Mo Fibers Research Articles

Microstructure of a eutectic NiAl—Mo alloy directionally solidified using an industrial scale and a laboratory scale Bridgman furnace

Abstract The eutectic NiAl-9Mo (at.%) alloy was directionally solidified using an industrial scale Bridgman furnace and using a liquid metal cooling Bridgman furnace to produce in-situ composites of aligned Mo fibers in an NiAl matrix. In the first part of the experiment, an investment casting shell mold system, produced for NiAl alloys, was used and investigated for the NiAl-9Mo alloy in the industrial scale furnace. Due to the lower temperature gradient of ∼2.8 K mm−1, the microstructure of the samples was dendritic. In the second part of the experiment, samples with well aligned Mo fibers were produced in the laboratory Bridgman furnace using a temperature gradient of ∼11 K mm−1 and growth rates of 0.33–0.66 mm min−1.

Microstructure and room temperature fracture toughness of directionally solidified NiAl–Mo eutectic in situ composites

The microstructure and room temperature fracture toughness of pseudo-binary NiAl–ⅹMo (ⅹ = 7.8, 9, 13 and 16 at. %) in situ composites were investigated. For all four alloys examined, uniform and well-aligned Mo fibrous structures were directionally solidified by the liquid metal cooling process at the growth rate of V = 6 μm/s, respectively. With the increase of the Mo content, the spacing of the Mo fibers decreased and the volume fraction of Mo fibers increased. Significant influence of Mo addition on the toughness was observed for the directional solidification alloys, and the fracture toughness was found to increase with volume fraction of Mo rods. Scanning electron microscopy was used to characterize the fracture behavior. Based on the results of theoretical analysis, the composite structures of these alloys provided improvement in fracture toughness over binary NiAl primarily by crack trapping and crack bridging. Interface debonding, crack deflection and microcrack linkage provided further resistance to crack growth.

Interface strength in NiAl–Mo composites from 3-D X-ray microdiffraction

The depth-dependent strain gradients near buried interfaces in a model system of NiAl–Mo composite were nondestructively probed with 3-D X-ray microdiffraction. Coupled with micromechanical analysis, our study shows that the relaxation of the residual thermal strains in the NiAl–Mo composites results in the formation of a near-surface “slip zone” with large strain gradients in both the reinforcing Mo fibers and NiAl matrix. Based on these results an approach to calculate the fiber–matrix interface strength for composite materials is suggested.

Mo Fiber Reinforced NiAl Composites Produced by Directional Solidification – Process, Microstructure and Mechanical Properties

ABSTRACTComposites with a eutectic composition NiAl-9at.%Mo were produced by controlled directional solidification (DS) so that refractory metallic Mo fibers were precipitated and aligned in the NiAl matrix parallel to the solidification direction through the eutectic reaction. Such NiAl composites can be used for structural applications at high temperatures (> 1000 °C), for example as blade material for modern gas turbines. The microstructure of the composites was examined by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The interface fine structure between Mo fiber and NiAl matrix was studied by high resolution TEM (HRTEM). Mechanical properties were measured by tensile tests at 700 °C and 1100 °C. Accordingly, a correlation of the DS parameters, microstructure and mechanical properties was established.

Indentation-induced localized deformation and elastic strain partitioning in composites at submicron length scale

Three-dimensional spatially resolved strains were mapped in a model NiAl/Mo composite after nanoindentation. The depth-dependent strain distributed in the two phases and partitioned across the composite interfaces is directly measured at submicron length scale using X-ray microdiffraction and compared with a detailed micromechanical stress analysis. It is shown that indentation-induced deformation in the composite material is distinct from deformation expected in a single-phase material. This difference arises in part from residual thermal strains in both phases of the composite in the as-grown state. Interplay between residual thermal strains and external mechanical strain results in a complex distribution of dilatational strain in the Mo fibers and NiAl matrix and is distinct in different locations within the indented area. Reversal of the strain sign (e.g., alternating tensile/compressive/tensile strain distribution) is observed in the NiAl matrix. Bending of the Mo fibers during indentation creates relatively large ∼1.5° misorientations between the different fibers and NiAl matrix. Compressive strain along the 〈0 0 1〉 direction reached −0.017 in the Mo fibers and −0.007 in the NiAl matrix.

