion resistance of laser beam powder bed fusion (LB-PBF) stainless steel 316 L. Samples built in vertical and horizontal orientations were subjected to three HT conditions (1050, 1150, and 1200 degrees C), enabling analysis of recrystallisation behaviour, grain morphology, and the mitigation of anisotropy. Tensile, low-cycle fatigue (LCF), and fatigue crack growth (FCG) testing revealed that full recrystallisation occurred at >= 1150 degrees C, reducing orientation-dependent discrepancies in strength and ductility. Despite improved isotropy, LCF testing demonstrated residual anisotropy, with samples built in the vertical orientation consistently outperforming those built in the horizontal orientation. Samples heat treated at 1050 degrees C retained a fine, columnar grain structure, resulting in superior resistance to crack growth due to increased yield strength and grain boundary density. In contrast, samples heat treated at 1150 and 1200 degrees C exhibited coarser, equiaxed grains with diminished fatigue crack resistance. Cyclic polarisation testing showed recrystallisation at the higher temperatures induced positive changes in corrosion performance, substantially increasing pitting potential compared to un-recrystallised microstructures, as found at 1050 degrees C. The findings highlight the trade-offs between strength, ductility, and fatigue resistance as a function of microstructure, offering insight into optimising HT protocols for LB-PBF SS316L components in fatigue-critical applications.

Thermo-mechanical fatigue (TMF) is a complex damage mechanism considered to be one of the key issues limiting the service lives of hot section components in a gas turbine engine. Turbine blades and nozzle guide vanes are particularly susceptible to this form of material degradation, which results from the simultaneous cycling of mechanical and thermal loads. In this research, a series of TMF tests were undertaken on a single crystal nickel-based superalloy, CMSX-4 under a variety of phase angles and a thermal cycle of 550-1050 degrees C, to holistically understand the evolving damage mechanisms that can occur under the various loading conditions. The generated data has shown that for the strain ranges tested, fatigue life is significantly affected by the employed phase angle. Furthermore, the length of time that the material is exposed to elevated temperature has a substantial influence on the material's microstructure, and thus, the dominant mode of damage that occurs.

Additive Manufacturing (AM), particularly laser beam powder bed fusion (LB-PBF), enables fabrication of complex thin-wall geometries, yet post-processing studies on such structures are limited. This work investigates heat-treatment annealing of thin-walled stainless steel 316L (SS316L) built via LB-PBF. A novel build geometry with wall thicknesses from 0.2–1.8 mm was used to examine microstructures before and after heat-treatment (HT) at 1050 °C and 1150 °C. In the as-built state, thinner walls showed grains oriented in <001> toward the wall centre, while thicker walls exhibited a <101> orientation due to a central band of preferential grain growth, typical of larger LB-PBF SS316L parts. Annealing at 1150 °C produced partial recrystallisation in all samples, reaching 86 % in the thickest walls, whereas 1050 °C annealing had little effect. Analysis of geometrically necessary dislocation density, low-angle boundaries (LAB), and high-angle boundaries (HAB) showed no correlation with recrystallisation behaviour. The primary factor limiting recrystallisation was Mn- and Si-based oxide distributions, which impeded grain boundary migration. This caused a stop-and-go growth mechanism, leading to abnormal grain growth in some cases. Findings highlight that chemical segregation, rather than dislocation structure, controls recrystallisation in thin-walled LB-PBF SS316L.

Cyclic hysteresis loops generated during high-temperature, isothermal, low-cycle fatigue testing of the nickel-based superalloy RR1000 revealed a gradual decrease in elastic modulus of up to 25 % compared to the initial value in dependence of the accumulated plastic strain. Based on the experimental observations, a material model was developed and implemented within a finite element solver to investigate the effect of degrading stiffness on crack growth laws and crack growth predictions. It was shown that the stress intensity factor diminished by 6 to 8 % and crack growth rates were 14 to 20 % higher when stiffness degradation was accounted for.

In this investigation, 0.19–1.8 wt.% of Ge was introduced into a ternary Zn-Mg-Al alloy. The introduction of Ge had a significant impact on the microstructure, leading to the formation of Mg2Ge. The area fraction of the eutectic phase diminished with increasing Ge additions. Small-scale test techniques were utilised to evaluate the mechanical properties due to the changes in microstructure. Zn-Mg-Al alloys were found to be inherently harder compared to standard hot-dip Zn-containing 0.2 wt.% Al. The hardness and strength of the Zn-Mg-Al alloys decreased with the increase in Ge additions.

