DS Alloy Research Articles

Low cycle fatigue behavior and life prediction of a directionally solidified alloy

Low cycle fatigue behavior and life prediction of a directionally solidified alloy

Role of stacking fault on mechanical properties of a DS alloy

In the present study, a novel strengthening mechanism that does not impair the plasticity has been ascertained in a directionally solidified nickel-based superalloy (DS alloy) during monotonic and cyclic deformations. The alloy exhibited excellent mechanical properties at 750°C. Sub-structural examinations revealed that the activation of the stacking fault-induced micro-twinning mechanism is responsible for the improved fatigue performance at 750°C. This work may lead us to design advanced nickel-based superalloys having good synergy of strength and plasticity for turbine blade applications.

Comparison of directional solidification of γ-TiAl alloys in conventional Al2O3 and novel Y2O3-coated Al2O3 crucibles

Abstract The effects of novel Y 2 O 3 -coated Al 2 O 3 (Y 2 O 3 /Al 2 O 3 ) crucibles on the microstructure and composition of directionally solidified TiAl alloys were investigated and compared with those of single layered Al 2 O 3 and Y 2 O 3 crucibles, based on which the corresponding alloy–crucible interaction mechanisms were discussed. The DS alloys exhibited a fully lamellar γ/α 2 structure interspersed with some Al 2 O 3 or Y 2 O 3 particles. Differently from that in the case of using Al 2 O 3 crucibles, no interfacial interaction layer was found in the ingots prepared using Y 2 O 3 /Al 2 O 3 crucibles. Dissolution and erosion were the main mechanisms responsible for the alloy–crucible interactions which increased with the heating temperature and interaction time. Nevertheless, the interaction extents when using Y 2 O 3 /Al 2 O 3 crucibles were much lower than using Al 2 O 3 crucibles, making the former promising candidate crucibles for the high quality DS of highly reactive TiAl alloys.

Effect of Process on Microstructure and Elevated Temperature Mechanical Behavior of Multi-Phase NiAl-Fe(Nb) Alloy

The microstructure and elevated temperature mechanical behavior of as-cast, HIPed and directionally solidified NiAl-Fe(Nb) Alloys have been investigated. The results show that the microstructure of as-cast NiAl-Fe(Nb) alloy consists of dendritic regions (b) and interdendritic regions (g/g¢) with non-continuous NbNiAl(Laves) phase segregating at the interfaces of b and g/g¢ phases. The HIP processing does not alter the microstructure of as-cast alloy. The longitudinal dendritic microstructure of the DS NiAl-Fe(Nb) alloy is regular. Room temperature elongations of as-cast and HIPed NiAl-Fe(Nb) alloys are all zero, however, 0.4% for the DS alloy. The strength and elongation of the HIPed alloy are higher than that of as-cast alloy. The yield strength and ultimate strength of the DS alloy are equivalent to that of as-cast alloy, but elongation of the DS alloy is higher than that of as-cast alloy, the elongation exceed 100% at 1273K, which indicates that the DS alloy can be deformed superplastically.

Microstructure evolution and room temperature deformation of a unidirectionally solidified Nb-22Ti-16Si-3Ta-2Hf-7Cr-3Al-0.2Ho (at.%) alloy

Unidirectional solidification (DS) experiments were performed on a Nb-22Ti-16Si-3Ta-2Hf-7Cr-3Al-0.2Ho (at.%) alloy and the growth rates were selected as 6, 10 and 16 mm/h, respectively. The effect of solidification rates on the microstructure development and room temperature deformation was investigated. The DS alloys were mainly composed of Nb solid solution (Nbss) and Nb 5Si 3 phases. All the Nbss/Nb 5Si 3 eutectic colonies aligned well along the growth direction of the DS specimens. A small amount of hex-Nb 5Si 3 phase was identified as intergranular phase when observed from the transverse section of the DS specimens. There was an orientation relationship between Nbss and hex-Nb 5Si 3, which was determined as [ 111 ] Nb / / [ 22 4 ¯ 3 ] h and the two almost parallel planes of ( 1 1 ¯ 0 ) Nb / / ( 1 1 ¯ 00 ) h with an include angle about 2.2°. The DS alloys showed improved room temperature compressive strength and fracture toughness as compared with the casting alloy.

