Fabrication and characterization of advanced materials using laser metal deposition from elemental powder mixture

Over the past decades of years, a great deal of money has been spent to machine large and complex parts from high-performance metals (i.e., titanium components for aerospace applications), so users attempt to circumvent the high cost of materials. Laser metal deposition (LMD) is an additive manufacturing technique capable of fabricating complicated structures with superior properties. This dissertation aims to improve the applications of LMD technique for manufacturing metallic components by using various elemental powder mixture according to the following three categories of research topics. The first research topic is to investigate and develop a cost-effective possibility by using elemental powder mixture for metallic components fabrication. Based on the studies of fabricating thin-wall Ti-6Al-4V using elemental powder mixture, comparative close particle number for Ti, Al and V powder could easily get industry qualified Ti-6Al-4V components. The particle number for each element in powder blends has been proved to be a key factor for composition control in the final deposit part. The second research topic focuses on the application improvements of elemental powder manufacturing. By fabricating AlxCoFeNiCu1-x (x = 0.25, 0.5, 0.75) high entropy alloys from elemental powder based feedstocks, it enhances the usage of elemental powder to fabricate novel materials with complex compositions. The third research topic extends the applications of using elemental powder mixture to the broader area. A functionally gradient material (FGM) path is developed to successfully join titanium alloy with γ-TiAl. This dissertation leads to new knowledge for the effective fabrication of unique and complex metallic components. Moreover, the research results of the dissertation could benefit a wide range of industries --Abstract, page iv

Structure and high temperature mechanical properties of novel nonequiatomic Fe-(Co, Mn)-Cr-Ni-Al-(Ti) high entropy alloys

Four non-equiatomic Fe-(Co, Mn)-Cr-Ni-Al-(Ti) high entropy alloys, namely Fe₃₆Mn₂₁Cr₁₈Ni₁₅Al₁₀, Fe₃₆Co₂₁Cr₁₈Ni₁₅Al₁₀, Fe₃₅Mn₂₀Cr₁₇Ni₁₂Al₁₂Ti₄, and Fe₃₅Co₂₀Cr₁₇Ni₁₂Al₁₂Ti₄ alloys, were produced by arc melting. Structures and compression mechanical properties of the as-cast alloys were examine

High-Entropy Coatings (HEC) for High-Temperature Applications: Materials, Processing, and Properties

High-entropy materials (HEM), including alloys, ceramics, and composites, are a novel class of materials that have gained enormous attention over the past two decades. These multi-component novel materials with unique structures always have exceptionally good mechanical properties and phase stability at all temperatures. Of particular interest for high-temperature applications, e.g., in the aerospace and nuclear sectors, is the new concept of high-entropy coatings (HEC) on low-cost metallic substrates, which has just emerged during the last few years. This exciting new virgin field awaits exploration by materials scientists and surface engineers who are often equipped with high-performance computational modelling tools, high-throughput coating deposition technologies and advanced materials testing/characterisation methods, all of which have greatly shortened the development cycle of a new coating from years to months/days. This review article reflects on research progress in the development and application of HEC focusing on high-temperature applications in the context of materials/composition type, coating process selection and desired functional properties. The importance of alloying addition is highlighted, resulting in suppressing oxidation as well as improving corrosion and diffusion resistance in a variety of coating types deposited via common deposition processes. This review provides an overview of this hot topic, highlighting the research challenges, identifying gaps, and suggesting future research activity for high temperature applications

Superfunctional materials by ultra-severe plastic deformation

Superfunctional materials are defined as materials with specific properties being superior to the normal functions of engineering materials. Numerous studies introduced severe plastic deformation (SPD) as an effective process to improve the functional and mechanical properties of various metallic and non-metallic materials. Moreover, the concept of ultra-SPD - introducing shear strains over 1,000 to reduce the thickness of sheared phases to levels comparable to atomic distances - was recently utilized to synthesize novel superfunctional materials. In this article, after a brief review of the recent advances in the SPD field, the application of ultra-SPD for controlling atomic diffusion and phase transformation and achieving superfunctional properties is discussed. The main properties achieved by ultra-SPD include (i) high-temperature thermal stability in new immiscible age-hardenable aluminum alloys, (ii) room-temperature superplasticity for the first time in magnesium and aluminum alloys, (iii) high strength and high plasticity in nanograined intermetallics, (iv) low elastic modulus and high hardness in biocompatible binary and high-entropy alloys, (v) superconductivity and high strength in the Nb-Ti alloys, (vi) room-temperature hydrogen storage for the first time in magnesium alloys, and (vii) superior photocatalytic hydrogen production, oxygen production, and carbon dioxide conversion on high-entropy oxides and oxynitrides as a new family of photocatalysts