Additive Manufacturing (AM) is receiving widespread attention from both industry and academia who are looking to benefit from the numerous advantageous possibilities that AM processes have to offer, such as the potential to design and produce highly complex bespoke geometries with minimal material wastage. Yet, despite this, AM also has some drawbacks. Some of the most significant include the presence of process-induced defects and the inherent surface roughness of an AM built component, both of which can have a considerable influence on the mechanical properties of the final product. This research will investigate the role of an as-built surface on the fatigue properties of AM Ti-6Al-4V manufactured by electron beam melting (EBM), laser powder bed fusion (L-PBF) and laser metal deposition with wire (LMD-w). Fatigue results have been generated alongside advanced surface profilometry, microstructural, defect and fractographic analyses that have revealed that whilst the surface roughness in the majority of instances is the primary factor impacting the fatigue performance on AM material, it cannot be considered alone. It was found that the inherent as-built (AB) surface finish was significantly different across the various AM processes, inducing a range of effective stress concentrations and thus, a contrasting impact on the resulting fatigue performance. Results from each variant have been compared against a machined and polished equivalent, to provide a further consideration as to whether the as-built surface would be suffice from a time and economical viewpoint. Statistical analysis of the generated results also allowed for an extrapolation of predicted fatigue lives in the very high cycle regime for the alternative AM Ti-6Al-4V variants.

Laser powder bed fusion (LPBF) is an additive manufacturing (AM) process capable of single-step fabrication of intricate and complex structures. However, there are multiple engineering challenges associated with the introduction of AM based parts into functional industrial applications due to the lack of understanding of the role that process parameters have on the structural integrity of additively manufactured (AM) components and the subsequent effect this has on the mechanical behaviour of such materials when subjected to cyclic loading conditions. The present work will investigate the low cycle fatigue (LCF) behaviour of LPBF stainless steel 316L components manufactured with different process parameters sets and how this effects the material built in different orientations and the resulting impact this has on the material's resistance to cyclic deformation. The LCF results are supported by microstructural, fractographic and advanced surface profilometry assessments to investigate the key parameters that control the resulting fatigue performance across three different build orientations. Finally, the generated mechanical data has also been interpreted through empirical fatigue lifing models, and the various data sets have been successfully correlated to enable an estimation of longer fatigue lives.

Additively manufactured, Electron Beam Melted (EBM) specimens of the titanium alloy, Ti-6Al-4V, have been produced using a process window determined through a normalised energy density method. Two batches were manufactured and compared using identical energy density values with differing beam current, power, and beam velocity. A stable process window has been demonstrated with a Vickers hardness range of 360-395 VHN resulting from alpha lath coarsening from 0.7 mu m up to 3 mu m. A range of macro morphologies have been reported and relate to the hatch overlap and beam velocity parameters. Base plate position does not appear to influence microstructure or micro-hardness. Prior-beta columnar and colony size increases with alpha lath width resulting from increased energy input; however, each grain type appears to respond differently to either beam velocity or hatch space variation. Average alpha lath width values show greater correlation to energy density, which demonstrates the dependence of grain formation on hatch overlap.

Powder recycling refers to the reuse of unused powder feedstock in the laser powder bed fusion (PBF-LB/M) process. This approach is crucial for the economic viability and sustainability of PBF-LB/M, as powder accounts for a large proportion of the total production cost. However, through powder recycling, the physical and chemical properties of powder are liable to change. This variation in powder properties can subsequently lead to knock-on effects on the mechanical properties of a fully built component.This research has investigated the changes that occur to stainless steel 316L (SS316L) powder as a result of recycling. This includes changes to powder size distribution (PSD), flowability, chemistry and phase composition. Likewise, the impact that these changes have will also be assessed in PBF-LB/M SS316L components manufactured from powders after different levels of recycling and subjected to alternative post processing routes such as hot isostatic pressing (HIP). This comprehensive investigation involves a thorough examination of both macro- and microstructures, encompassing detailed analyses of chemical composition, microstructural features, and defects. The study aims to elucidate differences in mechanical behaviour through a series of experiments, including uniaxial tensile tests, Charpy impact assessments, and low cycle fatigue (LCF) experiments. Additionally, the investigation will be complemented by pitting potential tests, providing a holistic understanding of the material's performance and characteristics.Although moderate changes to powder were observed for both PSD and chemistry, this was found to be negligible and not enough to result in any adverse changes to part performance. In addition, the microstructure of SS316L remained stable across differing levels of powder recycling. Whereas the porosity content increased marginally as the fine particle content of powder was reduced, this was not found to be sufficient to affect the LCF performance of the material. After powder recycling, increases in ductility and Young’s modulus were attributed to a reduction in oxides present in the microstructure, which were sources of localised damage and deformation.

