This study investigates the deformation mechanisms and failure behavior of the high-entropy alloy (HEA) AlCoCrFeNi, fabricated via binder jetting and subjected to various heat treatments. The alloy’s microstructure is analyzed using scanning electron microscopy (SEM), X-ray diffraction (XRD), and electron backscatter diffraction (EBSD), while the deformation mechanisms are probed through macro- and nano-indentation testing. The results reveal several novel insights: (i) second-stage heat treatments significantly alter the morphology of nanoprecipitates within the B2 matrix, (ii) the FCC/B2 interphase at grain boundaries influence intergranular deformation, (iii) aging treatments activate greater slip and shear band formation enhancing hardness and toughness, and (iv) sigma-phase precipitates formed by spinodal decomposition promotes transgranular failure. Collectively, these findings offer a mechanistic basis for tailoring failure resistance in AlCoCrFeNi HEAs via optimized multi-stage heat treatment strategies, thereby advancing their mechanical reliability in demanding applications.

A major challenge that hinders the application of computational models for simulating Additive Manufacturing (AM) processes to predict distortions and residual stresses is the exorbitant computational costs. A recently proposed multi-step incremental simulation approach, closely mirroring the sequential material addition in AM, offers a promising path to reducing the computational cost without sacrificing prediction accuracy. However, the effectiveness of this approach hinges on selecting an optimal step size that balances computational cost against solution error introduced during the solution mapping across the steps. This study presents a numerical optimization scheme to determine the optimal number of simulation steps. The scheme involves formulating a cost function that integrates both computation time and solution accuracy, and minimizing this function for a given AM simulation. The scheme is validated by performing simulations of Direct Energy Deposition (DED) and Wire Arc Additive Manufacturing (WAAM) of 316-L stainless steel components. Results show that assigning equal weight to solution error and computational cost in the optimization reduces simulation time by up to 37

The effect of boron addition on the mechanical properties of Ti-6Al-4V produced via Directed Energy Deposition (DED) is investigated through extensive mechanical and microstructural characterization. Ti-6Al-4V blocks with boron concentrations ranging from 0 wt% to 1.5 wt% were fabricated, and specimens extracted from these blocks were characterized. Up to 0.05 wt% boron, strength, and flow stress in both parallel and perpendicular to the build directions increased, while anisotropy was decreased, achieving an optimal balance of isotropy, strength, and ductility at 0.05 wt% boron. Beyond 0.05 wt%, additional boron reintroduced anisotropy, increasing strength in the build direction while reducing ductility. Microstructurally, boron refined grain and subgrain features, weakened crystallographic texture, and promoted the formation of TiB needles preferentially aligned with the build direction In-situ tensile testing revealed that these microstructural changes suppressed key hierarchical deformation mechanisms in Ti-6Al-4V, including phase boundary sliding (PBS), shear strain localization (SSL), and grain boundary-constrained material flow. The superior strength-ductility combination at 0.05 wt% boron is attributed to crystal texture weakening and an increased mean free path for dislocation glide due to Widmanstatten colony formation. At higher boron concentrations, the reduced ductility, increased anisotropy, and higher strength result from finer microstructural features and greater TiB stress partitioning.

In this study, the hierarchical deformation and anisotropic behavior of (alpha+/3) Ti alloys are investigated using a novel microstructure-informed multiscale constitutive model. State-of-the-art crystal plasticity finite element (CPFE) models, due to their emphasis on a single length scale, are inadequate for capturing the complex hierarchical behavior of additively manufactured (AM) (alpha+/3) Ti alloys, which are characterized by columnar grains and lamellar subgrain features at distinct length scales. To overcome this limitation, a decoupled multiscale framework was developed, integrating representative volume elements (RVEs) for both the columnar grain structure at the higher length scale and the lamellar subgrain microstructure at the lower length scale, with equal emphasis on each. The material behaviors at these scales were modeled using an anisotropic classical plasticity model and a mechanism-based CPFE model, respectively. The framework was experimentally validated for Directed Energy Deposition (DED) manufactured Ti6Al-4V. It was then used to investigate microscopic stress/strain fields, deformation localizations at grain and subgrain levels, and stress partitioning among neighboring grains. From the insights gained a new theory of anisotropy for AM (alpha+/3) Ti alloys is proposed.