3D x-ray microprobe investigation of local dislocation densities and elastic strain gradients in a NiAl-Mo composite and exposed Mo micropillars as a function of prestrain

3D spatially-resolved polychromatic microdiffraction was used to nondestructively obtain depth-dependent elastic strain gradients and dislocation densities in the constituent phases of a directionally solidified NiAl–Mo eutectic composite consisting of ∼500–800 nm Mo fibers in a NiAl matrix. Measurements were made before and after the composite was compressed by 5% and 11%. The Mo fibers were analyzed both in their embedded state and after the matrix was etched to expose them as pillars. In the as-grown composite, due to differential thermal contraction during cooldown, the Mo phase is under compression and the NiAl phase is in tension. After the prestrains, the situation is reversed with the Mo phase in tension and NiAl matrix in compression. This result can be explained by taking into account the mismatch in yield strains of the constituent phases and the elastic constraints during unloading. The dislocation density in both the Mo and NiAl phases is found to increase after prestraining. Within experimental uncertainty there is little discernible difference in the total dislocation densities in the Mo phase of the 5% and 11% prestrained specimens. However, the density of the geometrically necessary dislocations and the deviatoric strain gradients increase with increasing prestrain in both the Mo and NiAl phases.

Effects of boron on the microstructure and thermal stability of directionally solidified NiAl–Mo eutectic

Microalloying with 0.01 at.% B decreases the range of growth speeds over which a well-aligned fibrous eutectic microstructure can be obtained in directionally solidified NiAl–Mo. Compared to the undoped alloy, the size/spacing of the Mo fibers is larger, and the fiber density smaller, in the B-doped alloy. Annealing at 1400 °C coarsens the fibers by a mechanism involving fault migration and annihilation driven by diffusion along the fiber–matrix interface. The coarsening kinetics, given by the decrease in Mo fiber density with time, is exponential, and microalloying with B decreases the coarsening rate.

Spatially resolved strain measurements in Mo-alloy micropillars by differential aperture x-ray microscopy

Spatially resolved strain distributions in the NiAl matrix and the ∼550–1000 nm Mo fibers of a NiAl–Mo eutectic were investigated by microbeam x-ray diffraction. Position sensitive d-spacings for the individual phases were obtained from spatially resolved and energy-resolved Laue patterns. For embedded Mo fibers, the measured elastic strain is consistent with the predicted thermal mismatch strain between the NiAl and Mo phases. However, when the NiAl matrix is etched back to expose Mo micropillars, the d-spacing increases to that of unconstrained Mo, indicating release of the compressive residual strain in the Mo fibers.

Tensile properties and thermal expansion behaviors of continuous molybdenum fiber reinforced aluminum matrix composites

The tensile properties and thermal expansion behaviors of continuous molybdenum fiber reinforced aluminum matrix composites (Mo f/Al) have been studied. The Mo f/Al composites containing different volume percents of Mo fibers were processed by diffusion bonding. The strengths of unidirectional Mo f/Al composites were close to the rule-of-mixtures. The strengths of 0°/90° dual-directional composites increased with fiber content, while those of 45°/135° composites remained relatively low. The coefficients of thermal expansion (CTEs) of the composites decreased as the fiber content increased, close to the values of Mo fibers. With increasing temperature, the CTEs of unidirectional composites increased, while those of dual-directional composites decreased due to large accumulated thermal stresses. The CTEs of 45°/135° composites were lower than those of 0°/90° composites because of contraction effect. At temperatures above 250 °C, the CTEs of the dual-directional composites gradually increased due to matrix yielding and interfacial decohesion.

Thermal-expansion behavior of a directionally solidified NiAl–Mo composite investigated by neutron diffraction and dilatometry

The thermal expansion of directionally solidified NiAl–Mo eutectic alloys consisting of nanoscale Mo fibers embedded in a NiAl matrix was analyzed by neutron diffraction and dilatometry. From room temperature to 800°C, perpendicular to the fiber direction, the NiAl and Mo phases expand independently with average coefficients of thermal expansion (CTEs) of 16.0×10−6°C−1 and 5.8×10−6°C−1, respectively. Parallel to the fiber direction, they coexpand up to 650°C with an average CTE of 12.8×10−6°C−1, but above this temperature the Mo fibers expand more than the NiAl matrix. This anomalous behavior is the result of the load transfer to the Mo fibers when the NiAl matrix softens. The average CTE of the composite parallel to the fiber direction was determined by dilatometry to be 13.0×10−6°C−1, which is approximately 11% lower than the value predicted by a simple rule of mixtures using the CTEs of the constituent phases.