Abstract This article discusses several alternative mechanical test approaches that can be applied to additive manufacturing (AM) materials, both for smaller-scale assessments and for specimens that have been extracted from an AM component. This includes small punch testing, shear punch testing, and small ring testing.

Additive manufacturing (AM) processes are currently under consideration for marine based components, predominantly due to the numerous benefits that the techniques have to offer over more conventional manufacturing routes. However, there are multiple engineering challenges and questions associated with the introduction of AM based parts into safety critical applications related to the mechanical behaviour of such components. One of the main factors influencing the cyclic performance of a component is the surface finish. As-built AM parts typically exhibit a rough surface owing to partially melted powder being present at the surface and the layer-by-layer nature of the AM process, which together will likely hinder the fatigue response of the component. This behaviour is further influenced by the build orientation of the AM component, with alternative orientations providing a different surface profile alongside a contrasting microstructural morphology. Therefore, alternative finishing methods have been explored to maximise the fatigue performance of components whilst also considering cost and time. This research will explore the low cycle fatigue (LCF) behaviour of laser powder bed fused (LPBF) stainless steel 316L (SS316LN) built in two principal orientations (vertical (90°) and diagonal (45°)) and subsequently subjected to several post-manufacture finishing processes in order to identify the optimal finish for mechanical performance. The mechanical results are supported by microstructural, fractographic and advanced surface profilometry assessments, which have revealed that surface roughness can not be considered alone to be the controlling influence on LCF behaviour. An as-built surface finish will inherently provide a greater number of surface breaking stress raisers, however, a novel mass finishing polishing procedure has been found to produce a similar effective stress concentration factor compared to conventional longitudinal polishing, offering a more viable and less time consuming alternative. Several other key factors must also be considered when assessing the fatigue performance of LPBF built materials, including build direction and the resulting grain orientation, density of the additive structure and the material's sensitivity to the presence of notched features at the surface. Finally, the generated mechanical data has also been interpreted through empirical modelling, and the various data sets have been successfully correlated to enable longer fatigue life predictions.

With the continuous drive of the aerospace industry to implement additive manufactured (AM) components into the next generation of aero-engines, to benefit from the near net shape and weight saving potential that the technology has to offer, the requirement to understand their mechanical performance is also rising in parallel. This is further complicated by the highly localised and transient micro/macro structures that AM produced parts typically possess, raising a question mark over the suitability of more traditional mechanical test approaches where the bulk properties are heavily influenced by the presence of a single defect. As such, alternative experimental approaches, capable of establishing the properties of smaller more intricate structures and geometrically representative microstructures and cross sections, needs to be considered for process parameter down-selection. This paper will explore the suitability of several alternative mechanical test methodologies in characterising the mechanical behaviour of a nickel based superalloy, Inconel 718 (IN718), produced by laser powder bed fusion (LPBF), and establish which results correlate most favourably to those generated through more conventional means. For the first time, results will be presented from several mechanical test methodologies including small punch, shear punch, hardness, nano-indentation and profilometry based indentation plastometry experiments; a set of mechanical test approaches that have yet to be directly compared and discussed in a single study on an additively manufactured material. Findings will be supported by advanced microscopy in the form of field emission SEM and crystallographic texture maps produced through electron back-scattered diffraction.

High entropy alloys (HEAs) are a novel class of metallic materials that exhibit a unique blend of properties due to their chemical composition and atomic arrangement. This research aims to investigate the strain rate sensitivity (SRS) of two HEA CoCrFeMnNiTix (x = 0, 0.3) alloy compositions through the use of shear punch testing. This method has been proven to provide reliable results for both HEA materials, including the CoCrFeMnNiTi0.3 HEA composition which was found to be inherently brittle and contained both sigma-phase and Laves phase compounds with a hardness close to 14 GPa and a soft FCC phase. Among all the testing temperatures (room temperature to 400 degrees C) and deflection rates (0.2, 2 and 10 mm.min  1) used, only the CoCrFeMnNi HEA alloy was found to exhibit SRS at room temperature (m = 0.0333), while for the other HEA alloy variant and testing conditions, the SRS was found to be zero. From empirical correlations and finite element analysis (FEA), the calculated value for m ranged from 0.0333 to 0.0359, thus evidencing that the FEA simulations provide an accurate and suitable means of capturing the deformation behaviour of such alloys when subjected to shearing.

