Tensile behaviour of Fe3Al-based intermetallic alloy in the rolled state

Abstracts are not published in this journal This revised version was published online in November 2006 with corrections to the Cover Date.     

Tensile behaviour of Fe3Al-based intermetallic alloy in the rolled state

Abstracts are not published in this journal     

TEM observation of DO3 structure in Fe3 Al alloy with Mn addition

The structures of DO₃ Fe-28Al-1.5Mn alloy, including ordering degree, superdislocation, APD and APB, were investigated by TEM. The results showed that addition of manganese into DO₃ Fe₃Al could not change the ordered type of the alloy, but could reduce APD size and then reduce ordering degree of the alloy. The fourfold superdislocations were retarded in DO₃ Fe₃Al alloy after Mn addition. Undeformed alloy with Mn has mainly twofold superdislocations. As deformation increases, the twofold superdislocations slip and decompose into unit dislocations, and unit dislocations slip and slip cross, leading to better ductility. The deformation mechanism of DO₃ Fe-28Al-1.5Mn alloy was controlled at first by twofold superdislocation and at last by unit superdislocation.     

Effect of titanium substitution on the structure and properties of Fe3Al-based intermetallic alloys

Substitutions (0–16 at%) of titanium for iron in Fe₃Al-based alloys rapidly solidified by chill-block melt-spinning stabilize the D0₃ and B2 ordered structures. Rapid solidification results in total suppression of D0₃ order in binary alloys, whereas alloys containing titanium have D0₃ structure. D0₃ and B2 antiphase domain size, lattice parameter and hardness increase with increasing titanium content of the alloy. The deformation mode changes from single (unit) bcc dislocation in binary alloys to two-fold superdislocation configuration in D0₃Fe₃Al-Ti alloys. Mechanical antiphase boundaries are generated by the movement of these imperfect configurations. All the alloys exhibited cleavage tensile failure. The mechanical properties are correlated with the observed structural changes.     

Laser-welding behaviour of cast Ni3Al intermetallic alloy

Ductile nickel aluminide, Ni₃Al+B, is an intermetallic alloy with high strength and ductility making it a promising structural material for both elevated temperature and cryogenic temperature applications. In order to be able to use this alloy over a spectrum of temperature-critical applications, it must be capable of being joined or welded. The weldability of a cast nickel aluminide alloy containing boron was studied using laser welding. Welding was carried out at laser beam traverse speeds ranging from 42.33–254 mms^–1 in the bead-on-plate and butt-joint configurations. Two types of surface preparation, namely chemical cleaning and mechanical polishing, were used prior to laser welding. The quality of the laser welds was evaluated through mechanical tests (hardness and tensile), X-ray diffraction and microscopical observations. High-magnification examination of the welds revealed fine columnar structures in the weld zone. The hardness of the weld zone was substantially higher than that of the base metal. Microscopic examination also revealed the welds to contain shrinkage cracks. For a constant set of laser parameters, the chemically etched surfaces provided deeper penetration than the mechanically polished surface. The performance of the laser-welded joint is rationalized.     

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.     

Effect of hydrogen on the mechanical behaviour of carbon-alloyed Fe3Al-based iron aluminides

The effect of hydrogen on the mechanical behaviour of two carbon-alloyed iron aluminides was studied. Weakening of some carbide-metal interfaces in the presence of hydrogen was indicated. The effect of cathodic hydrogen charging on the microstructure has also been addressed.     

Creep behaviour of carbon containing Fe3Al alloy

Abstract

Tensile creep of a Fe–16 wt-%Al–0·5 wt-%C alloy was investigated over a temperature range of 773 to 873 K and stress range of 80 to 200 MPa. Creep curves exhibited very limited primary and secondary creep regimes. An extended tertiary creep regime was observed for all the test conditions. Stress dependence of minimum creep rate can be represented by a power-law equation with stress exponents being in the range 4 to 5. The activation energy for creep was found to be ～340 kJ mol^?1. The observed stress exponent and activation energy for creep suggest that creep is controlled by dislocation climb. Creep fracture in Fe₃Al–C alloy is predominantly by transgranular ductile mode by nucleation, growth and coalescence of microvoids formed at FeAlC_0·5 particle/matrix interface by decohesion as well as fracture of elongated particles. Extended tertiary creep observed in the alloy was analysed in the light of the mechanisms proposed for nickel based superalloys.     

Effect of order-disorder transformation modes on the anomalous yield behaviour of Fe3Al intermetallic compounds

Compression tests were performed to clarify the effects of transformation modes on the anomalous yield behaviour of hypo- and hyper-stoichiometric Fe₃Al alloys which show a first- and a second-order transition, respectively. There were great differences in the anomalous yield behaviour depending on the transformation modes. In the first-order transformation alloy, changes in the degree of order played an important role before phase separation, while precipitation of phase had a great influence on the anomalous behaviour after phase separation. In contrast, only the change in the degree of order was a dominant factor in the second-order transformation alloy.     