A comprehensive study is undertaken to investigate the evolution of microscopic deformation under in-situ uniaxial tensile testing of the Directed Energy Deposited (DED) Ti-6Al-4V alloy. The study uncovered a synergistic influence of the (α+β) lamellar microstructural features at different length scales, leading to distinct deformation mechanisms and barrier effects. At the smallest length scale, the deformation mechanisms are lattice slipping and phase boundary sliding (PBS), with a characteristic length scale comparable to the dimensions of α platelets and colonies. The slipping and PBS encountered complex barrier effects at β interlayers. At the intermediate length scale, a mechanism of strain localization manifests as shear bands along a 45o orientation to the uniaxial loading direction. The strain localization involves the activation of multiple slipping events and exhibits a propensity to choose large unobstructed phase-boundary pathways for its realization. At the highest length scale, grain boundaries introduce another barrier effect to both slipping and strain localization through intergranular deformation compatibility. The synergistic interplay between these three mechanisms constitutes a hierarchical response, enabling the material to effectively accommodate large plastic deformation. Moreover, the synergistic interplay influences the anisotropy of the material.

High-strength martensitic stainless steels such as 17-4 PH (SS 17-4 PH) generally exhibit poor ductility and strain-hardening rates. In this study, an Ultrasonic Impact Treatment (UIT) is combined with a powder bed fusion (PBF) manufacturing process with the objective of enhancing the mechanical properties of SS 17-4 PH. UIT is introduced as a surface peening step at regular intervals during the PBF process after depositing a set of predetermined number of layers. The coupon specimens extracted from the build are then subjected to a post-build heat treatment. Then the microstructural and the mechanical properties of these specimens are characterized. The characterizations reveal that the heavy plastic deformation induced by UIT drives microstructural relaxation and recrystallization under heat treatment, reducing defects extensively. The heavy plastic deformation also drives diffusional reversion of the austenite phase during deposition and an inhibition of the martensitic phase transformation during the post-build heat treatment. As a result, the UIT specimens subjected to post-build heat treatment exhibit remarkable enhancement in ductility along with high strength and strain-hardening rate compared to the non-UIT PBF specimens under as-built condition.

Additively manufactured metal alloys often have a hierarchical microstructure consisting of subgrain features. The subgrain features, being smaller in size than grains, significantly influence the mechanical properties and anisotropy of the material. Recognizing the critical role of subgrain features, this work proposes a novel generalized microstructure-informed constitutive modeling framework for hierarchical materials using a mechanism-based crystal plasticity formulation. Considering direct energy deposited (DED) stainless steel (SS) 316 L as the reference material we first categorize subgrain features as a network of solidification cells and a scattered distribution of secondary phases. Then we advanced the state-of-the-art mechanism-based crystal plasticity formulation by explicitly accounting for the geometric and mechanistic effects of both cellular network and scattered secondary phase subgrain features. Homogenization schemes are developed to implement the constitutive model on a finite element analysis (FEA) framework. The anisotropy is realized as a natural outcome of a preferred alignment of cells in the build direction. The model is then calibrated using experimental data. Limited validation of the model is also performed. Results show that explicitly accounting for the subgrain features in a constitutive model allows one to accurately capture the macroscopic and microscopic response of a hierarchical material.

Additively manufactured high-entropy alloys are in a critical stage of development. More studies are required on how their compositions, processing routes, and microstructural evolutions intertwine to influence their (mechanical) properties for various fields of applications. In this study, equiatomic AlCoCrFeNi high-entropy alloys were fabricated via two additive manufacturing routes: binder jetting 3-D printing (BJ3DP) and directed energy deposition (DED). The response of the samples to isothermal heat treatment was investigated and compared with the as-printed samples. Their microstructural evolutions were studied with scanning electron microscopy (SEM), energy-dispersive spectroscopy (EDS), X-ray diffractometer (XRD), and backscattered electron diffraction (EBSD). Nanoindentation was used to characterize their mechanical properties. The results show that the binder jetting and DED samples respond differently to isothermal heat treatments, as diffusion mechanisms like grain boundary precipitation, precipitate-free zones (PFZs), and Widmanstatten structures are observable in the binder jetting samples. The study provides significant insights into the differences in microstructural morphology, diffusion phenomena, and evolution of the alloy manufactured via the two additive manufacturing (AM) routes. The insights gained from this study will help to improve the properties and fabrication of AlCoCrFeNi HEA via the two additive manufacturing routes for applications.

Additive manufacturing (AM) has gained wide popularity for manufacturing geometrically complex metal parts. However, the mechanical properties of these materials under as-built conditions, and many times, after post-build heat treatments, have been found inferior to their conventionally manufactured counterparts. Combining Ultrasonic Impact Treatment (UIT) with AM processes has been reported recently as a promising technique to enhance quasi-static mechanical properties. In the present study, the effectiveness of UIT to enhance the low cycle fatigue properties of nickel-based superalloys such as Inconel 718 (IN718) is investigated. The results show that the application of UIT enhances low cycle fatigue (LCF) life substantially by about 110%. The stress-strain and nanoindentation behaviors also show an overall enhancement in strength and toughness. The mechanism responsible for the dramatic increase in LCF performance and strengthening is determined as an increased resistance to dislocation gliding and fatigue crack propagation due to UIT induced lattice distortion. The fracture surface analysis of the failed specimens shows a reduction in fatigue crack propagation rate confirming the enhancement in fatigue crack resistance.