Effect of withdrawal rate on microstructure and mechanical properties of a directionally solidified NiAl-based hypoeutectic alloy doped with trace Hf and Ho

The hypoeutectic alloy, with nominal composition NiAl–31Cr–2.9Mo–0.1Hf–0.05Ho (at.%), was directionally solidified at three different withdrawal rates by liquid metal (Sn) cooling technique. Microstructural examination reveals that directional solidification gave rise to a shift in the coupled zone for the eutectic growth towards the Cr(Mo) phase. With the withdrawal rates increasing from 3 mm/min to 15 mm/min, the volume fraction of primary dendritic NiAl increases from 21.1% to 25.9%, while the size and the arm spacing of NiAl primary dendrite reduces simultaneously. The room temperature (RT) fracture toughness and the tensile strength at RT and elevated temperature (1373 K) present the valley value at intermediate rate (8 mm/min) among the withdrawal rate range which could be attributed to the decrease in volume fraction of eutectic NiAl/Cr(Mo) microstructure and the refinement of microstructure resulted from the increase of withdrawal rates. In terms of RT tensile elongation, the DS alloy grown at different withdrawal rates all break with no plastic flow.

An Overview of the Anatomy of Crystal Plasticity Models

AbstractSingle crystals, polycrystals, and DS alloys can be modeled in the same framework by means of crystal plasticity. This paper wants to show the common features of the different approaches on the grain level, and the additional assumptions that are needed to derive polycrystal or DS models. Phenomenological rules are introduced for representing the hardening in the single crystal constitutive equations. Series of examples are given to illustrate the capabilities of the various approaches that are mainly related to the crystallographic character, and to the fact that the macroscopic yield locus is not predefined, but built from a collection of linear yield conditions.

Hydrogen Permeation Properties of Nb-Based Two-Phase Compounds

Nb-NiTi and Nb-CoTi eutectic alloys were directionally solidified in an optical floating zone furnace. Rod-type eutectic structures with Nb rods aligned parallel to the growth direction are obtained for Nb-41Ni-40Ti grown at relatively slow growth rates below 1.0mm/h, while lamellar-type eutectic structures are obtained for Nb-35Co-34Ti grown at the same condition. The hydrogen permeability for the Nb-41Ni-40Ti DS alloy with Nb rods perpendicular to the membrane surface is 2.60×10-8mol H2 m-1 Pa-1/2 at 673K, which is about 2.5 times higher than that of as-cast sample. No hydrogen embrittlement is observed between 573 and 673K, indicating that the Nb-NiTi rod-type eutectic structure effectively suppresses the hydrogen embrittlement of Nb during hydrogen permeation.

Microstructural Analysis of Laser-Beam-Welded Directionally Solidified INCONEL 738

The laser welding of a directionally solidified INCONEL 738 superalloy (DS IN738) was investigated. Microstructural analysis of the fusion zone (FZ) and the heat-affected zone (HAZ) revealed that cracking occurred mainly in HAZ with only some cracks being extended into the FZ. However, the frequently observed centerline cracking in FZ of DS and single crystal (SX) alloys did not occur, which was attributed to the presence of negligible volume fraction of γ-γ′ eutectic in FZ. Constitutional liquation of secondary solidification constituents (MC carbides, M3B2 borides, M2SC sulphocarbides, and γ-γ′ eutectic) and γ′ precipitate particles was found to be the major cause of grain boundary liquation and the resultant intergranular microfissuring in the HAZ. The extent of HAZ microfissuring was, however, observed to be smaller in samples welded along the transverse direction (perpendicular to solidification direction) than when welding was done along the longitudinal direction (solidification direction). Nonetheless, more severe HAZ cracking occurred in samples of similarly welded conventionally cast (CC) alloy, and the present results indicate that the severity of HAZ liquation cracking in IN738 superalloy can be reduced by using a DS version of the alloy.