The suitability of determining the strain rate sensitivity (SRS) of the CoCrFeMnNi high-entropy alloy (HEA) by small punch (SP) testing has been assessed at displacement rates ranging from 0.2 to 2 mm∙min −1 . The stress was found to increase as the displacement rate was raised from 0.2 to 2 mm∙min −1 , whereas the plastic strain distributions were similar in all cases. However, for a higher displacement rate of 10 mm∙min −1 , the sample was found to exhibit a drop in strength and ductility attributed to casting defects. The strain-rate sensitivity exponent ( m ) was found to be 0.1387 whilst the Finite Element Analysis (FEA) simulations predicted a slightly smaller value of 0.1313. This latter value is closer to m = 0.091 obtained from nanoindentation strain rate jump tests since the results are insensitive to the presence of small casting defects. The relationship between the experimental and the empirically derived predicted properties from the SP tests revealed a high level of agreement for maximum stress properties. The properties predicted at 2 mm∙min −1 (R 2 = 0.96) offered a stronger fit than at 0.5 mm∙min −1 (R 2 = 0.92). © 2022 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY license (http:// creativecommons.org/licenses/by/4.0/).

There is a clear economic benefit for the recycling of metallic powder during additive manufacturing (AM). Laser powder bed fusion (L‐PBF) is one such AM process and research has found that the properties of the powder feedstock can change when the powder is reused through mechanisms such as spatter generation and alterations in chemistry. Such changes to powder properties can accumulate and may lead to significant differences in the mechanical properties of the final component. Often the changes to part properties are reasonably small; however, there is not currently enough understanding of the specific links between powder properties and the characteristics of the end component for the effects of powder recycling to be discounted. Herein, the typical lifecycle of stainless steel 316L powder in L‐PBF, the changes that occur to the powder feedstock, and the effects that this may have on the mechanical properties of components manufactured with recycled powder are reviewed.

Duplex stainless steels (DSS) are a family of stainless steel alloys that benefit from the presence of two relatively equally proportioned phases, ferrite and austenite. The alloys are designed to have an enhanced resistance to corrosion and superior strength properties in comparison to more common stainless steel alloys such as 316L. Design engineers are now exploring the introduction of additively manufactured (AM) DSS into industrial components, to benefit from these enhanced capabilities provided by the alloy and the greater flexibility in design offered by AM. This research focuses on the mechanical and microstructural characterisation of the DSS 2205, manufactured by the AM process laser powder bed fusion (LPBF). Results have been generated through both uniaxial tensile testing and small punch (SP) testing on as built and heat-treated conditions, across a range of temperatures up to 750 °C. Microstructural assessments have been conducted using advanced microscopy to determine relevant phase distributions and texture morphologies present in the materials, and how these influence mechanical performance.

