Heat-Resistant Intermetallic Compounds and Ceramic Dispersion Alloys for Additive Manufacturing: A Review

Many industries such as aerospace, power generation, and ground transportation demand structural materials with high specific strength at elevated temperatures. Up until now, many types of heat-resistant materials including Ni-based superalloys, intermetallic compounds, and dispersion-strengthened alloys have been developed for specific applications in these industries. Moreover, with the recent development of additive manufacturing techniques, these industries can now benefit from the rapid prototyping abilities, geometric freedom, and increased mechanical properties that can be achieved through various additive manufacturing processes. With this in mind, the progress made in additive manufacturing of heat-resistant intermetallic compounds and ceramic dispersion alloys is herein examined. A brief introduction is provided on the target industries, applications, and the compositions of heat-resistant alloys of current research interest. Then, recent research on heat-resistant intermetallic compounds and ceramic dispersion alloys fabricated by additive manufacturing processes such as laser powder bed fusion, laser direct energy deposition, and electron-beam powder bed fusion are reviewed with information provided on microstructure, processing parameters, strengthening mechanisms, and mechanical properties. Finally, an outlook is provided with future research suggestions

A discrete element method representation of an anisotropic elastic continuum

A method for modeling cubically anisotropic elasticity within the discrete element method is presented. The discrete element method (DEM) is an approach originally intended for modeling granular materials (sand, soil, and powders); however, recent developments have usefully extended it to model stochastic mechanical processes in monolithic solids which, to date, have been assumed to be elastically isotropic. The method presented here for efficiently capturing cubic elasticity in DEM is an important prerequisite for further extending DEM to capture the influence of elastic anisotropy on the mechanical response of polycrystals, composites, etc. The system demonstrated here uses a directionally assigned stiffness in the bonds between adjacent elements and includes separate schemes for achieving anisotropy with Zener ratios greater and smaller than one. The model framework is presented along with an analysis of the accessible space of elastic properties that can be modeled and an artificial neural network interpolation scheme for mapping input parameters to model elastic behavior

A critical dislocation velocity for serration mechanism transition in a nickel-chromium solid solution alloy

The influence of strain rate across three orders of magnitude (1.70 × 10−5/s to 1.43 × 10−2/s) along with the effect of the plastic strain accumulation (up to 10%) on the serrated plastic flow were investigated in the nickel-chromium (Ni-Cr) solid solution alloy Nimonic 75 by performing constant-strain-rate tension testing at 600 °C. As the strain rate decreased, the critical strain for the onset of serrations transitioned from normal behavior to inverse behavior. The serrated flow was characterized as Type A+B serration at high strain rate (1.43 × 10−2/s). In the intermediate strain-rate regime (1.43 × 10−3/s and 1.45 × 10−4/s), Type B serrations were observed and followed by a transformation to Type C+B serrations. At the low strain rate (1.70 × 10−5/s), the plastic flow immediately displayed Type C serrations, which later evolved into Type C+B serrations. Regardless of the strain rate, plastic strain, or dislocation density, a critical dislocation velocity falling in the range of 1.2 × 10−6 – 2.2 × 10−6 m/s was identified to signify the onset of Type C serration, whereby the mobile dislocations break free from the solute cloud for short bursts of deformation. Finally, a novel model by solute rearrangement across dislocation cores was used to understand how the critical dislocation velocity is quantitatively determined by the rate at which solute atoms are able to hop across the glide plane as a partial dislocation core moves through the lattice

