Multi‐Principal‐Element Alloys with Improved Oxidation and Wear Resistance for Thermal Spray Coating

Abstract

Thermal-spray technology is commonly used for structural components by building up a protective coating layer on their surfaces. Choosing a suitable sprayed metal can improve corrosion resistance, [1,2] oxidation resistance, [3±5] wear resistance [6] and/or heat insulation, and thus extend the life of protected components.Three methods have been developed to give a protective coating to resist oxidation: glass-ceramic coating, [7±11] aluminiding the surface by reaction between Al and metal substrates, [12±15] and over-layer coating such as MCrAlY (M=Fe, Co, Ni) superalloys. [16±21] Glass-ceramic or aluminide layers are hard and brittle at low temperatures, and easily delaminated from the substrate due to the mismatch of the coefficient of thermal expansion. Over-layer coating of MCrAlY alloys is the favorite one for industry to protect structural components from oxidation at high temperatures. However, its lower hardness, about HRC 20~30, renders them poor in wear resistance when particulates or counterparts are involved in the environment. Thus, traditional coating materials can not provide an ideal coating both for oxidation resistance and wear resistance. In this study, a new alloy design concept ª;multi-principle-element alloysº, was explored to create alloys with excellent combinations of properties for some critical applications such as dies, molds and turbine blades. Processing, microstructure and properties were investigated to evaluate such multi-principle-element alloys as AlSiTiCrFeCoNiMo 0.5 (designated as 8E) and AlSiTiCrFeNiMo 0.5 (designated as 7E).As-cast microstructure and properties: Figure 1 shows the XRD patterns of as-cast 8E and 7E alloys. It reveals that their microstructures consist primarily of an ordered BCC phase and two FCC phases. The ordered BCC phase of 8E alloy had a lattice constant a = 2.87 , and the two FCC phases had a = 3.52 and 4.14 , respectively. 7E alloy had the phases with nearly the same constants as 8E alloy. It is indeed a surprise to see that these alloys did not to have many complex phase constituents since many possibilities of binary or ternary phases might be expected for such a large number of principle elements. Figure 2 shows the SEM pictures of the as-cast microstructures of 8E and 7E alloys. They were of a typical dendritic cast structure. EDS analysis indicates that Si, Ti, Cr, Fe, and Co and Mo were the main elements in the dendrite whereas Ni and Al were the main elements in the interdendrite regions. It is interesting to see that Co played as a neutral element and partitioned almost equally in both regions. The dendrite phase corresponded to the BCC phase shown in the XRD pattern and the interdendrite one was related with the FCC phases since the former was rich in Mo and Cr elements both of which have the BCC structure and are the first two elements highest in melting point. COMMUNICATIONS 74

Microstructure and Properties of Al<SUB>0.5</SUB>CoCrCuFeNiTi<I><SUB>x</SUB></I> (<I>x</I>=0&ndash;2.0) High-Entropy Alloys

“…Refer to Ref. 7) for further details of this preparation method. The polished, aqua regia etched specimens were observed with an optical microscope and scanning electron microscope (SEM, JEOL JSM-5410).…”

“…6) Consequently, alloys containing a higher number of principal elements will more easily yield the formation of random solid solutions during solidification, rather than intermetallic compounds or other complicated phases. 5,[7][8][9] AlCoCrCu 0:5 Ni, Al x CoCrCuFeNi, AlCoCrCuFeMoNiTiVZr, AlCoCrFeMo 0:5 NiSiTi and AlCrFeMo 0:5 NiSiTi high-entropy alloys are examples exhibiting a quite simple as-cast microstructure and promising performance. [7][8][9][10][11] Among the previous alloys investigated, Al 0:5 -CoCrCuFeNi alloy is an eminent alloy for structural applications, which possesses simple FCC structure and good hot and cold workability.…”

“…5,[7][8][9] AlCoCrCu 0:5 Ni, Al x CoCrCuFeNi, AlCoCrCuFeMoNiTiVZr, AlCoCrFeMo 0:5 NiSiTi and AlCrFeMo 0:5 NiSiTi high-entropy alloys are examples exhibiting a quite simple as-cast microstructure and promising performance. [7][8][9][10][11] Among the previous alloys investigated, Al 0:5 -CoCrCuFeNi alloy is an eminent alloy for structural applications, which possesses simple FCC structure and good hot and cold workability. Moreover, it exhibits a wear resistance close to SKD 61 steel with a hardness of 567 HV due to its large work hardening capacity although its as-cast hardness is 223 HV.…”

High-entropy alloys: a critical assessment of their founding principles and future prospects

“…However, a number of potential applications have been highlighted in the literature. These include their use as protective coatings owing to their good corrosion and wear resistance 1,80,84,115,170,171,[219][220][221][222][223][224][225][226][227][228][229][230][231] (see review of laser-deposited HEA coatings by Zhang et al 232 ), as alloys for bulk metallic glasses, 11,[233][234][235][236] and even as materials for hydrogen storage 237 and diffusion barriers. 1,181,182 There has also been interest in their magnetic 1,10,97,[238][239][240][241][242][243] and thermoelectric properties.…”

High-Entropy Alloys: A Current Evaluation of Founding Ideas and Core Effects and Exploring “Nonlinear Alloys”

“…The second foundational concept is to favor SS phases over IM compounds by controlling the configurational entropy in complex alloys. [4][5][6][7][8] A composition-based definition gives HEAs as any alloy with five or more principal elements with atom fractions between 0.05 and 0. 35, and an alternative definition gives an HEA as any alloy with an ideal configurational entropy ‡1.5R, where R is the gas constant.…”