Feasibility studies on Laser Powder Bed Fusion of powders mixtures based on Aluminium alloys or High Entropy Alloys

L'abstract è presente nell'allegato / the abstract is in the attachmen

High Entropy Materials: Challenges and Prospects

This book is a reprint of a special issue of Metals (ISSN 2075-4701), titled High Entropy Materials: Challenges and Prospects. It is a compilation of nine articles from different aspects of high-entropy materials. The book primarily focuses on high-entropy alloys, the first emergent high-entropy materials, but also covers high-entropy ceramics and high-entropy composites, which are the extensions of high-entropy alloys. The articles on high-entropy alloys cover some important facets in the field such as phase structures, mechanical properties, laser beam welding, design of soft magnetic alloys, and potential as biomaterials. In addition, there are one article introducing the potential of using high-entropy carbides as hard metals for machining, and one another on high-entropy composite studying the microstructures and tribological properties of the FeCoNiCuAl-TiC composite. The goal of this reprinted book is essentially two-fold. In the first place, it offers a platform for researchers in the broad field of high-entropy materials to communicate their views and recent research on the subject. Next, it reports challenges in the sub-fields of high-entropy materials and inspires researchers to continue to practice diligence to resolve these challenges and advance high-entropy materials solidly. We hope that readers in the field feel encouraged, inspired, and challenged by the book, and readers outside the field can grasp some basic ideals of high-entropy materials and their potential to the society as a family of novel materials

Design and Validation of Novel Potential High Entropy Alloys

The design approach and validation of single phase senary refractory high entropy alloys (HEAs) MoNbTaTiVW and HfNbTaTiVZr were presented in first part of this dissertation. The design approach was to combine phase diagram inspection of available binary and ternary systems and Calculation of Phase Diagrams (CALPHAD) prediction. Experiments using X-ray diffraction and scanning electron microscopy techniques verified single phase microstructure in body centered cubic lattice for both alloys. The observed elemental segregation agrees well with the solidification prediction using Scheil model. The lattice constant, density and microhardness were measured to be 0.3216 nm, 4.954 GPa and 11.70 g/cm3 for MoNbTaTiVW and 0.334 nm, 5.5 GPa and 9.36 g/cm3 for HfNbTaTiVZr. To elaborate the single-phase stability of HEAs, CrxMoNbTaVW was examined over a certain range of Cr content in the second part of this dissertation. The change in composition led to different BCC structures with different microstructures and physical properties. Microstructure characterizations were performed using X-ray diffraction and scanning electron microscopy. Chemical micro-segregation during solidification predicted using the Scheil model generally agrees with the experimental results. The lattice constant, density, and Vickers\u27 micro-hardness of the high-entropy alloy samples in the as-cast state are measured and discussed. For CrxMoNbTaVW, x=2.0 case appears exceeding the upper limit of maintaining a single BCC phase HEA, determined by the XRD patterns. The elemental dependence of the mixing thermodynamic properties (entropy, enthalpy and Gibbs energy) in BCC phase in the senary system is analyzed. The calculated entropy of mixing and enthalpy of mixing for CrMoNbTaVW are 14.7 J/K/mol and −662.5 J/mol respectively. Phase predictions and characterizations on as-solidified septenary refractory high-entropy alloy, CrMoNbReTaVW, are presented in the third part of the dissertation. The simulated solidification process predicts a single body-centered-cubic (BCC) crystal structure with the tendency of compositional segregation. X-ray diffraction results confirm the “single-phase-like” BCC structure, while further experimental characterizations reveal the existence of multiple grains with significantly different compositions yet the same crystal structure and similar lattice. For better understanding of corrosion properties of high entropy alloys, the CALPHAD method was further used to simulate the Pourbaix diagram and the corrosion layer evolutions under equilibrium conditions for CoCrFeNi based HEAs in the last part of the dissertation. The oxidation layer pitting and forming potential were calculated and compared favorably with published experimental results on CoCrFeNi, CoCrFeNiCu and CoCrFeNiAl0.5 HEAs

From high-entropy alloys to complex concentrated alloys

High-entropy alloys (HEAs) and related concept of complex concentrated alloys (CCAs) expand the diversity of the materials world and inspire new ideas and approaches for the design of materials with an attractive combination of properties. Here, we present a critical review of the field with the intent of summarizing the principles underlying their birth and growth. We highlight the major accomplishments and progresses over the last 14 years, especially in the discovery of new microstructures and mechanical properties. Finally, we outline the main challenges and provide guidance for future works

