MC Carbide Decomposition Research Articles

Order phase transition of HIP nickel-based powder superalloy during isothermal aging

Nickel-based powder superalloys have become one of key materials for aerospace turbine engines due to their excellent comprehensive properties. The microstructure stability during long-term isothermal aging is closely related to mechanical properties. The correlation between order phase transition and mechanical properties of a HIP (hot isostatic pressing) nickel-based powder superalloy after long-term isothermal aging treatment was investigated. The results show that the decomposition of MC carbides and precipitation, growth and coarsening of γ' phases occur with duration of isothermal aging. Tertiary γ' phases precipitates by the way of element diffusion with γ' phases coarsening. In addition, strengthening mechanism is quantitatively calculated, and the relative contribution of γ' phases to yield strength is largest, and multi-mode size of γ' phases are favorable to yield strength and hardness of HIP nickel-based powder superalloy.

Athermally induced MC carbide decomposition in gradient M50 bearing steels during electric pulse treatments

Athermally induced MC carbide decomposition in gradient M50 bearing steels during electric pulse treatments

Influence of solution temperature on microstructure and mechanical properties of DZ411 alloys with different Ta compositions

The influence of solution treatment temperature on microstructure and mechanical properties of DZ411 alloys with different Ta compositions (2.72, 3.10, and 4.00 wt.%) is studied in details. With the increase of Ta composition, the average values of the primary dendrite arm spacing (PDAS) in superalloys before heat treatment are found to increase from 145.9 ± 3.7 to 209.8 ± 7.6 μm, the area fractions of carbides increase from 0.59 ± 0.05% to 0.65 ± 0.05%, the area fractions of γ-γ′ eutectics increase from 4.6 ± 0.05% to 8.1 ± 0.05%, and the cubicity of the γ′ phase also increases. After heat treatment, since the γ′ phase shows more superior high temperature performance in alloys with higher Ta composition, the solution temperature of alloys with higher Ta composition should be increased to promote the dissolution of the coarse γ′ phase. The evolution of carbides is mainly divided into two types: the decomposition of MC carbides: MC + γ → M23C6 + γ′ and the precipitation of fine M23C6 carbides. The increase of cubicity of γ′ phase will increase the interfacial energy of two phases, which can lead to the pile-up of dislocation and improve the microhardness of alloys. The fine M23C6 carbides will hinder the initiation and propagation of cracks, and the interaction of fine carbide particles reduces the stress concentration and improve the fracture strength of alloys. Therefore, the addition of Ta increases the mechanical properties including both tensile properties and microhardness of alloys with higher Ta composition increases more obviously after heat treatment at higher temperature.

Material characterization of long-term service-exposed GTD-111 nickel based superalloy

In this study, the influence of long-term service exposure on the microstructure of a GTD-111 precipitation hardened Ni-based superalloy extracted from turbine blades after 55,000 and 75,000 h without any interval rejuvenations is investigated. This alloy consists of multi phases including γ, γ′ (Ni3Al) strengthening precipitates, carbides, and eutectic phases. The long-term operation caused dramatic microstructural degradations, such as decomposition of MC carbides, formation of M23C6 carbides along grain boundaries, grain boundary oxidation and cracking, coarsening of primary γ′, dissolving of secondary γ′, and formation of TCP-type phases which profoundly impacted its failure mechanisms. The results showed that after 75,000 h of service, the grain boundaries which reached the surface were oxidized and even some led to crack initiation due to high thermal cyclic loads in the cooling holes. The obtained results also illustrate that with increasing service exposure times, the size of primary γ′ increased to about 1.5–2 µm and secondary γ′ dissolved. Dissolution and coarsening of γ′, widening of grain boundaries, decomposition of carbides, and phase transformations were analyzed by optical and scanning electron microscope and energy dispersive spectroscopy. Furthermore, the coating on the blade’s surface is investigated microstructurally to understand its severe degradation due to the high temperature and pressure such as microstructural degradation and cracking, and widening of interdiffusion layer.

