Microstructure and Oxidation Behavior of Nb-Si-Based Alloys for Ultrahigh Temperature Applications: A Comprehensive Review

Nb-Si-based superalloys are considered as the most promising high-temperature structural material to replace the Ni-based superalloys. Unfortunately, the poor oxidation resistance is still a major obstacle to the application of Nb-Si-based alloys. Alloying is a promising method to overcome this problem. In this work, the effects of Hf, Cr, Zr, B, and V on the oxidation resistance of Nb-Si-based superalloys were discussed. Furthermore, the microstructure, phase composition, and oxidation characteristics of Nb-Si series alloys were analyzed. The oxidation reaction and failure mechanism of Nb-Si-based alloys were summarized. The significance of this work is to provide some references for further research on high-temperature niobium alloys.

Electron Beam Melting of Niobium Alloys from Blended Powders

Niobium-based tungsten alloys are desirable for high-temperature structural applications yet are restricted in practice by limited room-temperature ductility and fabricability. Powder bed fusion additive manufacturing is one technology that could be leveraged to process alloys with limited ductility, without the need for pre-alloying. A custom electron beam powder bed fusion machine was used to demonstrate the processability of blended Nb-1Zr, Nb-10W-1Zr-0.1C, and Nb-20W-1Zr-0.1C powders, with resulting solid optical densities of 99+%. Ultimately, post-processing heat treatments were required to increase tungsten diffusion in niobium, as well as to attain satisfactory mechanical properties.

Bayesian Calibration of Multiple Coupled Simulation Models for Metal Additive Manufacturing: A Bayesian Network Approach

Modeling and simulation for additive manufacturing (AM) are critical enablers for understanding process physics, conducting process planning and optimization, and streamlining qualification and certification. It is often the case that a suite of hierarchically linked (or coupled) simulation models is needed to achieve the above task, as the entirety of the complex physical phenomena relevant to the understanding of process-structure-property-performance relationships in the context of AM precludes the use of a single simulation framework. In this study using a Bayesian network approach, we address the important problem of conducting uncertainty quantification (UQ) analysis for multiple hierarchical models to establish process-microstructure relationships in laser powder bed fusion (LPBF) AM. More significantly, we present the framework to calibrate and analyze simulation models that have unmeasurable variables, which are quantities of interest predicted by an upstream model and necessary for the downstream model in the chain that are difficult or impossible to observe experimentally. We validate the framework using a case study on predicting the microstructure of binary nickel-niobium alloys processed using LPBF as a function of processing parameters. Our framework is shown to be able to predict segregation of niobium with up to 94.3% prediction accuracy in test data.

Plasma Electrolyte Polishing of Titanium and Niobium Alloys in Low Concentrated Salt Solution Based Electrolyte

Today, world enterprises specializing in the production of titanium and niobium alloy products and having modern high-tech equipment are still forced to use outdated, costly and environmentally unsafe methods of polishing and surface cleaning, based on electrochemical treatment in toxic solutions. The complex geometric shape of the processed products does not allow the use of mechanical polishing methods, due to high labor costs and low efficiency. These materials are difficult to process, and the processes of their polishing require the use of toxic electrolytes. An alternative to existing methods of electrochemical polishing is plasma electrolyte polishing in aqueous solutions of salts with a concentration of up to 5%. We have developed fundamentally new modes of plasma electrolyte polishing of products from titanium and niobium alloys using simple electrolytes based on an aqueous solution of fluoride salt, the use of which can significantly increase the surface quality class. The advantage of the plasma electrolyte polishing over classical electrochemical polishing is its safety due to the use of an electrolyte based on an aqueous solution of salt. The paper presents the results of an investigation of the characteristics and morphology of the surface after processing using the developed method. Modes that provide the maximum change in roughness with minimal metal removal were obtained.

Dynamic Recrystallization Mechanisms of Nickel Niobium Alloys during Hot Working

This present work examines the influence of niobium in solid solution on the microstructural evolution of pure nickel at various deformation conditions. On this purpose, high-purity nickel and six model nickel-niobium alloys (Ni–0.01, 0.1, 1, 2, 5 and 10 wt. % Nb) were subjected to hot torsion test to large strains within the temperature range from 800 to 1000 °C at strain rates of 0.03, 0.1 and 0.3 s–1. Microstructural analyses were carried out using both optical and scanning electron microscopy-based electron back-scattered diffraction technique. The overall results showed the key role played by the Nb amount when coupled with various DRX mechanisms involved, i.e. DDRX, CDRX, and GDRX with respect to the prescribed deformation conditions, in reducing grain size and retarding DRX kinetics from which the microstructures of the examined materials such as Ni 2 and 10 wt. % Nb were seen evolving in different ways. In all these deformed materials, a transition from discontinuous dynamic recrystallization to continuous dynamic recrystallization was observed at low temperature and high strain rate whereas only discontinuous dynamic recrystallization occurred at high temperature.