Additively manufactured metals and alloys often have a hierarchical microstructure consisting of subgrain features. The subgrain features are typically a network of solidification cells and a scattered distribution of secondary phases. Since the subgrain features have a characteristic length scale smaller than a single grain, they have a significant influence on the material’s mechanical properties including its anisotropy. For instance, recent studies have reported that the subgrain features are responsible for the exceptional combination of strength and ductility exhibited by the direct energy deposited (DED) stainless steel (SS) 316L compared to its conventionally manufactured counterpart. In this work, a generalized microstructure-informed constitutive modeling framework is developed for hierarchical materials to capture the influence of subgrain features on mechanical properties using DED SS 316L as the reference material. A mechanism-based crystal plasticity formulation is used for the development of the framework. The formulation explicitly accounts for the geometric and mechanistic effects of both cellular network and scattered secondary phase subgrain features. Homogenization schemes are developed to implement the constitutive model on a finite element analysis (FEA) framework. The anisotropy is realized as a natural outcome of a preferred alignment of cells in the build direction. The model is then calibrated and validated experimentally.

The columnar grain morphology (grain shape and orientation) in polycrystalline metals is generally considered to contribute to mechanical anisotropy due to the interaction of dislocations with grain boundaries. However, past investigations have failed to reach a consensus on the role of grain morphology on property anisotropy. In this study, the influence of columnar morphology as a potential source for anisotropy is investigated for directed energy deposited (DED) stainless steel 316L (SS 316L). Macroscopic stress-strain behaviors and microscopic slip band evolution of DED SS 316L specimens were obtained under uniaxial tensile testing. The stress-strain behaviors show that the influence of grain morphology on anisotropy is negligible. The characteristics of slip band evolution reveal that the influence of grain morphology on anisotropy depends on the outcome of two competing choices for slip selection: the least resistant glide path versus the one that offers the greatest compatibility with the intergranular deformation.

Hetero-deformation induced (HDI) strengthening is investigated as the mechanism responsible for the exceptional combination of strength and ductility exhibited by directed energy deposited (DED) stainless steel 316L (SS316L). Recent studies have reported that the exceptional properties are due to the influence of cell wall features, i.e. austenitic cell walls and secondary delta ferrites on the deformation of cell interiors. The influences manifest as barrier effects causing deformation localization and slip band refinement. While these findings allude to the possibility of HDI strengthening, the role of the properties of these individual features is unclear. To investigate the role of individual features in terms of their properties and establish HDI, we performed a nanoindentation-based study. First, using low -load (< 1500 mu N) nanoindentations, the hardness of the individual features were determined. Contour maps of local hardness generated using these hardness values showed a striking resemblance to its cellular subgrain morphology, revealing a microstructure with hetero-zones. Then, the synergistic influence of the hetero-zones was revealed from the relationship between the hardness of individual features and the effective hardness (determined with high-load indentations of greater than 7 N), thus establishing HDI strengthening. Finally, by analyzing the hardness variation with respect to the distance from the feature interface, the presence of an interface affected zone (IAZ), one that has been hypothesized in recent studies, was experimentally established, and its width estimated. The findings of this study suggest that DED SS316L is a heterostructured material with great promise for deriving exceptional properties by enhancing HDI strengthening.

Recent studies have established that the exceptional combination of strength and ductility of directed energy deposited stainless steel 316 L is primarily due to the barrier effect against mobile dislocations introduced by cellular subgrain feature boundaries. Interestingly, the morphology of the feature, and therefore its boundaries, exhibits a significant variation with process parameters. This article presents an experimental study to determine the mechanism responsible for the formation of cellular subgrain feature with different morphologies and the influence of morphology on mechanical properties. Two builds with cellular subgrain features of different morphologies, tubular and vermicular, are manufactured using different process parameters and scanning strategies. The microstructural analysis of the specimens shows very different element segregation and secondary phase distribution. The mechanism responsible for the formation of different morphologies is the differences in the solidification modes. The barrier effect has two contributions; a hard barrier effect introduced by delta ferrite secondary phases and a soft barrier effect introduced by austenite cell wall regions. Irrespective of different morphologies, the two builds exhibited similar macroscopic mechanical behavior. This indicates that the total barrier effect exerted by the cellular subgrain feature determines the mechanical properties of directed energy deposited stainless steel 316 L, not necessarily its morphology.

