Design of New Face-Centered Cubic High Entropy Alloys by Thermodynamic Calculation

A new face-centered cubic (fcc) high entropy alloy system with non-equiatomic compositions has been designed by utilizing a CALculation of PHAse Diagram (CALPHAD) - type thermodynamic calculation technique. The new alloy system is based on the representative fcc high entropy alloy, the Cantor alloy which is an equiatomic Co- Cr-Fe-Mn-Ni five-component alloy, but fully or partly replace the cobalt by vanadium and is of non-equiatomic compositions. Alloy compositions expected to have an fcc single-phase structure between 700 A degrees C and melting temperatures are proposed. All the proposed alloys are experimentally confirmed to have the fcc single-phase during materials processes (> 800 A degrees C), through an X-ray diffraction analysis. It is shown that there are more chances to find fcc single-phase high entropy alloys if paying attention to non-equiatomic composition regions and that the CALPHAD thermodynamic calculation can be an efficient tool for it. An alloy design technique based on thermodynamic calculation is demonstrated and the applicability and limitation of the approach as a design tool for high entropy alloys is discussed.11sciescopuskc

Design of Cost-Effective Fe-Based Amorphous Coating Alloys Having High Amorphous Forming Ability by Thermodynamic Calculation

In this study, new cost-effective Fe-based amorphous coating alloys having high amorphous forming ability were developed by varying the Fe content, while their microstructure, hardness, and corrosion resistance were also evaluated. Chemical compositions that have the lowest driving force of formation of crystalline phases such as Fe3P, Fe3C, and alpha-Fe were obtained from thermodynamically calculated phase diagrams of the representative FexAl2(P10.83C7.47B1.7)(98-x) alloy system at a crystallization temperature of 443 A degrees C. Considering the intersections of driving force curves of Fe3P and Fe3C, Fe3P and alpha-Fe, and Fe3C and alpha-Fe, the Fe contents were found to be 77.8, 76.2, and 75.8 at.%, respectively. The microstructural analysis results of 1.5-mm-diameter suction-cast Fe-based alloys indicated that the Fe76.5Al2(P10.83C7.47B1.7)(21.5) alloy had a fully amorphous microstructure, whereas crystalline phases were formed in other alloys. This alloy showed a better hardness and corrosion resistance than conventional thermal spray coating alloys, and its production cost could also be reduced by using less expensive alloying elements, which could provide a good way to practically apply this alloy to Fe-based amorphous thermal spray coatings.11sciescopuskc

Effects of Phosphorus and Carbon Contents on Amorphous Forming Ability in Fe-based Amorphous Alloys Used for Thermal Spray Coatings

Cost-effective Fe-based amorphous alloys used for thermal spray coatings were developed by varying contents of P and C, and their microstructure, hardness, and corrosion resistance were analyzed. In order to achieve chemical compositions having high amorphous forming ability, thermodynamically calculated phase diagrams of Fe-Al-P-C-B five-component system were used, from which compositions of super-cooled liquid having the lowest driving force of formation of crystalline phases were obtained. The thermodynamic calculation results showed that only phases of Fe3P and Fe3C were formed in the Fe78Al2P(18.3-x)C (x) B-1.7 alloy system. Considering driving force curves of Fe3P and Fe3C, the carbon contents were selected to be 6.90 and 7.47 at. pct, when the thermodynamic calculation temperatures were 697 K (414 A degrees C) and 715 K (442 A degrees C), respectively. According to the microstructural analysis of suction-cast alloys, the Fe78Al2P10.83C7.47B1.7 alloy showed a fully amorphous microstructure, whereas the Fe78Al2P11.40C6.9B1.7 and Fe78Al2P10.3C8.0B1.7 alloys contained Fe3P and Fe3C phases. This Fe78Al2P10.83C7.47B1.7 alloy showed the better hardness and corrosion resistance than those of conventional thermal spray coating alloys, and its production cost could be lowered using cheaper alloying elements, thereby leading to the practical application to amorphous thermal spray coatings. DOI: 10.1007/s11661-013-1615-0 (C) The Minerals, Metals & Materials Society and ASM International 201311sciescopu

Interpretation of High-Temperature Tensile Properties by Thermodynamically Calculated Equilibrium Phase Diagrams of Heat-Resistant Austenitic Cast Steels

High-temperature tensile properties of three heat-resistant austenitic cast steels fabricated by varying W, Mo, and Al contents were interpreted by thermodynamically calculated equilibrium phase diagrams of austenite, ferrite, and carbides as well as microstructural analyses. A two-step calculation method was adopted to cast steel microstructures below the liquid dissolution temperature because the casting route was not an equilibrium state. Thermodynamically calculated fractions of equilibrium phases were well matched with experimentally measured fractions. Ferrites existed at room and high temperatures in both equilibrium phase diagrams and actual microstructures, which has not been reported in previous researches on austenitic cast steels. In the W2Mo1Al1 steel, 38% and 12% of ferrite existed in the equilibrium phase diagram and actual microstructure, respectively, and led to the void initiation and coalescence at ferrites and consequently to the serious deterioration of high-temperature strengths. The present equilibrium phase diagrams, besides detailed microstructural analyses, effectively evaluated the high-temperature performance by estimating high-temperature equilibrium phases, and provided an important idea on whether ferrite were formed or not in the heat-resistant austenitic cast steels.11sciescopuskc

