Secondary recrystallisation of MA 957 oxide dispersion strengthened ferritic superalloy

Abstract

Secondary recrystallisation has been studied in MA 957 oxide dispersion strengthened ferritic superalloy following heat treatment at a temperature of ~1320°C. Selected area channelling patterns in the scanning electron microscope were used to characterise the crystallographic texture of grains of large aspect ratio produced by zone annealing and microbeam electron diffraction was used to determine the orientations of submicrometre, equiaxed grains behind the (secondary) recrystallisation front. MA 957 exhibits a dominant 〈110〉 texture in the as extruded condition, but this texture is notably absent after recrystallisation. At high zoning speeds the material displays a 〈113〉 texture, but an increasing proportion of 〈111〉 texture component is evident as the zoning speed is reduced. The texture development can be tentatively rationalised in terms of the influence of solute segregation processes and grain orientation on interfacial mobility.

MST/1555     

Transient liquid phase diffusion bonding of oxide dispersion strengthened nickel alloy MA758

Abstract

Transient liquid phase diffusion bonding has been used to join an oxide dispersion strengthened (ODS) nickel alloy (MA758) using an amorphous metal interlayer with a Ni–Cr–B–Si composition. A microstructural study was undertaken to investigate the effect of parent metal grain size on the joint microstructure after isothermal solidification. The ODS alloy was bonded both in fine grain and recrystallised conditions at 1100°C for various hold times. The work shows that the final joint grain size is independent of the parent alloy grain structure and the bonding time. However, when the alloy is bonded in the recrystallised condition and given a post-bond heat treatment at 1360°C, the joint grain size increases and a continuous parent alloy microstructure across the joint region is achieved. If MA758 is bonded in the fine grain condition and then subjected to a recrystallising heat treatment at 1360°C, the grains at the joint appear to increase in size with increasing bonding time. The joint grains are generally larger than those produced when the alloy is bonded in the recrystallised condition. The differences in microstructural developments across the joint are discussed in terms of stored strain energy of the parent metal grains.     

Dynamic recrystallisation in hot deformed oxide dispersion strengthened MA956 and MA957 steels

Abstract

The hot deformation behaviour of the oxide dispersion strengthened stainless steels MA956 and MA957 has been studied. The alloys are made by a mechanical alloying process which leaves them in a very fine grained, cold deformed state immediately after consolidation. It is found that deformation is accompanied by dynamic recrystallisation, even when the deformation temperatures are far less than the ordinary recrystallisation temperatures of the two alloys. These and other results on the strength and anisotropy of the alloys are interpreted in terms of their microstructures.

MST/3043     

Effect of bonding variables on TLP bonding of oxide dispersion strengthened superalloy

Transient liquid phase (TLP) bonding has evolved as a successful alternative joining technique for high service temperature components (e.g., vanes and blades for aircraft gas turbine engines) made from superalloys when neither fusion welding nor solid-state bonding techniques are successful. However, study shows that the optimization of bonding variables is critical to achieve a metallurgically sound joint free from deleterious intermetallic constituents in the joint region. In this study, the influence of bonding pressure and interlayer thickness on microstructural developments at the joint region of TLP bonded oxide dispersion strengthened (ODS) superalloy MA758 was examined. A commercial interlayer based on the Ni–Cr–B (MBF-80) system was used and results showed that bonding pressure and interlayer thickness affected the final width of the joints. A theoretical study revealed that the TLP bonding time can also be reduced when there is an increase in bonding pressure.     

Diffusion bonding of ferritic oxide dispersion strengthened alloys to austenitic superalloys

The superior high temperature mechanical strength and oxidation resistance of ferritic oxide dispersion strengthened (ODS) tubular alloys are compromised by the difficulties encountered in joining. Conventional fusion welding techniques generate a weld fusion zone which is devoid of the mechanical strength exhibited by the base material. Therefore, more sophisticated solid state joining techniques, such as diffusion bonding, must be employed when joining ODS materials. This paper describes a series of solid state diffusion bonding experiments carried out between two tubular ferritic ODS alloys and two high temperature austenitic alloys. Careful control of bonding conditions produced pressure retaining joints between one of the tubular ODS alloys and both austenitic alloys. The successful joint design was incorporated into the manufacture of a tubular creep component, which enabled a series of internally pressurized creep tests to be carried out. The microstructure developed at the bond interface of each of the four separate material couples is described and the high temperature performance of the pressure retaining joints is discussed.     