Microstructure evolution and phase transformation in a nickel-based superalloy with varying Ti/Al ratios: Part 2 – Phase transformation

Nickel-based superalloy Waspaloy is one of the promising candidate materials for power plant components served in higher operation temperatures. Experimental alloys based on Waspaloy with varying Ti/Al ratios were produced and subjected to thermal exposure at 800 °C for up to 10,000 h. The phase transformations, mainly focusing on the η-Ni3Ti phase precipitation behavior and decomposition of two types of MC carbides during the prolonged ageing process, were investigated in this paper. η phases precipitated in the center of extra-large γ’ (EL-γ′), with or without facilitated by the small size MC carbides that were embedded within EL-γ’ as well. There were different decomposition routes for MC carbides with different sizes. Most of the MC carbides with larger size followed the usual transformation to M23C6 carbides, whilst the decomposition of MC carbides with smaller size was related to the nucleation of η phase and/or other phases such as σ phase in some cases. It is found that slightly lowering Ti/Al ratio (above 0.74) could not rule out the precipitation of η phase. The precipitation kinetics of η phase were discussed in terms of the density and stability of small size MC carbides. Furthermore, the effects of Ti/Al ratios on η formation were evaluated.

Service damage mechanism and performance attenuation of nickel-based alloy turbine blades

Multi-scale morphology observations and fatigue performance tests were carried out to study the damage evolution mechanism and fatigue performance attenuation behavior of K403 turbine blades in service. During service in field, the dendrite separation and breakage, γ' phase polymerization and rafting, MC carbide decomposition, harmful phase precipitation and grain boundary weakening are having adverse effects on the fatigue performance of turbine blades. Meanwhile, the loss of alloying elements in matrix causes the alloy matrix to soften. In addition, a large number of pores and microcracks formed in the process of service further deteriorate the service performance of turbine blades. Therefore, after long-term operations, the solid solution strengthening, precipitation strengthening, dispersion strengthening and grain boundary strengthening effects of K403 turbine blades are all weakened, which led to serious degradation of the fatigue performance of these turbine blades with the fatigue life reduced. Besides, the crack initiation source of turbine blade is gradually transformed from subsurface metallic pore initiation to carbide initiation.

Microstructure evolution and stress rupture properties of K4750 alloys with various B contents during long-term aging

The relationship among B content, microstructure evolution and stress rupture properties of K4750 alloy during long-term aging were investigated. After aging at 800 ℃ for 1000 h, the decomposition degree of MC carbides of K4750 alloys with 0B, 0.007 wt. % B and 0.010 wt. % B were basically identical, which indicated that B has no inhibition on MC carbide decomposition during long-term aging. The MC carbide decomposition was accompanied by the formation of M23C6 carbides and a small number of η phases, which was controlled by the outward diffusion of C and Ti combined with the inward diffusion of Ni and Cr from the γ matrix. In addition, M23C6 carbides in boron-free alloy were in continuous chain and needle-like η phases were precipitated near them, while M23C6 carbides in boron-containing alloys remained in granular distribution and no η phases precipitation around them. Adding B could delay the agglomeration and coarsening of M23C6 carbides during long-term aging, which was because the segregation of B at grain boundary retarded the diffusion of alloy elements, thus weakened the local fluctuation of chemical composition near grain boundary. The stress rupture samples of K4750 alloys with various B contents after aging at 800 ℃ for 1000 h were tested at 750 ℃/380 MPa. The results indicated that the stress rupture properties of boron-containing alloys were significantly better than that of boron-free alloy, which could be attributed to the increase of grain boundary cohesion strength and the optimization of M23C6 carbide distribution due to the addition of B.

Effect of Co on Microstructure and Stress Rupture Properties of K4750 Alloy

The effects of substituting Co for Fe on the microstructure and stress rupture properties of K4750 alloy were studied. The microstructure of the alloy without Co (K4750 alloy) and the alloy containing Co (K4750-Co alloy) were analyzed. Substitution of Co for Fe inhibited the decomposition of MC carbide and the precipitation of η phase during long-term aging treatment. In K4750-Co alloy, the morphology of MC carbide at the grain boundary (GB) remained dispersed blocky shape and no η phase was observed after aging at 750 °C for 3000 h. However, in K4750 alloy, almost all the MC carbides at GBs broke down into granular M23C6 carbide and needle-like η phase. The addition of cobalt could delay the decomposition of MC carbides, which accordingly restricted the elemental supply for the formation of η phase. The stress rupture tests were conducted on two alloys at 750 °C/430 MPa. When Co is substituted for Fe in K4750 alloy, the stress rupture life increased from 164.10 to 264.67 h after standard heat treatment. This was mainly attributed to increased concentration of Al, Ti and Nb in γ′ phase in K4750-Co alloy, which further enhanced the strengthening effect of γ′ phase. After aging at 750 °C for 3000 h, substitution of Co for Fe can also cause the stress rupture life at 750 °C/430 MPa to increase from 48.72 to 208.18 h. The reason was mainly because MC carbide degradation and η phase precipitation in K4750 alloy, which promoted the initiation and propagation of micro-crack during stress rupture testing.

