Long-term Microstructural Stability Research Articles

Principles of self-annealing in silver processed by equal-channel angular pressing: The significance of a very low stacking fault energy

Experiments were conducted to evaluate the long-term microstructural stability of silver after processing using equal-channel angular pressing (ECAP). The results show that an ultrafine-grained microstructure is produced by ECAP at room temperature but there is self-annealing in the form of recovery and recrystallization during long-term storage at room temperature. In practice, the very low stacking fault energy of silver results in a high degree of dislocation dissociation and thereby hinders recovery by cross-slip and climb. The experiments examine the evolution of microstructure and the mechanical behavior as a function of the storage time after different numbers of ECAP passes. The results demonstrate that the degree and kinetics of self-annealing depend upon the number of passes imposed in ECAP.

TEM analysis of Cr–Mo–W–B phase in a DS nickel based superalloy

Rene 80 is a precipitation hardened nickel-based superalloy, which is widely used to manufacture components of gas turbine engines for aerospace and power generation applications. It is designed to withstand service temperatures of up to 980 C [1] and possesses remarkable hot corrosion resistance, long-term microstructural stability, and hightemperature strength produced primarily by precipitation hardening by ordered intermetallic Ni3(Al,Ti) c0 precipitates. Directional solidification (DS) processing of the alloy results in further improvement of its elevated temperature properties and permits the use of a higher service temperature, which improves the thermal efficiency of gas turbines [2–5]. It has been reported that in DS Rene 80, the tensile strength is increased by about 10–15% and the creep rupture life is extended by 2–4 times relative to conventionally cast material [5]. Boron is an essential minor elemental addition to these heat resistant materials primarily to improve their creep rupture properties [3, 4]. Nevertheless, it has been recognized that the mode in which boron exists on grain boundary regions, either in austenitic solid solution form or selectively partitioned into second phase particles, can significantly influence the creep properties of superalloys [6, 7]. Experimental study is crucial in establishing the presence and nature of intergranular borides in multi-component superalloys due to the influence of interfacial elemental segregation that is not generally considered during thermodynamic equilibrium calculations of theoretical models [8]. The objective of this investigation was to perform transmission electron microscopy (TEM) study of heat-treated DS Rene 80 superalloy to understand the nature of boron rich second phase particles present along its grain boundary regions. The composition of the DS Rene 80 material used in this study was (wt%) 0.2C, 14.1Cr, 9.52Co, 4.0W, 3.98Mo, 0.02Nb, 0.18Fe, 2.9Al, 4.98Ti, 0.028Zr, 0.013B balance nickel. The alloy was subjected to standard solution heat treatment for 2 h at 1200 C and was subsequently aged at 1025 C for 16 h. Thin foils from the aged material for TEM study were prepared by mechanical polishing and dimpling to a thickness of less than 10 lm followed by ion milling in a Gatan precision ion polishing system (PIPS). TEM microchemical and crystal-structure analyses were performed in a field emission gun JEOL 2100F scanning transmission electron microscope equipped with Oxford energy dispersive spectrometer (EDS), Gatan imaging filter (GIF) system, and electron energy-loss spectrometer (EELS). High-resolution TEM micrographs were filtered using an inverse fourier fast transform (FFT) program in Gatan Digital Micrography software. A TEM boron post-edge electron energy loss image of the heat-treated alloy consisting of c0 precipitates and particles of a different phase along the grain boundary and inside the grain identified with arrows is shown in Fig. 1a. Microchemical analysis by X-ray energy dispersive spectroscopy indicated that the particles are rich in Cr, Mo, W, and B (Fig. 1b), and have much different composition from the adjoining regions (Fig. 1c). The presence of boron in the particles was further confirmed by EELS analysis (Fig. 1d) and EELS-based energy filtered transmission electron microscopy (EFTEM) elemental mapping (Fig. 1e). TEM Electron diffraction pattern (EDP) analysis was performed by systematic titling of particle of this phase along certain crystallographic directions to determine its crystal structure. Figure 2a is a [001] zone-axis EDP showing a fourfold symmetry of the phase. Figure 2b and c were obtained by H. R. Zhang O. A. Ojo (&) Department of Mechanical and Manufacturing Engineering, University of Manitoba, Winnipeg, MB, Canada R3T 5V6 e-mail: ojo@cc.umanitoba.ca

Microstructure and long-term creep properties of 9–12% Cr steels

Advanced microstructure characterisation and microstructure modelling has demonstrated that long-term microstructure stability in 9–12% Cr steels under technical loading conditions is equivalent to precipitate stability. Mo and W can have a positive influence on long-term creep strength of 9–12% Cr steels by Laves phase precipitation hardening. Unexpected breakdown of long-term creep stability of a number of alloys is caused by precipitation of the complex Z-phase nitride, which may completely dissolve fine V and Nb containing MX nitrides. High Cr contents of 10% and above in the steels accelerate Z-phase precipitation.

Aging effects on microstructural and mechanical properties of select refractory metal alloys for space-reactor applications

Refractory alloys based on niobium, tantalum and molybdenum are potential candidate materials for structural applications in proposed space nuclear reactors. Long-term microstructural stability is a requirement of these materials for their use in this type of creep dominated application. Early work on refractory metal alloys has shown aging embrittlement occurring for some niobium and tantalum-base alloys at temperatures near 40% of their melting temperatures in either the base metal or in weldments. Other work has suggested microstructural instabilities during long-term creep testing, leading to decreased creep performance. This paper examines the effect of aging 1100h at 1098, 1248 and 1398K on the microstructural and mechanical properties of two niobium (Nb–1Zr and FS-85), tantalum (T-111 and ASTAR-811C) and molybdenum (Mo–41Re and Mo–47.5Re) base alloys. Changes in material properties are examined through mechanical tensile testing coupled with electrical resistivity changes and microstructural examination through optical and electron microscopy analysis.