High temperature oxidation and sulfate induced corrosion of an oxide dispersion strengthened Ni base superalloy

The oxidation and sulfate induced corrosion behavior of an oxide dispersion strengthened nickel base superalloy, MA 6000, has been studied at 900°C and 1000°C. The experimental program involved thermogravimetry. X-ray diffraction and scanning electron microscopy. The kinetics of high temperature oxidation in air increased with increasing temperature. The experimental activation energy for oxidation in air indicated that a chromia-rich scale formed the protective oxide layer. In the case of sulfate induced corrosion, however, increasing temperature decreased the scale growth rate. This result has been explained by considering the relative stability of nickel sulfide at 900° and 1000°C. Metallographic studies of the scales are also presented.     

Joining of Co coated ferritic stainless steel to ceramic solid oxide cells by a novel Ag-SiO2 braze

A novel Ag-SiO₂braze for Solid Oxide Cells was successfully developed and applied to join Co coated AISI 441 ferritic stainless steel to YSZ-Ni O half cells. The braze showed an improved wettability on the coated steel and the YSZ compared to pure Ag. A defect-free steel/YSZ joint was obtained using an Ag-1SiO₂braze jointed at 1000 ℃ for 30 min in air. The microstructure of the joints was characterized by scanning electron microscopy and transmission electron microscopy wit...     

Properties,processability and weldability of a novel affordable creep resistant nickel base superalloy

Abstract

A recently designed heat resistant alloy for operation ～750°C in power plant, named FT750_DC, of composition Ni–20Cr–5Fe–3·5W–2·3Al–2·1Ti–0·07C–0·4Si–0·005B (wt-%), is fully characterised. Creep rupture and damage is investigated, as well as Charpy impact toughness. The alloy processability is also assessed: castability (comparison between measured microsegregation and Scheil simulation), forgeability (hot tensile ductility, strain hardening and recrystallisation behaviour), γ′ aging heat treatment (gamma-prime precipitation kinetics) and weldability (GTAW/TIG). It is demonstrated that this material, made affordable by avoiding the use of expensive alloying elements, with a creep rupture life in excess of 100 000 h at 750°C under 100 MPa, is easily castable, forgeable and weldable. It can therefore be favourably compared to concurrent alloys for fossil fuel or nuclear power plant applications (like the coal fired ultra supercritical steam power plant, or the nuclear high temperature gas cooled reactor) currently under assessment, such as alloys 230, 617, 625, 740, HX, etc.     

Role of A2 and A50 process on the oxide dispersion strengthened ferritic alloy fabricated by mechanical alloying

This study has investigated the effects of adding occasion of Stearic Acid (SA) on the characteristics of powder and properties of bulk with nominal composition of Fe–12Cr–2.5 W–0.4Ti–0.3Y₂O₃, which was fabricated by mechanical alloying and vacuum sintering. SA was milling with the powder mixture with 50 h (added before milling, A₅₀) or 2 h (added after 48 h milling, A₂). The resulted showed: SA could inhibit the agglomeration and retard the alloying process. Powders through A₂ process achieved alloying with high powder yield, while the A₅₀ powders presented alloying extent, and the yield of A₀ powder presented low powder yield. SA added in A₅₀ was almost dissolved into the matrix for the long milling duration, but SA added in A₂ was mostly volatilized during heating. A₂ bulk alloy was of better strength for the finer, more uniform grain and second phase and higher density than A₅₀ bulk.     

Joining of tungsten carbide to nickel by direct diffusion bonding and using a Cu–Zn alloy

The objective of this work was to study various aspects of liquid and solid state diffusion bonding of cylindrical samples of WC (with 6% Co) and commercially pure nickel (99.5%) produced by direct bonding and brazing using a 25 μm thick 70Cu 30Zn (wt%) alloy as joining element. Joining experiments were carried out on WC/Ni and WC/Cu Zn/Ni combinations at temperature of 980 °C using 1, 15, 25 and 35 min holding times in argon (Ar). The results show that it is possible to create a successful joint at temperature and times used. Joining occurred by the formation of a diffusion zone. The joining interface is feasible because it presents a homogeneous interface with no several interfacial cracking and porosity. In both combinations, it can be observed a diffusion of cobalt decreasing in the direction of the metal, as well as, the diffusion of nickel decreasing in the direction of the ceramic.     