Effects of tungsten and molybdenum on high-temperature tensile properties of five heat-resistant austenitic stainless steels

Heat-resistant austenitic stainless steels have been intensively used worldwide for turbo-chargers requiring excellent high-temperature properties to sustain their structures at very high exhaust gas temperatures. Five heat-resistant austenitic stainless steels were fabricated by varying contents of W and Mo, and their high-temperature tensile properties were investigated by analyzing phases (liquid, austenite, ferrite, and carbides) existing at high temperatures. Effects of replacement of expensive alloying element, W, by Mo or reduction in W were also examined. The increase in contents of W and Mo resulted in linearly increased volume fractions of M7C3 and total carbides, while the volume fraction of MC carbide hardly showed any relation, and showed a good correspondence with high-temperature yield and tensile strengths. The steel where 2 wt% of W was replaced by 2 wt% of Mo showed the best 900 degrees C-tensile properties, thereby confirming the successful achievement of partly replacement of W by Mo. In addition, the low-W-containing steels showed excellent 900 degrees C-tensile properties, which also indicated that the reduction in 1-2 wt% of W was accepted for saving costs of alloying elements. (C) 2016 Elsevier B.V. All rights reserved.11sciescopu

Interpretation of high-temperature tensile properties by thermodynamically calculated equilibrium phase diagrams of heat-resistant austenitic cast steels

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Effects of Mn and Mo addition on high-temperature tensile properties in high-Ni-containing austenitic cast steels used for turbo-charger application

Since turbo-chargers require more excellent high-temperature properties to maintain their structures at further higher exhaust gas temperatures up to 1050 degrees C, a 20 wt%-Ni-containing austenitic cast steel (N20 steel) has been suggested as a promising candidate cast steel. However, this steel is very expensive because it contains a large amount of expensive Ni. In order to partly replace expensive Ni by inexpensive Mn and to improve high temperature tensile properties in the N20 steel, three austenitic cast steels were fabricated by replacing 6 wt% of Ni by 6.9 wt% of Mn or by adding 2-4 wt% of Mo. Thermodynamically calculated fractions of equilibrium phases (austenite, ferrite, and M7C3 carbide) were matched with experimentally measured fractions, although they were somewhat overestimated. The N14 steel where 6 wt% Ni was replaced by 6.9 wt% of Mn did not contain any ferrite, and showed comparable or more excellent high-temperature tensile properties than those of the N20 steel, which indicated the successful replacement up to 6 wt% Ni by Mn, together with alloying cost reduction of 10%. The Mo addition also favorably affected high-temperature properties because Mo worked for increasing both M7C3 fraction and austenite matrix hardness. Simultaneously considering mechanical properties and alloying costs, therefore, these Mo-containing N14 steels can be fully adopted for high-performance turbo-chargers requiring excellent high-temperature properties.11sciescopu

Effects of Mn and Mo addition on high-temperature tensile properties in high-Ni-containing austenitic cast steels used for turbo-charger application

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Effects of Cr and Nb addition on high-temperature tensile properties in austenitic cast steels used for turbo-charger application

In automotive turbo-chargers, excellent high-temperature properties are needed for retaining their structures at extremely high temperature of exhausted gas, and thus austenitic cast steels have been actively developed. In this study, high-temperature tensile properties of austenitic cast steels containing different W, Nb, and Cr were interpreted by deformation and fracture mechanisms related with carbides and austenite matrix. Replacement effects of W by Nb or Cr were also investigated because of expensive alloying cost of W. The high-temperature tensile test results revealed that the Cr-added steel containing many M7C3 carbides showed the higher strength than the Nb-added steel containing hard MC carbides. This was because the strength could be enhanced by distributing carbides having slightly lower hardness such as M7C3 carbides in the strengthened matrix as it could hold hard carbides, like in the Cr-added steel. The Cr-added steel (1 wt% W) also showed the higher strength and elongation at 900 degrees C than the basic-composition steel (2 wt% W), which indicated that the partly replacement of W by Cr was successfully achieved. (C) 2016 Elsevier B.V. All rights reserved.11Nsciescopu

Replacement of Ni by Mn in High-Ni-Containing Austenitic Cast Steels used for Turbo-Charger Application

High-temperature tensile properties of austenitic cast steels fabricated by replacing Ni by Mn in a 20 wt pct Ni-containing steel were investigated. In a steel where 8 wt pct Ni was replaced by 9.2 wt pct of Mn, 17.4 and 9.8 pct of ferrite existed in equilibrium phase diagrams and actual microstructures, respectively, because a role of Mn as an austenite stabilizer decreased, and led to deterioration of high-temperature properties. When 2 to 6 wt pct Ni was replaced by 2.3 to 6.9 wt pct Mn, high-temperature properties were comparable to those of the 20 wt pct Ni-containing steel because ferrites were absent, which indicated the successful replacement of 6 wt pct Ni by Mn, with cost reduction of 27 pct.11sciescopu