Review: Multi-principal element alloys by additive manufacturing

Multi-principal element alloys (MPEAs) have attracted rapidly growing attention from both research institutions and industry due to their unique microstructures and outstanding physical and chemical properties. However, the fabrication of MPEAs with desired microstructures and properties using conventional manufacturing techniques (e.g., casting) is still challenging. With the recent emergence of additive manufacturing (AM) techniques, the fabrication of MPEAs with locally tailorable microstructures and excellent mechanical properties has become possible. Therefore, it is of paramount importance to understand the key aspects of the AM processes that influence the microstructural features of AM fabricated MPEAs including porosity, anisotropy, and heterogeneity, as well as the corresponding impact on the properties. As such, this review will first present the state-of-the-art in existing AM techniques to process MPEAs. This is followed by a discussion of the microstructural features, mechanisms of microstructural evolution, and the mechanical properties of the AM fabricated MPEAs. Finally, the current challenges and future research directions are summarized with the aim to promote the further development and implementation of AM for processing MPEAs for future industrial applications

Transient creep-fatigue crack growth in creep-brittle materials: Application to Alloy 718

This study presents incremental finite element computations of creep-fatigue crack growth in Alloy 718 at 650°C in air. Alloy 718 is representative of creep-brittle materials, in which viscoplastic deformation is restricted near the tip of a growing crack. The computations predict crack growth using a unique combination of an irreversible cohesive zone formulation and a strain gradient viscoplastic material model based on the Kocks–Mecking formulation. Cohesive zone damage parameters are estimated using sustained loading and constant-amplitude cyclic loading experiments. Computations of crack extension under three different waveforms containing overloads all predict post-overload retardation. The amount of retardation depends strongly on the overload ratio, consistent with experiments in the literature using similar waveforms. Analysis of the crack-tip fields demonstrates retardation is associated with unloading in the highly deformed material near the advancing crack tip. Dynamic recovery and geometrically necessary dislocations are shown to significantly influence post-overload crack extension

EDP2PDF: a computer program for extracting a pair distribution function from an electron diffraction pattern for the structural analysis of materials

Pair distribution function (PDF) analysis is a powerful technique to understand atomic scale structure in materials science. Unlike X-ray diffraction (XRD)based PDF analysis, the PDF calculated from electron diffraction patterns (EDPs) using transmission electron microscopy can provide structural information from specific locations with high spatial resolution. The present work describes a new software tool for both periodic and amorphous structures that addresses several practical challenges in calculating the PDF from EDPs. The key features of this program include accurate background subtraction using a nonlinear iterative peak-clipping algorithm and automatic conversion of various types of diffraction intensity profiles into a PDF without requiring external software. The present study also evaluates the effect of background subtraction and the elliptical distortion of EDPs on PDF profiles. The EDP2PDF software is offered as a reliable tool to analyse the atomic structure of crystalline and non-crystalline materials

Controlling the relaxation versus rejuvenation behavior in Zr-based bulk metallic glasses induced by elastostatic compression

Elastostatic compression (ESC) has received considerable research attention as a tool to study rejuvenation and relaxation processes for bulk metallic glasses (BMGs). However, little is understood about the conditions that control whether rejuvenation or relaxation will occur, and whether conditions exist that can give structural stability. We address these questions by applying ESC at 90% of the yield stress to both cast and laser powder bed fusion (LPBF) manufactured Zr-based BMG samples in the as-cast, as-built, and different annealed states. The structural state and mechanical properties for each material condition were characterized by differential scanning calorimetry and microhardness, respectively, and two representative groups were also used for compression testing. Initial relaxation or rejuvenation was observed for elastostatically compressed as-cast samples, and the behavior reversed over 72 h of ESC. In contrast, no ESC effect was observed for the as-built LPBF samples. It was found that the onset of either relaxation or rejuvenation by ESC could be better predicted if samples were annealed into a controlled initial state. Five different types of initial response to ESC were observed, corresponding to different initial energy state ranges. Materials in the highest and lowest initial energy states were stable against structural changes by ESC. Close to the highest energy state, rejuvenation was dominant, while relaxation took place close to the lowest energy state. At intermediate initial energy states, both relaxation and rejuvenation were observed after ESC loading, suggesting that the glass structure easily finds different local minima in the potential energy landscape. In all cases, relaxation was associated with BMG hardening and rejuvenation was associated with softening. Overall, the results of this study provide new insights into how ESC impacts the structural state and mechanical properties of BMGs