Recrystallization and fatigue failure of DS alloy blades

This paper analyzed cracks and fractures of DS alloy blades. The failure modes and causes were studied on the basis of fracture surfaces and metallographical examination, and prevention measures were given. The results showed that the cracks and fractures failed in the same mode—fatigue failure caused by surface recrystallization of the blades. The surface recrystallization formed during the solution heat treatment because the blades deformed plastically.

Microstructure control of TiAl alloys containing β stabilizers by directional solidification

Phase transformation at elevated temperatures in Ti-Al-X(X=Mo, Re, W) systems has been investigated by analyzing the dendrite morphology of arc-melted buttons. It was found that the addition of β stabilizers, such as Mo, Re and W, shifted the β phase field at liquid/solid temperatures to the high Al composition side. So, the β phase formed as a primary crystal even in higher Al compositions in Ti-Al-X ternary systems compared with Ti-Al binary. The addition of W was found to be the most effective β (bcc) stabilizer among Mo, Re, W, and the Ti-47Al-2W composition was selected for β solidification. In the directional solidification of Ti-47Al-2W alloy at the growth rate of 90 mm/h using the Bridgman type DS apparatus, it was found that the lamellar orientation in the columnar grains was parallel or inclined with the angle of 45° to the growth direction. This means that the β phase forms at the solid/liquid interface in the Ti-47Al-2W alloy. The tensile elongation of DS alloys was clearly improved compared with that of the polycrystalline alloy with a fully lamellar microstructure, while maintaining the same yield stress level.

The prospects of copper alloys for VNS applications

The center post (CP) is a critical component of the VNS spherical tokamak (VNS ST). DS and PH copper alloys are considered as candidate materials for the center post conductor. The behavior of the conductor materials under mechanical, thermal and radiation conditions peculiar to the CP structure is viewed and its life time is predicted

The prospects of copper alloys for VNS applications

The center post (CP) is a critical component of the VNS spherical tokamak (VNS ST). DS and PH copper alloys are considered as candidate materials for the center post conductor. The behavior of the conductor materials under mechanical, thermal and radiation conditions peculiar to the CP structure is viewed and its life time is predicted

Fabrication of large-grained binary stoichiometric Ni 3Al

Little is known about recrystallization in binary stoichiometric Ni{sub 3}Al because of the difficulty in deformation due to grain boundary brittleness. This is part of a larger study aimed measuring the grain boundary strengths in Ni{sub 3}Al for which the authors needed larger bicrystals, with specific misorientations. However, it is hard to grow them by solidification from the melt, for example by Bridgman or conventional floating zone methods, because binary stoichiometric Ni{sub 3}Al solidifies incongruently. Diffusion bonding of two single crystals to form bicrystals is another technique but it often results in poor-quality bonded interfaces. Thus, they tried to fabricate large-grained sheet by recrystallization from which bicrystal tensile specimens would be cut. For this to work the grain size should be larger than 5 mm. In this paper, they present the details of a procedure to grow large-grained Ni{sub 3}Al by deforming and recrystallizing the ductile DS alloys and some findings about the recrystallization in binary stoichiometric Ni{sub 3}Al.

High temperature mechanical properities and creep crack initiation of DS CM186LC for nozzle guide vane

The second generation DS alloy, CM186LC is used in the as-cast and double aged condition which has creep-rupture properities equivalent to the first generation single crystal alloys CMSX-2 and CMSX-3. In production, cast vane components have to be subjected to a brazing treatment for joining into pairs. The effect of the brazing treatment and modified brazing treatment (heat treatment) on mechanical properities at high temperature was studied in accordance with microstructure. Brazing treatments gave no effect on tensile properities and creep failure mode of DS CM186LC, although a small decrease in stress-rupture life was observed. Creep failure was related to the solidified microstructure. Creep cracks began at the grain boundary normal to the applied stress, especially at the γ/γ’ eutectic phase on grain boundaries. Most of γ/γ’ eutectics which had solidified at the last stage of casting, had microporosity which became a crack initiation site during creep. MC carbide reaction with the matrix γ was observed in the creep failed specimens.