The effect of H2-anneal on the adhesion of Al2O3 scales on a Fe3Al-based alloy

Abstract

The cyclic (1,100°C, air) and isothermal (1,000°C, O₂) oxidation behavior of a Fe-28Al-5Cr (at%) alloy, with and without a prior H₂-anneal heat treatment at 1,200°C for 100 h, was studied. Changes in interfacial chemistry were evaluated using Scanning Auger Microscopy after removal of the oxide film in ultra high vacuum. This was achieved by making a scratch on the specimen surface, which caused spallation of the film at various locations along the scratch. The scale thickness and the temperature drop at which spallation took place during cooling were utilized to semi-quantitatively compare the adherence of the scales. Porosity at the scale–alloy interface and the scale microstructure were determined from scanning electron microscope observations. It was found that H₂-anneal greatly improved scale adhesion and resulted in a pore-free and sulfur-free interface. The effects were similar to that of a 0.1 at% Zr-containing alloy, except that the improvement in scale adhesion was not as great as that from Zr doping. This implies that oxide/alloy interfaces are not intrinsically strong and the effect of reactive elements, such as Zr, is more than preventing impurity from segregating to the interface. Results are also compared with the effect of H₂-anneal on other model alloys, such as NiCrAl, FeCrAl and NiAl, and on single crystal superalloys.     

TEM analysis of diffusion brazement microstructure in a Ni<Subscript>3</Subscript>Al-based intermetallic alloy

Transmission electron microscopy study of brazed joint microstructure in a Ni₃Al-based intermetallic alloy IC 6 was performed. A continuously distributed microconstituents consisting of γ solid-solution, M₃B₂-type and M₂₃B₆-type borides, which were likely formed by eutectic-type reaction(s) from residual liquated insert during cooling from brazing temperature, were observed along the joint centreline. Consideration of possible incipient melting due to eutectic-type transformation reaction of these phases is pertinent to development of post braze heat treatment for modifying the brazement microstructure.     

Tensile creep behaviour of NiAl-Cr(Zr) multiphase intermetallic alloy

Abstract

The microstructure of a multiphase NiAl-33.5Cr-0.5Zr intermetallic alloy was examined by SEM with energy dispersive spectroscopy and TEM. The tensile creep behaviour of the hot isostatically pressed NiAl-33.5Cr-0.5Zr alloy was studied. The results of the creep test indicated that all of the creep curves under the present test have similar characteristics: a short primary creep stage, a dominant tertiary creep stage, and nearly identical creep strains (~45%). The apparent stress exponent and the apparent activation energy were analysed and discussed. The mechanism of the creep deformation was also analysed by the observation of TEM.     

Effect of recrystallization on environmental embrittlement of a Ni3Al-based alloy

The effects of recrystallization on room temperature tensile properties and susceptibility to test environments (air versus vacuum) of a Ni3Al-based alloy have been investigated. The results indicate that recrystallization treatment not only increases ductility but also reduces susceptibility to test environment of the alloy. The fracture mode has also been observed in the scanning electron microscope (SEM). It was found that air promoted transgranular cleavage fracture and reduced the amount of ductile fracture for unrecrystallized and partially recrystallized microstructures. In completely recrystallized specimens, the ductile fracture mode remained unchanged when test environment changed from vacuum to air. This revised version was published online in November 2006 with corrections to the Cover Date.     

The fracture behaviour of intermetallic TiAl alloys with and without warm pre-stressing

The fracture behaviour of near- (NG) and fully lamellar (DFL) intermetallic TiAl alloys with and without warm pre-stressing (WPS) is investigated by testing three point bending specimens with fatigue pre-cracks at room temperature and at 700 °C. Detailed fractographic observations and FEM calculations are carried out to find the critical step of cleavage fracture. The results show that the cleavage fracture is induced by direct propagation of the pre-crack and that the stress at the crack tip is the decisive controlling factor. The WPS process improves the fracture initiation toughness of both the NG and the DFL microstructure, but deteriorates slightly the increase of the K-resistance curves, especially for the DFL microstructure. The compressive residual stress induced by WPS plays the main role in improving the fracture initiation toughness.     

Ductile/brittle grain-boundary fracture behaviour of L12-type intermetallic compounds

The fracture behaviour of grain boundaries in L1₂-type intermetallic compounds was investigated to explain recent experimental results reported by Takasugiet al. and Inoueet al. Inspection of these results revealed a simple rule; grain boundaries are ductile in Kurnakov compounds having an order-disorder transition below their melting point and a wide solid-solution field. On the other hand, grain boundaries are brittle in Berthollide compounds having no order-disorder transition below their melting point. The ductile grain-boundary behaviour in Kurnakov compounds is considered to be produced by disordered grain boundaries, intrinsically ductile and inherited when formed in disordered states.     

Effects of intermetallic particles on cavitation during superplastic forming of aluminium alloy

An Al-4.6%Mg-1.5%Mn-0.27%Fe alloy was specially processed with friction stir processing followed by cold rolling. Half of the sheet thickness contains a large number of blocky or irregular-shaped Al₆(Mn,Fe) coarse intermetallic particles, while the other half, smaller and more spherical ones. The particle-induced cavitation upon uni-axial tension at 475°C with 2?×?10^?4–2?×?10^?2?s^?1 strain rates was investigated. The density of nuclei for cavities was estimated based on quantitative image analysis of the particles, and the strain controlled growth rate, calculated assuming the cavities are nucleated before or in the early stage of straining. The number, size and morphology of intermetallic particles are found to control the cavitation by determining the nucleation rate, but the strain controlled growth rate appears unaffected.