A machine learning method to quantitatively predict alpha phase morphology in additively manufactured Ti-6Al-4V

Quantitatively defining the relationship between laser powder bed fusion (LPBF) process parameters and the resultant microstructures for LPBF fabricated alloys is one of main research challenges. To date, achieving the desired microstructures and mechanical properties for LPBF alloys is generally done by time-consuming and costly trial-and-error experiments that are guided by human experience. Here, we develop an approach whereby an image-driven conditional generative adversarial network (cGAN) machine learning model is used to reconstruct and quantitatively predict the key microstructural features (e.g., the morphology of martensite and the size of primary and secondary martensite) for LPBF fabricated Ti-6Al-4V. The results demonstrate that the developed image-driven machine learning model can effectively and efficiently reconstruct micrographs of the microstructures within the training dataset and predict the microstructural features beyond the training dataset fabricated by different LPBF parameters (i.e., laser power and laser scan speed). This study opens an opportunity to establish and quantify the relationship between processing parameters and microstructure in LPBF Ti-6Al-4V using a GAN machine learning-based model, which can be readily extended to other metal alloy systems, thus offering great potential in applications related to process optimisation, material design, and microstructure control in the additive manufacturing field

Additive manufacturing of crack-free Al-alloy with coarsening-resistant τ<inf>1</inf>-CeAlSi strengthening phase

Wrought aluminium alloys popular for automotive and aerospace applications are susceptible to solidification cracking when fabricated via laser powder bed fusion (LPBF). Another long-standing and common issue for these alloys is microstructure coarsening and corresponding strength loss caused by elevated temperature exposure. To tackle these challenges, this study designs and develops a class of 1–4 wt% Ce modified Al6061 alloys. The best alloy, with 3 wt% Ce, achieves crack-free fabrication via LPBF due to a reduction in the solidification temperature range and a new solidification pathway that achieved 0.9 solid mass fraction at just 14 °C below the solidification onset. Furthermore, a fine microstructure consisting of coarsening-resistant τ1-CeAlSi eutectic forms, and after hot isostatic pressing, the tensile strength and elongation of the 3 wt% Ce alloy can reach 153 ± 6 MPa and 18.3% at room temperature and 89 ± 6 MPa and 32.5% at 200 °C, respectively. The observed ductility is attributed to nanoscale dispersion of discrete, coarsening resistant τ1-CeAlSi particles within grains and to the presence of large columnar α-Al grains. Meanwhile, solidification cracking was inhibited by continuous grain boundary τ1-CeAlSi eutectic accumulation, which converted to discrete nanoscale τ1-CeAlSi after hot isostatic pressing. This research uncovers a simple and effective approach of designing Al-alloys for LPBF with great potential for both room temperature and high temperature applications in automotive and aerospace industries

An Ultrafast and Stable High-Entropy Metallic Glass Electrode for Alkaline Hydrogen Evolution Reaction

A new type of high-entropy alloy with a composition of Pt25Pd25Ni25P25(at.%) and an amorphous structure, referred to as a high-entropy metallic glass (HEMG), was developed by a scalable metallurgical technique for efficient hydrogen evolution reaction (HER). The achieved overpotential was as low as 19.8 mV at a current density of 10 mA cm-2while maintaining an ultrareliable performance for 60 h in 1.0 M KOH solution, exhibiting 5- and 10-times higher performance than those of traditional Pt60Ni15P25and Pd40Ni40P20metallic glasses, respectively, and also surpassing the benchmark performance of commercial Pt/C nanoparticles and pure Pt sheet. Experimental and theoretical results revealed that the enhanced HER activity was ascribed to a synergistic function of multiprincipal components that optimized the electronic structure to accelerate the rate-determining steps in HER. Moreover, the unique long-range disordered structure provided a high density of unsaturated atomic coordination that was able to improve the amount of electrochemically active sites. This bulk HEMG strategy paves the way for the development of flexible freestanding electrodes for water splitting applications