TO DEVELOP NANOSTRUCTURED THERMAL BARRIER COATINGS

Advanced turbine engines require the application of thermal barrier coatings (TBCs) to provide still higher reliability, thermal insulation effect and longer lifetime under harsh operating conditions. TBCs with nanostructure proved to be promising to deliver the desired property and performance. To exploit full potentials of the current widely used yttria-partially-stabilized zirconia (YSZ), nano-sized YSZ powders were developed and used as the ceramic source material. By controlling the deposition processes, novel TBCs with outstanding nanostructure such as nano-sized grains and pores were produced by atmospheric plasma spray (APS) and electron beam physical vapor deposition (EB-PVD), respectively. The incorporated nanostructure in TBCs resulted in substantial increase in thermal barrier effect and their lifetime. The long-term microstructure stability of the nanocoating was also investigated.

Ｃｏを含まないＮｉ基単結晶超合金のクリープ強度

The Pebble Bed Modular Reactor (PBMR) is a small atomic power reactor which can be used in areas where the electric power transmission network is not well developed. It generates electricity by gas-turbines that used He-gas as a coolant. Ni-based superalloys are the primary materials used in gas-turbines. However if conventional Ni-base superalloys are to be used for PBMR, turbine maintenances will be difficult due to Co, as it can produce 60Co a recitative material that possesses a long half-life. Therefore, Co-free Ni-base superalloys are required for PBMR. In this paper, a Co-free single-crystal (SC) superalloy, TMS-82+(-Co), was developed; it was the same chemical composition as the 2nd generation SC superalloy, TMS-82+, except that it is Co-free creep tests were performed on the developed alloy, TMS-82+, under three conditions; 900°C/392 MPa, 1000°C/245 MPa, and 1100°C/137 MPa; CMSX-4, a popular SC alloy, was used as comparison. TMS-82+(-Co) displayed superior creep-rupture life in all creep tests; however, detrimental TCP were observed in the microstructure of the sample that underwent creep testing at 1100°C/137 MPa. This, the developed Co-free SC alloy should be further modified for long-term microstructural stability. Elimination of phase formation will allow the alloy to be utilized in the PBMR.

Precipitate Stability in Creep Resistant Ferritic Steels-Experimental Investigations and Modelling.

Predictions of long-term microstructure stability of creep resistant ferritic 9-12% Cr steels up to 200 000-300 000 h at temperatures up to 600-650°C are highly interesting for safe power plant operation. At technically interesting creep conditions the microstructure stability is mainly controlled by the stability of precipitate particles. Predictions of precipitate stability have to rely on i) Microstructure characterisation methods to measure volume fractions and mean particle sizes of individual precipitate types, and ii) Microstructure models to predict the evolution of precipitate volume fractions and mean sizes as functions of temperature, time and applied stress. Characterisation methods, which allow on-line particle type discrimination in 9-12 % Cr steels include energy filtered transmission electron microscopy (EFTEM) and scanning electron microscopy (SEM) with atomic number contrast. Modelling of precipitate stability based on thermodynamic equilibrium calculations and multicomponent diffusion databases is demonstrated. A multi-component coarsening model gives accurate predictions of coarsening rates for MX and Laves phase precipitates in steel P92 with fit values for the interfacial energy in the expected range. For M23C6 carbides in steel P92 the model results in unexpectedly low apparent values for the interfacial energy. Modelling of published data for steel P91 indicate much higher coarsening rates for M23C6 carbides, and the fit value for the interfacial energy is as expected. A possible explanation for the low apparent value of the interfacial energy for M23C6 carbides in steel P92 is the content of boron in the steel.

Effect of Trace Elements on Creep Properties of 0.06C-2.25Cr-1.6W-0.1Mo-0.25V-0.05Nb Steel.

The effect of trace elements such as Mn and B on creep properties of 0.06C-2.25Cr-1.6W-0.1Mo-0.25V-0.05Nb has been investigated from the standpoints of a long-term microstructural stability. The chemical analysis of extracted residues and TEM observation show that the M23C6 carbide and/or M7C3 carbides, precipitated by tempering are replaced by M6C with a concentration of W; i.e., the amount of W in solution reduces during creep. On the other hand, MX type carbides such as VC and NbC are very stable during long-term aging and contribute to the creep strength by obstructing the dislocation annihilation. One of the most significant results is that a reduction in Mn-content lowers the minimum creep rate, resulting in an increase in the creep rupture time. The nucleation and/or growth of M6C are retarded with reducing Mn-content, thereby the increase in dissolved W seems to enhance the resistance to creep deformation. Another significant result is that an increase in B-content has delayed the transition from the primary creep to a tertiary creep stage. The principal roles of B are stabilizing M23C6 or M23(C, B)6 on former austenite grain boundaries and retarding the dynamic recrystallization during creep. In addition, in the specimens with higher amount of B, the bainite lath interface is covered by MX and some filmy precipitates with high density, thereby the softening resistance is enhanced.

12Cr鋼の長時間クリープ強度特性と組織安定性

12Cr鋼の長時間クリープ強度特性と組織安定性