Author
G. Appa Rao
Bio: G. Appa Rao is an academic researcher from Defence Metallurgical Research Laboratory. The author has contributed to research in topics: Superalloy & Hot isostatic pressing. The author has an hindex of 8, co-authored 10 publications receiving 517 citations.
Papers
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TL;DR: In this article, the role of strain rate on the microstructural evolution during hot deformation of a hot isostatically processed nickel base superalloy was analyzed using EBSD analysis.
126 citations
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05 Nov 2006-Materials Science and Engineering A-structural Materials Properties Microstructure and Processing
TL;DR: In this paper, the authors show that the formation of deleterious phases such as prior particle boundaries (PPBs) occur more predominantly in the HIPed material produced from the powder with high oxygen content.
Abstract: Inert gas atomized (IGA) superalloy Inconel 718 powders with varied oxygen levels of 275, 180 and 140 ppm were consolidated by hot isostatic pressing (HIPing) at 1200 °C/120 MPa/3 h. The microstructural characterization of as-HIPed alloys has shown that the densification phenomenon of the powder does not depend on its oxygen content. However, the formation of deleterious phases such as prior particle boundaries (PPBs) occur more predominantly in the HIPed material produced from the powder with high oxygen content. It was also observed that the recrystallization and formation of annealing twins in the HIPed alloy are greatly influenced by the oxygen content. Transmission electron microscopy (TEM) studies have revealed that the precipitation of γ″, γ′ and δ phases does not depend on oxygen level of HIPed alloy during heat treatment as per AMS 5662J standard schedule but the MC carbides enriched with Nb and Ti precipitated preferentially at the PPBs for the alloy with high oxygen content of 275 ppm. In contrast, the carbides were found to be precipitated more uniformly in the matrix of the alloy with low oxygen content of 140 ppm. Tensile properties of the as-HIPed and HIP + heat treated alloys have shown that the yield strength (YS), ultimate tensile strength (UTS) do not get influenced by the oxygen content, but the ductility was found to be deteriorated drastically at elevated temperatures with increasing the oxygen content of the alloy. Stress rupture properties of the heat treated alloys at 650 °C under a stress level of 690 MPa have maintained a direct relation with the oxygen content, as the alloy with 275 ppm of oxygen content has shown inferior rupture life of 27 h with 2.1% ductility as compared to 84.5 h of life with 4.5% ductility and 116 h of life with 6% ductility offered by the alloys with 180 and 140 ppm of oxygen content, respectively. A better combination of mechanical properties achieved by use of prealloyed powder with low oxygen content makes it possible to explore the near net shape advantage of HIP technology to its maximum potential for alloy 718 components.
108 citations
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TL;DR: In this article, the consolidation behavior and mechanical properties of nanocrystalline equiatomic AlFeTiCrZnCu high entropy alloy after mechanical alloying were investigated by cold pressing with conventional sintering, vacuum hot pressing and hot isostatic pressing techniques.
Abstract: The present study is aimed to investigate the consolidation behaviour and mechanical properties of nanocrystalline equiatomic AlFeTiCrZnCu high entropy alloy after mechanical alloying. The consolidation was achieved by cold pressing with conventional sintering, vacuum hot pressing and hot isostatic pressing techniques. The microstructure and mechanical properties were evaluated. The hardness and compressive strength of nanocrystalline equiatomic AlFeTiCrZnCu high entropy alloy after vacuum hot pressing are 9.50 and 2.19 GPa and those after hot isostatic pressing are 10.04 and 2.83 GPa, respectively. The wear resistance is found to be higher than the commercially used materials such as Ni-hard faced alloy.
99 citations
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25 Feb 2006-Materials Science and Engineering A-structural Materials Properties Microstructure and Processing
TL;DR: In this article, the HIPed superalloy Inconel 718 powder was consolidated by hot isostatic pressing (HIPing) at 1200°C under 120 MPa pressure for 3h.
Abstract: Inert gas atomized (IGA) superalloy Inconel 718 powder was consolidated by hot isostatic pressing (HIPing) at 1200 °C under 120 MPa pressure for 3 h. The HIPed alloy heat treated as per the aerospace materials specification (AMS) 5662J standard schedule, viz. solution treatment (ST) at 980 °C for 1 h/water quenching (WQ) to room temperature (RT) and a two-step ageing treatment (AT) at 720 °C for 8 h/furnace cooling (FC) at 55 °C h −1 to 620 °C and holding at 620 °C for 8 h and air cooling (AC) to room temperature has exhibited the yield strength (YS) and ultimate tensile strength (UTS) comparable to that of the conventionally processed (forged and heat treated) IN 718. However, its ductility and stress rupture properties at 650 °C were found to be poor due to the presence of prior particle boundary (PPB) networks decorated with highly stable oxides (Al 2 O 3 and TiO 2 ) and brittle MC (Nb, Ti)C carbides. To mitigate this problem, the HIPed alloy was subjected to solution treatment at 1270 °C for 1 h followed by re-HIPing at 1100 °C/130 MPa/3 h before heat-treating it as per AMS 5662J standard schedule. Optical and scanning electron microscopy (SEM) of the alloy processed under modified HIPing and heat treatment conditions have shown the dissolution of MC-carbides, breaking up of PPB networks and formation of equiaxed grains with an average diameter of about 50 μm. Transmission electron microscopy (TEM) of this alloy has revealed uniform distribution of γ″ and γ′ strengthening precipitates in the γ-matrix and the presence of δ(Ni 3 Nb) phase as well as very fine oxide particles near the grain boundaries. The tensile properties of the alloy processed under modified conditions have shown quite satisfactory levels of YS and UTS combined with a significantly improved elongation (EL) values at room temperature (19.5%) and at 650 °C (8.0%). The improvement in alloy ductility was found to correlate well with the fractography of the tensile tested specimens, which showed the predominance of transgranular fracture with fine dimples at room temperature and fine dimples together with the particle boundary facets at 650 °C. The stress rupture properties of modified processed alloy at 650 °C and at a stress level of 690 MPa have shown a vastly improved rupture life of 80 h combined with 5% ductility. The improvement in tensile and stress rupture properties accomplished by the modified processing makes it possible to explore the near net shape capability of HIP technology to its full potential in the development of alloy 718 components.