Abstract The development and adoption of nonconventional and small scale mechanical test methods has risen in recent decades. This has primarily been driven by the requirement to measure mechanical properties at smaller length scales to enable an assessment of remnant life capabilities, alloy-down selection, and localized property sampling of larger complex components. This development is typified by the small punch (SP) test, one of the most prominent forms of these smaller scale approaches, which is receiving widespread attention from a multitude of industrial sectors that recognize the significant cost benefits it has to offer. With numerous worldwide laboratories now utilizing the SP test for material characterization for all manner of purposes, this growing interest and exploitation led to the recent formulation of an ASTM standard (2020) and equivalent EN standard (2021), illustrating the role that small scale testing has in the wider testing community. Indeed, other methods now being adopted include miniaturized uniaxial, shear punch, hydraulic bulge, small ring, and impression testing. This list is by no means exhaustive, however, with many more innovative techniques now being developed. This special issue of ASTM International’s journal Materials Performance and Characterization showcases the diversity and innovative nature of the research that is currently developing across the world on a broad scope of materials and applications in this ever-expanding field. It is with great pleasure that the guest editors present this Special Issue on Mechanical Characterization of Small Scale Specimens. The call for papers attracted a large number of abstracts from leading scientists, engineers, and researchers from a wide range of international institutions. After scrutiny and their acceptance, a total of 10 full-length research articles have been included in the final issue. The issue opens with two articles on SP testing. The first paper is written Dr. Martin Abendroth and colleagues; Dr. Abendroth of Freiburg University is a highly esteemed figure in the world of small scale testing who has applied the SP test to investigate the properties of iron-based shape memory alloys. Dr. Abendroth’s team are the first to attempt this, and this paper is certainly one of the highlights of the issue. The second article on SP testing is written by Dr. Romy Welschen—a nuclear materials scientist at the Nuclear Research and Consultancy Group (NRG) in Petten—and colleagues, who presents a novel SP test fabrication and setup that works under hot-cell conditions for evaluating the properties of irradiated materials, another article of particular interest to the SP testing community. The third contribution is made by Prof. Steve Brett, an honorary professor of the University of Nottingham and prominent member of the small scale testing community, and colleagues on the recent developments in creating a workshop agreement on impression creep testing. Prof. Brett has worked in this field for the past two decades and has led the efforts in formally standardizing the test procedure and making it accessible to a wider group of researchers. The next three papers focus on the use of miniature creep specimens and their application in deriving the high temperature properties of in-service materials. Two of these three papers are written by Dr. Akito Nitta’s team from Kobe Material Testing Laboratory in Tokyo, who firstly extracted miniature creep specimens from the wall of a service exposed boiler tube and were able to evaluate the internal pressure creep rupture life. The second paper from the same team was able to successfully correlate the stress–rupture behavior of an ultra-miniature creep specimen to the behavior generated from more standard test approaches. The final paper on this theme was submitted by Dr. Masatsugu Yaguchi from the Central Research Institute of Electric Power Industry, also located in Tokyo. Dr. Yaguchi has been researching high temperature damage mechanisms for the past 30 years, and Dr. Yaguchi’s contribution proposes a remaining creep life prediction method for Grade 91 steel weldments, as derived from small scale specimens taken from the base metal rather than the weld itself. The next two contributions look at the application of miniaturized tensile testing. The first of these is a contribution provided by Dr. Sreekar Karnati of Missouri University of Science and Technology and colleagues, who utilized a tensile specimen design that is 1/230th of the volume of the smallest ASTM recommended tensile geometry and coupled this with digital image correlation for localized strain mapping. The second paper is written by Prof. Hezong Li of Hebei University of Engineering and colleagues, and it focuses on the use of interrupted miniature tensile tests to understand the fracture evolution mechanisms of a Grade 91 steel at elevated temperature. The ninth contribution is provided by Prof. Shin-ichi Komazaki of Kagoshima University and colleagues, another key member of the growing small scale testing community of researchers. This paper details the use of a new fatigue testing method–the small bulge fatigue test–where oil is used to exert a load on the miniaturized test sample and utilizes the test method to examine the suitability of the approach in assessing fatigue damage of actual pre-exposed in-service components. The tenth and final paper is provided by Dr. Noritake Hiyoshi, an associate professor at the University of Fukui, and colleagues, and it details the efforts in compiling a standard for the miniature testing of solders and emphasizes the importance of why such a standard is needed in this field. The guest editors would like to express their sincere gratitude to all authors and reviewers who have provided contributions to this special issue. Without their high-level research and excellent contributions, this special issue would not have been possible. Finally, we wholeheartedly thank Alyssa Conaway of ASTM International for handling issues related to the process of publishing an outstanding journal. It has been a long process, but the guest editors are delighted with the final collection of articles that have been compiled. We sincerely hope that you enjoy these papers and look forward to engaging with you in future research projects and international events. Best regards, Prof. Robert Lancaster Dr. Spencer Jeffs

This work compares the mechanical and corrosion properties of 316L steel manufactured by Laser Powder Bed Fusion (LPBF) and post treated by Hot Isostatic Pressing (HIP) to wrought 316L. HIP is often used by default on LPBF components to reduce porosity and obtain the best mechanical properties, however, if the HIP temperatures are too high, there is a risk of reducing mechanical strength and corrosion resistance. The purpose of this work was to investigate the HIP parameters and understand the trade-off in properties. By choosing various HIP temperatures (700 ?, 1125 ?, 1200 ?), pressures (100 MPa, 137 MPa and 200 MPa) and hold times, optimal cycles were investigated based on the most favourable mechanical properties (density, hardness, tensile and low-cycle fatigue), and pitting corrosion resistance. Microstructural features associated with LPBF such as melt pools, melt pool boundaries and sub granular cells were observed. These features were found to disappear with longer and higher temperature treatments, accompanied by increased grain sizes. Low and mid temperature point HIP treatments resulted in higher ultimate tensile strength but lower fracture elongation. The decreasing hardness and tensile strength trends were consistent with decreased grain boundary strengthening and decreased dislocation strengthening (with dis-appearing sub grain boundary and granular cells). Only one HIP condition, consisting of a low temperature and medium pressure, produced samples that achieved runout under low cycle fatigue testing for both the lower and higher stresses. Despite this, most higher temperature HIP cycles reduced the fatigue resistance. This was again attributed to the coarsening of the microstructure at the higher temperature treatments. The spread of the pitting potentials of HIP treated samples was reduced by 52.46 % compared to the as-built material, although none were better overall compared to the wrought material. Of all the properties, porosity appears to play the most influential role on pitting corrosion, and to this extent, despite having a larger variation in results, some of the treated parts demonstrated improved pitting resistance and some demonstrated improved repassivation potentials compared to wrought 316L.