Manufactura y caracterización de aleaciones de alta entropía

High entropy alloys are a new kind of multicomponent alloys, consisting of five or more metallic elements with equiatomic proportions. Despite the large number of alloying elements, HEA can exhibit simple solid solution phases, such as face- and body-centered cubic phases. In this work, the AlxCrCuFeNiTi (x = 0, 0.45, 1, 2.5, 5 mol) alloy was fabricated by mechanical alloying to determine the effect of aluminum on the phase evolution during the process and its impact on the mechanical properties. Grinding of the powders was carried out at 300 rpm during 180 minutes. The powders resulting from milling were pressed at 250 kg/cm2. The pressed samples were sintered at 1,300°C during 1 hour. From results it can be seen that with increasing Al concentration, the alloys undergo a transformation from a single FCC phase to mixture of FCC and BCC phases, as well as the precipitation of FeAl3, Al3Ni, TiAl and Ti3Al intermetallics. The alloy that achieved the highest hardness was the one with the highest Al content. These alloys harden significantly with the addition of Al, due to the BCC phase formation and intermetallic compounds.Las aleaciones de alta entropía son una nueva clase de aleaciones multicomponentes, que consisten en cinco o más elementos metálicos con proporciones equiatómicas. A pesar del gran número de elementos de aleación, las HEA pueden exhibir fases de solución sólida simples, como las fases cúbicas centrada en las caras y centrada en el cuerpo. En este trabajo se fabricó la aleación AlxCrCuFeNiTi (x = 0, 0.45, 1, 2.5, 5 mol) mediante aleado mecánico para determinar el efecto del aluminio en la evolución de fases durante el proceso y su impacto en las propiedades mecánicas. La molienda de los polvos se realizó a 300 rpm durante 180 minutos. Los polvos resultantes de la molienda se prensaron a 250 kg/cm2. Las muestras prensadas se sinterizaron a 1300°C durante 1 hora. De los resultados se tiene que, al aumentar la concentración de aluminio, las aleaciones sufren una transformación de una sola fase FCC a una mezcla de fases FCC y BCC, así como la precipitación de intermetálicos de FeAl3, Al3Ni, TiAl y Ti3Al. La aleación que alcanzó la mayor dureza fue la de mayor contenido de aluminio. Estas aleaciones se endurecen significativamente con la adición de aluminio, debido a la formación de la fase BCC y por la formación de intermetálicos

High temperature corrosion of some selected stainless steels and Ni-base alloys – an advanced microscopy study

The thesis deals with high temperature corrosion behavior of some selective stainless steels and Ni-base alloys with applications in power generation technologies, e.g. boilers fired by biomass and waste. The initial stages of KCl-induced oxidation behavior of two alumina formers (alloys Kanthal APMT and TH1) and one chromia former (alloy Sanicro 25) were analyzed in an O2/H2O environment using in-situ ESEM method. Besides, the effects of thermal cycling on the oxidation behaviour of a Ni-base alloy HR-214 was studied in air at 1200\ub0C. The in-situ oxidation experiments provided an opportunity to view dynamic processes occurring during the oxidation process ′′live′′. The in-situ results were validated by ex-situ exposures, i.e. reference tube furnace. The alloys were corroded in the matter of minutes in the studied environment. Quite evidently, the severest oxidation attack occurred locally in the vicinity of KCl particles, where oxide crusts and oxide shells/rims (consisting of Fe-, Cr- and Al- oxides) were formed. STEM studies showed that all the three alloys formed a thin base oxide scale. Chlorine-induced oxidation caused chlorination of the alloys as evidenced by detection of chlorine below the protective scales.In the case of the Ni-base alloy HR-214, both isothermal and cyclic exposures led to the formation of a duplex oxide morphology, composed of a columnar alumina layer overlaid by a complex Ni(Al,Cr)2O4 spinel. It was evident that thermal cycling resulted in the formation of vertical cracks in the multi-layered scale. Additionally, STEM/EDX revealed outwards transport of Cr through the cracks/ alumina grain boundaries, which caused thickening of the outer spinel layer. Moreover, an attempt was made to develop the newly introduced TKD method to study nano-sized oxide scales. This was conducted by (a) designing a dedicated sample holder, (b) specimen preparation and (c) acquisition parameters. These efforts made it possible to achieve data-rich TKD orientation maps (with indexing rates > 85%). Thus, the technique was effectively employed to obtain useful information from the microstructure and microtexture of the fine-grained oxide scales. Besides, the technique provided information concerning the crystallographic orientation relationship at oxide/oxide and oxide/alloy interfaces. Keywords: high-temperature materials; oxidation; KCl; STEM; STEM/EDX; TKD