On the tensile creep behaviour of a directionally-solidified Ni3Al-based alloy

High temperature tensile creep behaviour of a directionally-solidified Ni3Al-based alloy is presented. The study involved selection of nine alloy systems based on Ni3Al. The alloys contained varying amounts of Cr and Ta, fixed amounts of 1·5 at.% Hf and 0·5 at.% Zr and doped with 0·2 at.% each of C and B. The alloys were vacuum arc-melted into buttons and homogenized at 1050°C for 68 h. The test pieces of the alloys were hot compression tested at 600, 700, 800 and 900°C. The yield strength data of some of the alloys were superior to conventionally cast Mar-M 200, a cast nickel-base superalloy widely used in gas turbine structural applications. The best alloy system was chosen based on consistent performance in the hot compression studies. The alloy so chosen was directionally solidified and vacuum-homogenization-treated for 20 h at various selected temperatures. Optimum creep properties were observed at 1120°C, 20 h treatment. The minimum creep rate data of the DS alloy showed relatively higher values even at lower temperatures and stress levels as compared to Mar-M 200. Hence, the alloy is less promising in replacing nickel-based superalloys used as structural materials in gas turbine applications.

New Alloy Developments in Single Crystal and DS Alloys

New Alloy Developments in Single Crystal and DS Alloys

A deformation mechanism map for IN738LC superalloy

This letter reports new data on the isothermal creep properties of the DS alloy IN738LC, which is used as a blading material for the first-row high-pressure stage of gas turbines

The influence of silicon and helium on the microstructural stability of ion-irradiated incoloy DS alloys

In order to test their void swelling behaviour under irradiation, several alloys based on the solid-solution nickel alloy Incoloy DS (18Cr-38Ni-Fe) with additions of 0.05, 0.43, 0.92 and 2.24 wt% Si have been studied using 4 MeV helium and 46 MeV nickel ion irradiation in the Harwell VEC. For irradiations of 60 dpa with 10 appm He the void swelling decreased from ~ 0.9% to negligible levels with increasing silicon content. After irradiation to 90 dpa following injection with 10 appm He the 2.24% Si alloy showed <1.0% swelling at an apparent peak swelling temperature of 625° C. This alloy was subsequently irradiated to check the swelling response with concentrations of helium and hydrogen appropriate to fusion-reactor conditions. Following irradiation to 60 dpa after 1000 appm He injection the swelling peak was shifted to 575° C where a swelling maximum of 4% was observed. At 625° C with 1000 appm He alone, swelling was 2.0% compared with 1.2% in samples injected with 1000 appm He +1000 appm H. This small reduction in swelling was associated with a higher cavity (bubble) concentration in the hydrogen implanted sample. Fine-scale precipitation of Ni 3Si(γ'), η-carbide and G-phase was observed after irradiation together with helium bubbles attached to the η- and G-phase precipitates. The precipitation and void swelling was significantly greater in irradiated samples containing 1000 appm He than in those with 10 appm and irradiated to 90 dpa. It is concluded that although the Incoloy DS alloy possibly has a potential for fission-reactor core applications it has little to commend it for fusion-reactor use where the high swelling response, microstructural instability and likely long-term induced activation arising from the higher nickel content are clearly undesirable factors.

Mechanical property anisotropy in superalloy El-929 directionally solidified by an exothermic technique

Directional solidification of the nickel-based superalloy El-929 was carried out by employing the exothermic technique for preparing several 150mm long × 55mm diameter rods. Specimens machined from the blanks cut at 0°, 45°, 75° and 90° to the chill surface were tensile and stress-rupture tested at different temperatures. The air-melted DS alloy, when loaded parallel to the growth direction, shows considerable improvement in stress-rupture life and tensile ductility as compared with the vacuum induction melted, forged and heat-treated alloy. However, these property advantages rapidly degrade with the increasing deviation of the load axis from the growth direction.