Thermodynamic analysis of precipitation behavior of M23C6 carbide in Nimonic 105 superalloy

AbstractThe Time–Temperature–Precipitation diagram ofM23C6carbide formation was established for a Nimonic 105 Ni-based superalloy by means of microstructural observation and mathematical-thermodynamic analysis. The results showed that during heat treatment at 750 °C,M23C6carbide usually forms at grain boundaries due to the decomposition ofMC carbides, indicating that the grain boundaries promoteM23C6carbide precipitation. Also, by increasing temperature or time, diffusion rate increases and new diffusional paths (such as γ matrix and twinning) are provided leading to the nucleation of these phases inside the matrix by means of γ→M23C6+ γ′ reaction. According to thermodynamic analysis, the interactions betweenM23C6formation Gibbs free energy and diffusion activation energy of elements, especially carbon, definesM23C6precipitation behavior. The peak of the Time–Temperature–Precipitation diagram was calculated to be about 900 °C based on experimental results, and about 927 °C based on thermodynamic results. These two temperatures can be considered as approximately equal.

Effects of solutioning and ageing treatments on properties of Inconel-713C nickel-based superalloy under creep loading

As nickel-based superalloys have been widely used in turbine blades of turbo-chargers in automobile industries, this article has presented the creep behavior of the Inconel-713C nickel-based superalloy. For this objective, forced-controlled creep testing has been performed at the temperature of 850°C and under the stress of 585.5MPa. Then, the effect of the solutioning process on creep (time-dependent) properties of the superalloy was investigated. Optical and scanning electron microscopies were utilized to study the material microstructure, before and after creep testing. The X-ray diffraction (XRD) spectrometry was also used to detect different phases in the superalloy. Results showed that solutioning at 1200°C for 1h had a lowering effect on the creep rupture time of the Inconel-713C superalloy, as the mean size of the γ′ particles crystallite was about 6.8nm. When the superalloy was aged for 16h at 930°C, an insignificant effect with respect to the as-cast sample could be observed, due to the precipitation of NbC carbides (at grain boundaries) and the coarsening behavior of γ′ particles. Consequently, different microstructures led to different creep fracture mechanisms for this alloy. The decomposition of MC carbides to M23C6 was also observed for all samples after creep testing.

Small-scale specimen testing for fatigue life assessment of service-exposed industrial gas turbine blades

Service-exposed industrial gas turbine blades made from conventionally cast nickel-base superalloy IN738 were investigated in order to study the effect of service-induced microstructural changes on the tensile and cyclic material properties. Optical microscopy and scanning electron microscopy were used for the microstructural characterization of the gas turbine blades. The microstructural examinations showed a degraded microstructure in the airfoil section, such as γ′ coarsening, MC carbide decomposition as well as M23C6 carbide formation. Uniaxial tensile and isothermal low cycle fatigue tests were carried out at 850°C using small-scale specimens manufactured from the blade root and airfoil section. The service-induced microstructural changes in the airfoil section yielded to a reduction of the tensile properties. Furthermore, a lower fatigue life of airfoil specimens compared to the blade root specimens was observed. Based on the determined fatigue data the residual fatigue life of the gas turbine blades was estimated. Finally, the fatigue crack paths were studied by optical microscopy.

An Investigation of the Microstructural Evolution of a Service-Exposed Turbine Blade

A field-emission scanning electron microscope with an energy-dispersive X-ray spectroscopy system was used to study the microstructural evolution of nickel-based superalloy K465 during service, including γ′ degeneration, decomposition of MC carbides, and formation of creep cavities. Results show that the primary γ′ precipitates having regular and cubic morphologies are agglomerated together by elastic interaction energy and gradually evolve to more stable γ′ plates. The γ′ particles in the interdendritic region have higher coarsening rates than those in the dendrite core. The sizes of γ′ precipitates at various locations of the blade, characterized by quantitative metallography, are employed to estimate the temperature distribution of the airfoil. MC carbides react with the surrounding matrix, forming M6C carbide and γ′ film. The interfaces between the carbides, both at grain boundaries and in grains, and the γ′ films are preferential sites for the nucleation and fast propagation of creep cavities.

