Combined strengthening from nanotwins and nanoprecipitates in an iron-based superalloy

Tailoring nanoprecipitates for ultra-strong high-entropy alloys via machine learning and prestrain aging

This work aims to study the combined effect of resistance spot welding (RSW) and precipitation hardening on tensile shear load bearing capacity (TSLBC) of A286 superalloy. The tensile shear test specimens were welded in the solution treated condition, and then subjected to different precipitation hardening treatments. The impoverishment in Ti, due to its segregation towards the interdendritic region (studied by performing SEM/EDX analysis), causes the development of gamma prime depleted zones in the dendrite core of the weld nugget, which is associated with a decrease of the mechanical properties with respect to the base metal. The segregation of Ti towards the interdendritic region of the weld nugget has also influence on the eta phase formation, which appears later (with longer aging times) in the weld nugget than in the base metal. The aging time at which the maximum experimental TSLBC is reached is, for both aging temperatures, 100 h. The experimental TSLBC of the tensile shear test specimens hardened at lower aging temperature is greater (except for the shortest aging time) than that of those hardened at higher aging temperature.

Combined effect of resistance spot welding and precipitation hardening on tensile shear load bearing capacity of A286 superalloy

The precipitation of η phase and its effects on stress rupture properties of K4750 alloy were investigated. During aging at 850 °C for 50–500 h, thin plate-like η phase were firstly precipitated at grain boundaries (GBs) and around MC carbides, then grew into grain interiors to form a Widmanstadter morphology. The η phase around MC carbides was considered to be the product of MC degradation which was in reaction of MC+ γ→ M23C6+ η. The η phase had an orientation relationship with γ matrix: 0001 η / / 111 γ , 11 2 ‾ 0 > η / / 1 1 ‾ 0 > γ . As the growth of η phase, γ′ phase was absorbed by η phase and led to the formation of γ′ precipitation free zones (PFZs). This was mainly because both γ′ and η phases were enriched in Ti and Ni, and the dissolution of γ′ released Ti and Ni for the formation of η phase. As the aging time prolonged, stacking faults (SFs) were induced in some coarsening γ′ phase. The SFs within γ′ offered sites for the nucleation of η phase, which facilitated the formation of η phase in grain interiors. To clarify the effects of η phase on stress rupture properties, samples were carried out the standard heat treatment (SHT), SHT+850 °C/300 h and SHT+850 °C/300 h + SHT, respectively. And η phase could only be observed in the sample having been subjected to SHT+850 °C/300 h. After above three heat treatments, the stress rupture life of K4750 alloy tested at 750 °C/360 MPa was 506.00 h, 99.55 h and 447.08 h, and the elongation was 13%, 22% and 8.7%, respectively. The change of stress rupture properties was closely related to the formation of η phase and the degradation of MC carbides, which would be analyzed in this paper.

Precipitation of η phase and its effects on stress rupture properties of K4750 alloy

Study of influence of gamma prime and eta phases on corrosion behaviour of A286 superalloy by using electrochemical potentiokinetic techniques

In this research, the mechanism of eta (η-Ni3Ti) phase precipitation in iron-nickel-based A286 superalloy was assessed during aging heat treatment in the temperature range between 650 and 900 °C for the times of 1-30 h. Optical microscopy, scanning electron microscopy, differential thermal analysis, and x-ray diffractometry were used to describe the η phase transformation. The results showed that the major precipitates at temperatures below 840 °C were γ′ and η. The η phase started to precipitate at the expense of the γ′ phase after prolonged aging. The η phase existed in the samples aged at temperature higher than 760 °C with cellular morphology. The η volume fraction increased with increasing heat treatment time. In addition, when the aging temperature was increased from 760 to 820 °C, the η volume fraction increased and then decreased after 840 °C. The η phase morphology also changed from cellular to Widmanstatten-type during aging. The time-temperature-precipitation diagrams of these morphologies are presented. The results indicated the differences in precipitation mechanisms of η phase at 840 and 860 °C.

The Mechanism of η Phase Precipitation in A286 Superalloy During Heat Treatment

In this paper, the shear deformation behavior of A286 Iron-based superalloy was studied with an emphasis on the influence of η phase on shear strength. The η (Ni3Ti) phase precipitates at high temperature heat treatment or during services at the expense of gamma prime phase. According to the microstructural features, no evidences of η phase were found at 650 and 720°C. η phase precipitated at 780 and 840°C and the amount of it increased with an increase the time and temperature. Because of using the alloy as fasteners, investigation of shear properties and the influence of η phase on it are indispensable. The shear strength of the alloy with different volume fractions of η was examined. It was found that, with an increase of η volume fraction, the ultimate shear strength decreases. The shear punch fracture surfaces were also examined by the scanning electron microscopy. The fracture surfaces of sheared samples indicated that low and high volume fraction of η phase result in interior cracks and grain boundary decohesion, respectively. In fact, the fracture of weak grain boundary films (η phase) produces this kind of decohesive cracking.

Effect of η Phase on Mechanical Properties of the Iron-based Superalloy Using Shear Punch Testing

On the Role of Niobium Addition on Volume Fraction of η Phase and Hardness of A286 Iron–Nickel-Based Superalloy

In this study, the effects of aging times and temperatures on the microstructure, hardness and compression strength of Al0.7CoCrFeNi high entropy alloy have been investigated. The alloy was cast in a vacuum induction melting furnace, homogenized at 1250 ˚C for 6 h; then aged at 700 to 1000˚C for 2-8 h. The as-cast structure is dendritic and includes FCC(A1) and BCC(A2, B2) phases with the hardness of 497 HV. During ageing, B2 precipitates at the grain boundaries at 700˚C and the hardness increases about 7%. The ratio of BCC to FCC phases on the basis of XRD in as-cast alloy is approximately equal which is increased by ageing at 700˚C. At 800˚C, the formation of a destructive and hard phase of σ cause to increases the hardness to 543 HV and the ratio of (A2+B2)/A1 has decreased. At 1000 ˚C, the ratio of (A2+ B2)/A1 increases, and the peak intensity of σ decreases, so that the hardness value decreases to 385 HV. The results of hot deformation test showed that the alloy at the strain rate of 10-3 s-1 and temperatures of 800, 900, 1000 and 1100˚C has a yield strength of 306, 179, 91 and 50 MPa, respectively. 

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Masumeh Seifollahi

Papers

The Mechanism of η Phase Precipitation in A286 Superalloy During Heat Treatment

Effect of η Phase on Mechanical Properties of the Iron-based Superalloy Using Shear Punch Testing

On the Role of Niobium Addition on Volume Fraction of η Phase and Hardness of A286 Iron–Nickel-Based Superalloy

Effect of Aging Treatment on Microstructure and Mechanical Properties of Al0.7CoCrFeNi High Entropy Alloy