July/August_AMP_Digital

A D V A N C E D M A T E R I A L S & P R O C E S S E S | J U L Y / A U G U S T 2 0 2 0 1 6 (a) (b) H uman civilization has witnessed various inventions related to met- als and alloys, which have played a significant role in the technological evolu- tion over different eras. Since the Bronze Age, alloys were developed tradition- ally following the “base metal” theory, where the number of principal elements was mostly two [1] . Efforts to improve ma- terial performance has led to approach- es for developing new alloys utilizing more than two principal components. In that course, the class of alloys termed as high-entropy alloys (HEAs) were designed almost a decade ago and have received significant attention [2] . HEAs consist of multiple (mostly five) principal elements which are stabilized in the crystal lattice driven by entropy, and exhibit excellent mechanical properties [3] . Historical evo- lution of the engineering metals and al- loys is presented in Fig. 1a, which clearly HEAs are critical, as these can easily be affected by the variation in the homoge- neity of alloying elements. Several cri- teria, such as mixing enthalpy (∆ H mix ), atomic size difference ( δ ), and valence electron concentration (VEC), are also utilized in predicting the formation and stability of HEAs [3] . In this regard, AM of- fers high-quality production of HEAs without segregation, owing to the la- ser-induced rapid heating and cooling of elemental powders. The optimized processing parameters further assist in controlling the entropy stabilized formation of HEAs. This is clearly illus- trated in Fig. 2, where several HEAs fab- ricated by AM processes are compiled. The figure shows AM manufactured HEAs consisting of parent elements Ni, Co, Cr, Fe, and Mn, tabulated based on the alloy phase formation. The demon- strated family of Ni-based HEAs is in- dustrially relevant by virtue of excellent high-temperature corrosion, wear, and abrasive resistance. The addition of oth- er alloying elements such as V, Ti, Mo, and W further improves the hardness and toughness in the listed HEAs by forming additional secondary phases. Typical applications of these AM pro- cessed HEA components primarily in- clude hypersonics in aviation, offshore drilling, and cutting tools in the oil & gas industry, among other extreme environments. Although the benefits of AM are quite clear in processing HEA compo- nents, the processing method still pos- es challenges due to the emergence of microstructural features such as aniso- tropic microstructure, metallurgical defects, etc., which have notable effects on the properties of HEAs. Therefore, it is extremely critical to analyze those distinct features for improving the qual- ity of HEAs, and in this regard, high-res- olution microscopy plays a significant role. This article describes the import- ant correlations between process- ing parameters, microstructure, and properties. Furthermore, the utility of data-driven approaches in high-resolu- tion microscopy to ease the predic- tions and characterization of micro- structural behavior of materials are also discussed. illustrates the rise of HEAs in recent years. Interestingly, the evolution of HEAs co- incides with the latest advancements in materials processing using additive man- ufacturing (AM) [4] . The progression of pro- cessingtechnologiestowardsAMisseenin Fig. 1b, demonstrating the relative impor- tanceofvariousprocessingroutesformet- al manufacturing over the years. While theindustrieshaveadoptedseveralmeth- ods for fabrication, AMmethods present a unique opportunity due to their capabil- ity of producing large three-dimensional components with high-precision design. In addition to AM of HEAs, which is the fo- cus of this article, AM also covers a broad spectrum of materials, such as metals, ceramics, composites and polymers. Fol- lowing these trends in the materials tech- nology, it can be confidently said that the AM processing of HEAs would lead the paradigm shift in the advanced materials and processing. The fundamen- tal principle of de- signing 3D structures by AM includes a lay- er-by-layer addition of material bonded by rapid melting and solidification. Due to fast cooling rates, AM methods inhibit the long-range diffusion of the elements, thus restricting the for- mation of undesired inter-metallic com- pounds and facilitat- ing the formation of the solid solution [5] . Since the HEAs con- sist of multiple ele- ments of dissimilar densities and high melting points, the most important chal- lenge in processing HEAs is to achieve an excellent solubil- ity during melting and to avoid compo- sitional segregation during solidifications. The formation and phase stability in Fig. 1 — Historical evolution of (a) engineering materials indicating the birth of high-entropy alloys, (b) different processing techniques illustrating the rise of additive manufacturing.