Quantitative microstructural evolution and corresponding stress rupture property of K465 superalloy

The damage assessment and life prediction of cast turbine blades are closely related to various microstructural degradation caused by thermal exposure at service temperatures. However, limited systematic investigations about quantitative characterization of microstructural evolution in superalloys were reported. In this paper, cast superalloy K465 was thermal exposed from 900°C to 1050°C for 100h to 1500h, then the microstructural evolution were characterized and the stress rupture tests were conducted under 975°C/225MPa. The experimental results indicated that γ′ precipitates showed high microstructural stability at 900°C and below, while the decomposition of MC carbides into M6C carbides proceeded gradually in the temperature range from 900°C to 1210°C. Moreover, blocky M6C and M23C6 carbides precipitated in the interdendritic region and along grain boundary in different temperature ranges. The plate-like phases precipitated in the temperature range from 900°C to 950°C and 1000°C to 1050°C were identified as μ phase and M6C carbides, respectively. The amount of μ phase was much higher than that of M6C carbides. The precipitation of μ phase enriched in W and Cr were mainly attributed to the decrease of stress rupture lives for K465 superalloy after thermal exposure in the temperature range from 900°C to 950°C, compared with those after thermal exposure in the temperature range from 1000°C to 1050°C, which contained even lower γ′ volume fraction with more degradation. This study is helpful to better understand the various microstructural evolution at different temperatures and to optimize the design and assessment of service induced degradation of turbine blades made of K465 superalloy.

Microstructure evolution during heat treatment of superalloys loaded with different amounts of carbon

Abstract

Microstructural evolution of a carbon modified single crystal Ni-base superalloy

The effects of heat treatments and long term exposures of C-containing modifications of CMSX-4 were examined. Some of the alloys examined also contained additions of boron or nitrogen. Significant changes to as-cast carbide morphologies were observed. Long-term exposure caused significant MC carbide decomposition and formation of Cr-rich secondary carbides on or near to decomposing carbides. Increased topologically close-packed phase formation occurred in some cases, while, in other alloys no topologically close-packed phases were observed. All three modifications experienced more γ′ coarsening than baseline CMSX-4.

MC Carbide Decomposition during Thermal Exposure of Polycrystalline Ni-Base Superalloys

MC decompositions during thermal exposure have been investigated in three conventional cast Ni-base superalloys, GTD111, IN738LC, and CM247LC. MC decomposition in GTD111 and IN738LC depends on exposure temperature. While MC decomposed into M23C6 at 982°C, η formed from MC after exposure at 927°C and 871°C. Ta and Ti separated from MC during thermal exposure made η phase to form instead of γ'. The decomposition of the MC in CM247LC depends on their morphology and position. Segregation during casting process affected the morphology and composition of MC type carbide in this alloy. Acicular M6C was found near scriptal MC in the dendrite. Blocky MC near γ-γ' eutectic decomposed into M23C6. Both M23C6 and M6C can be found on grain boundary after thermal exposure in CM247LC.

A Comparison of Aging Kinetics of New and Rejuvenated Conventionally Cast GTD-111 Gas Turbine Blades

Two industrial gas turbine blades made from a conventionally cast Ni-base superalloy GTD-111, one new and the other rejuvenated, were removed from the same machine after a particular operational cycle for an examination in order to determine the effect of rejuvenation on the material’s behavior during service. It was found that service-induced changes in the microstructure, such as γ′-phase coarsening and coalescence, excessive grain-boundary secondary M23C6 carbides formation, and primary MC carbides decomposition, were noticeably more advanced in the rejuvenated blade. The stress-rupture life of the rejuvenated blade decreased significantly compared to that of the new blade after the same number of hours in service. The cause of this decrease appears to be related to a release of additional amounts of carbon and carbide-forming elements into the matrix during rejuvenating heat treatment as a result of the primary MC carbide decomposition.

The effects of tantalum on the microstructure of two polycrystalline nickel-base superalloys: B-1900 + Hf and MAR-M247

Changes in the γ/γ'/carbide microstructure as a function of Ta content were studied in conventionally cast B-1900 + Hf and both conventionally cast and directionally solidified MAR-M247.* The effects of tantalum on the microstructure were found to be similar in both nickel-base superalloys. In particular, the γ' and carbide volume fractions increased approximately linearly with tantalum additions in both alloys. The γ' phase compositions did not change as tantalum additions were made with the exception of an increase in the tantalum level. Bulk tantalum additions increased the tantalum, chromium, and cobalt levels of the γ phase in both alloy series. The increase in the concentrations of the latter two elements was attributed to a decrease in the γ phase fraction with increasing bulk tantalum level and nearly constant γ' /γ partitioning ratios. It was demonstrated that the large increase in the γ ' volume fraction was a result of tantalum not affecting the partitioning ratios of the other alloying elements. The addition of tantalum led to a partial replacement of the hafnium in the MC carbides, although the degree of replacement was reduced by the solutionizing and aging heat treat-ment. In addition, chromium-rich M23C6 carbides formed as a result of MC carbide decomposition during heat treatment.