The central goal of this chapter is to present an outline of the plan and current status of an effort to connect Additive Manufacturing (AM) process parameters with parameters describing the functional performance of produced parts. The term “functional performance” here represents primarily mechanical or thermal or electrochemical performance. The described effort represents an overview of the main research activities within a new multi-year grand-challenge project initiated at the US Naval Research Laboratory (US-NRL) in late 2016, in collaboration with groups from various academic institutions.

Coupling additive manufacturing (AM) with interlayer peening introduces bulk anisotropic properties within a build across several centimeters. Current methods to map high resolution anisotropy and heterogeneity are either destructive or have a limited penetration depth using a non-destructive method. An alternative pseudonondestructive method to map high resolution anisotropy and heterogeneity is through energy consumption during milling. Previous research has shown energy consumption during milling correlates with surface integrity. Since surface milling of additively manufactured parts is often required for post-processing to improve dimensional accuracy, an opportunity is available to use surface milling as an alternative method to measure mechanical properties and build quality. The variation of energy consumption during the machining of additive parts, as well as hybrid AM parts, is poorly understood. In this study, the use of net cutting specific energy was proposed as a suitable metric for measuring mechanical properties after interlayer ultrasonic peening of 316 stainless steel. Energy consumption was mapped throughout half of a cuboidal build volume. Results indicated the variation of net cutting specific energy increased further away from the surface and was higher for hybrid AM compared to as-printed and wrought. The average lateral and layer variation of the net cutting specific energy for printed samples was 81% higher than the control, which indicated a significantly higher degree of heterogeneity. Further, it was found that energy consumption was an effective process signature exhibiting strong correlations with microhardness. Anisotropy based on residual strains were measured using net cutting specific energy and validated by hole drilling. The proposed technique contributes to filling part of the measure gap in hybrid additive manufacturing and capitalizes on the pre-existing need for machining of AM parts to achieve both goals of surface finish and quality assessment in one milling operation.

Variable ram blowout prevention (VRBOP) valves are elastomeric material-based flow control devices used in offshore oil drilling applications as the primary safety mechanism to respond to high wellbore pressure emergency situations. During their operation, the elastomer deforms and distorts extensively to form a tight seal around the drillpipe. Because of the large deformation and distortion of the elastomer, developing a Lagrangian-based finite element analysis model to simulate the operation of a VRBOP valve is quite challenging. The finite elements of the Lagrangian finite element mesh degrade in quality because they deform with the material when the deformation becomes excessive. This leads to poor convergence of the numerical solution. In this study, we first demonstrate that the numerical convergence issues can be resolved by using a suite of modeling techniques: explicit integration scheme, Ogden secondorder hyperelastic constitutive model for the elastomer, and the selection of appropriate values for element sizes and other modeling parameters. After resolving the convergence issues, we used the model to study the sealing efficiency and material failure of a VRBOP valve for two different operating temperatures and drillpipe diameters. The sealing efficiency is studied using two performance criteria: the uniformity of the sealing pressure around the drillpipe and the magnitude of the overall deformation of the elastomer. For the material failure analysis of the elastomer, we used multiple failure criteria. The results of this study provide many new insights that have the potential to improve VRBOP valve design. For instance, results show that elevated temperature improves the sealing efficiency of a VRBOP valve because of the higher flexibility of the elastomer at elevated temperatures. Likewise, the wellbore pressure also improves the sealing efficiency. However, all these improvements in sealing pressure come with the risk of a higher probability of material failure.

Microstructure-informed design approach is set to revolutionize the design of metals and alloy components for aerospace applications. In this approach, a designer utilizes the influence of individual microstructural features on microscopic deformation to yield desirable macroscopic properties. Therefore, the development of advanced experimental capabilities that enable detailed characterization of microscopic deformation of material test specimens is critical to realize this paradigm shift in practice. However, extracting the complex characteristics of microscopic deformation hidden in raw image data is quite challenging. In this article, we propose an automated data extraction and analysis method based on instance segmentation and tracking of microstructural features using deep learning (DL) and image processing algorithms. The method consists of a trained mask Region-based Convolutional Neural Network (mask R-CNN) DL model combined with a regional instance segmentation approach for the instance segmentation of features, an intersection over union based multi-object tracking method to track segmented instances as they deform, and kinematics models to extract the material characteristics from the geometrical data of the deforming instances. The method is then validated by characterizing the microscopic deformation of an additively manufactured 316L stainless steel coupon specimen under quasi-static tensile loading. Our study presents a general framework for advancing deep learning algorithms to solve complex problems in the field of experimental mechanics.