Diffusion bonding of titanium alloy to micro-duplex stainless steel using a nickel alloy interlayer: Interface microstructure and strength properties

In the present study, diffusion bonding of titanium alloy and micro-duplex stainless steel with a nickel alloy interlayer was carried out in the temperature range of 800–950 °C for 45 min under the compressive stress of 4 MPa in a vacuum. The bond interfaces were characterised by scanning electron microscopy, electron probe microanalyzer and X-ray diffraction analysis. The layer wise Ni₃Ti, NiTi and NiTi₂ intermetallics were observed at the nickel alloy/titanium alloy interface and irregular shaped particles of Fe₂₂Mo₂₀Ni₄₅Ti₁₃ was observed in the Ni₃Ti intermetallic layer. At 950 °C processing temperature, black island of β-Ti phase has been observed in the NiTi₂ intermetallics. However, the stainless steel/nickel alloy interface indicates the free of intermetallics phase. Fracture surface observed that, failure takes place through the NiTi₂ phase at the NiA–TiA interface when bonding was processed up to 900 °C, however, failure takes place through NiTi₂ and β-Ti phase mixture for the diffusion joints processed at 950 °C. Joint strength was evaluated and maximum tensile strength of ∼560 MPa and shear strength of ∼415 MPa along with ∼8.3% ductility were obtained for the diffusion couple processed at 900 °C for 45 min.     

In vitro and in vivo studies on a novel solid dispersion of repaglinide using polyvinylpyrrolidone as the carrier

In order to improve the dissolution and absorption of the water insoluble drug repaglinide, a solid dispersion was developed by solvent method using polyvinylpyrrolidone K30 (PVP K30) as the hydrophilic carrier for the first time. Studies indicated that both solubility and the dissolution rate of repaglinide were significantly increased in the solid dispersion system compared with that of repaglinide raw material or physical mixtures. The repaglinide solid dispersions with PVP K30 solid state was characterized by polarizing microscopy, differential scanning calorimetry (DSC), X-ray diffraction (XRD), and Fourier transform infrared spectroscopy (FT-IR). DSC and XRD studies indicated that repaglinide existed in an amorphous form in the solid dispersion. FT-IR analysis demonstrated the presence of intermolecular hydrogen bonding between repaglinide and PVP K30 in the solid dispersion. In the in situ gastrointestinal perfusion experiment, solid dispersion was shown to remarkably enhance the absorption of repaglinide in stomach and all segments of intestine. In vivo pharmacokinetic study in rats showed that immediate and complete release of repaglinide from the solid dispersion resulted in rapid absorption that significantly increased the bioavailability and the maximum plasma concentration over repaglinide raw material. These results demonstrated PVP K30 was an appropriate carrier for solid dispersion of repaglinide, with increased dissolution and oral absorption.     

In vitro and in vivo studies on a novel solid dispersion of repaglinide using polyvinylpyrrolidone as the carrier

In order to improve the dissolution and absorption of the water insoluble drug repaglinide, a solid dispersion was developed by solvent method using polyvinylpyrrolidone K30 (PVP K30) as the hydrophilic carrier for the first time. Studies indicated that both solubility and the dissolution rate of repaglinide were significantly increased in the solid dispersion system compared with that of repaglinide raw material or physical mixtures. The repaglinide solid dispersions with PVP K30 solid state was characterized by polarizing microscopy, differential scanning calorimetry (DSC), X-ray diffraction (XRD), and Fourier transform infrared spectroscopy (FT-IR). DSC and XRD studies indicated that repaglinide existed in an amorphous form in the solid dispersion. FT-IR analysis demonstrated the presence of intermolecular hydrogen bonding between repaglinide and PVP K30 in the solid dispersion. In the in situ gastrointestinal perfusion experiment, solid dispersion was shown to remarkably enhance the absorption of repaglinide in stomach and all segments of intestine. In vivo pharmacokinetic study in rats showed that immediate and complete release of repaglinide from the solid dispersion resulted in rapid absorption that significantly increased the bioavailability and the maximum plasma concentration over repaglinide raw material. These results demonstrated PVP K30 was an appropriate carrier for solid dispersion of repaglinide, with increased dissolution and oral absorption.