91 citations
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TL;DR: In this article, the effect of deformation temperature on the work hardening behavior of hot isostatically processed (HIPed) experimental nickel superalloy was investigated by carrying out isothermal constant true strain rate compression experiments over wide temperature ranges (1000°C, 1050 °C, 1100 °C and 1150 °C).
82 citations
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TL;DR: In this article, a review of critical aspects of high-entropy alloys, including core effects, phases and crystal structures, mechanical properties, high-temperature properties, structural stabilities, and corrosion behaviors are discussed.
Abstract: High-entropy alloys (HEAs) are alloys with five or more principal elements. Due to the distinct design concept, these alloys often exhibit unusual properties. Thus, there has been significant interest in these materials, leading to an emerging yet exciting new field. This paper briefly reviews some critical aspects of HEAs, including core effects, phases and crystal structures, mechanical properties, high-temperature properties, structural stabilities, and corrosion behaviors. Current challenges and important future directions are also pointed out.
2,005 citations
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TL;DR: It is shown that intermetallic phases are consistent with HEA definitions, and the strategy developed here includes both single-phase, solid solution HEAs and HEAs with intentional addition of a 2nd phase for particulate hardening.
Abstract: We develop a strategy to design and evaluate high-entropy alloys (HEAs) for structural use in the transportation and energy industries. We give HEA goal properties for low (≤150 °C), medium (≤450 °C) and high (≥1,100 °C) use temperatures. A systematic design approach uses palettes of elements chosen to meet target properties of each HEA family and gives methods to build HEAs from these palettes. We show that intermetallic phases are consistent with HEA definitions, and the strategy developed here includes both single-phase, solid solution HEAs and HEAs with intentional addition of a 2nd phase for particulate hardening. A thermodynamic estimate of the effectiveness of configurational entropy to suppress or delay compound formation is given. A 3-stage approach is given to systematically screen and evaluate a vast number of HEAs by integrating high-throughput computations and experiments. CALPHAD methods are used to predict phase equilibria, and high-throughput experiments on materials libraries with controlled composition and microstructure gradients are suggested. Much of this evaluation can be done now, but key components (materials libraries with microstructure gradients and high-throughput tensile testing) are currently missing. Suggestions for future HEA efforts are given.
651 citations
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TL;DR: In this paper, the starting material, manufacturing processes, heat treatment and characterization procedures of mechanical properties are presented, and the microstructure is crucial for the mechanical properties of IN718.
602 citations
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TL;DR: In this article, the aging characteristics of Inconel 718 have been studied and the effect of the particles on the deformation mechanism and particularly on the softening mechanism of softening mechanisms.
352 citations
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15 Apr 2008-Materials Science and Engineering A-structural Materials Properties Microstructure and Processing
TL;DR: Inconel™ 718 (IN718) has been deposited using laser rapid forming (LRF) from the gas atomized (GA) and plasma rotation electrode preparation (PREP) powders, and the mechanical properties of LRF IN718 were evaluated and compared in between as-deposited and heat-treated state as discussed by the authors.
Abstract: Inconel™ 718 (IN718) has been deposited using laser rapid forming (LRF) from the gas atomized (GA) and plasma rotation electrode preparation (PREP) powders. The mechanical properties of LRF IN718 were evaluated and compared in between as-deposited and heat-treated state. The results show that the existence of the porosities in as-deposited samples, caused by the hollow particles in the GA powders, results in the low ductility and stress rupture properties for LRF GA IN718, since it will promote the occurrence of the micro-porous coalescence failure in the tensile samples. However, the ultimate tensile strength for heat-treated LRF GA IN718 is comparable to that of the wrought IN718, which is 1.5 times of that of the as-deposited samples. It is found that there exists a continuous thin film of Nb-rich MC carbides along the grain boundaries on the fracture surface of the stress rupture samples, which makes cracks initiate and propagate along this path easily, which also results in the poor stress rupture life for LRF GA IN718. The porosities and microcracks in LRF sample were successfully eliminated by using PREP powders, which leads to a substantial improvement in both tensile and stress rupture properties of LRF IN718.
325 citations