Microstructures and mechanical properties of a directionally solidified NiAl–Mo eutectic alloy

A NiAl–Mo ternary eutectic alloy, having the nominal composition Ni–45.5Al–9Mo (at.%), was directionally solidified in a high-temperature optical floating zone furnace. Well-aligned rod-like microstructures were obtained, consisting of NiAl matrix and 14% (by volume) continuous Mo fibers having a square cross-section. With increasing growth rate (from 20 to 80 mm/h), the spacing and size of the Mo fibers decreased, from ≈2 to 1 μm (fiber spacing) and 800 to 400 nm (fiber size). X-ray microprobe analyses revealed that the NiAl matrix contained essentially no Mo (<0.1 at.%) and had the composition Ni–45.2Al (at.%), while the composition of the Mo fibers was Mo–10.1Al–3.9Ni (at.%). From electron backscatter diffraction patterns, the following orientation relationships were obtained: 〈1 0 0〉 NiAl//〈1 0 0〉 Mo, and {0 1 1} NiAl//{0 1 1} Mo. The growth directions and Mo/NiAl interface boundaries were found to be parallel to 〈1 0 0〉 and {0 1 1}, respectively, in both Mo and NiAl. Nanoindentation was used to probe the mechanical properties of the individual phases in the eutectic microstructure and the modulus and hardness of the NiAl matrix were determined to be 180 and 2.9 GPa, respectively, and those of the Mo fibers 270 and 4.8 GPa. Tensile tests were used to investigate the temperature dependence of the strength and ductility of the composite. Its ductile-to-brittle transition temperature was found to be ∼675 °C, and its yield strength about 25–30% higher than that of 〈1 0 0〉 NiAl single crystals over the temperature range 600–1000 °C.

Microstructures and Mechanical Properties of NiAl-Mo Composites

ABSTRACTIn-situ composites consisting of ∼14 vol.% continuous Mo fibers embedded in a NiAl matrix were produced by directional solidification in a xenon-arc-lamp, floating-zone furnace. The fiber spacing and size were controlled in the range 1–2 μm and 400–800 nm, respectively, by varying the growth rate between 80 and 20 mm/h. Electron back-scatter diffraction patterns from the constituent phases revealed that the growth directions and interface boundaries exhibited the following orientation relationships: 〈l00〉NiAl//〈100〉Mo and {011}NiAl//{011}Mo. The temperature dependence of the tensile strength and ductility were investigated and the NiAl-Mo composite was found to be both stronger and have a lower ductile-brittle transition temperature than the unreinforced NiAl matrix.

Thermal cycling induced deformation and damage in Mo-fiber reinforced aluminum composites

Unidirectional Mo-fiber reinforced aluminum composites were subjected to thermal cycling tests. The axial length of the composite increases with increasing thermal cycles. The Mo fibers remain intact and the interfacial reaction between fiber and matrix is insignificant after thermal cycling. To relax the internal stress built up in the composite due to differential thermal expansion of the composite constituents, cracks are developed along the fiber–matrix interface and propagate into the aluminum matrix. The formation of interfacial cracks causes reduced density with increasing thermal cycles. Thermally cycled specimens also show reduced tensile strength, which could be related to the presence of interfacial cracks in the composite.

Rotation of Mo-fibers in directionally solidified Ni-Al-Mo eutectic alloys

The interface structure of α-Mo fib ers in directionally solidified Ni-Al-Mo eutectic alloys has been investigated by transmission electron microscopy and computer-aided graphical techniques. The Mo fibers have square or rectangular cross sections with well defined flat facets and fundamentally have the Bain orientation relationship with respect to the γ matrix lattice. The crystal lattices of the Mo fibers, however, are often rotated with respect to the matrix lattice about their longitudinal axes. Correspondingly, the facet planes deviate from the exact rational (1 1 0)γ′//(1 0 0)α relationship. A computer aided graphical plotting of the atom arrangement at the interfaces suggests that good atomic matching is kept even after the rotation of fibers. High resolution electron microscopy revealed the incorporation of single atomic structural ledges to keep the coherency of the lattices of the matrix and rotated fibers. The relation between the lattice rotation and the facet plane rotation is well explained by the moire and O-lattice models.

Validation of multi-concentric cylinder model for sapphire/Mo/NiAl composites

Fiber coatings have been extensively modeled and proposed as a means to mitigate the thermal residual stresses in advanced composites, including the sapphire-fiber reinforced NiAl (sapphire/NiAl) system. While the finite element models have been useful for screening candidate coating materials, few, if any, experimental studies have been performed to validate the results of the models. The residual stress distribution in sapphire/NiAl composites with three thickness’ of Mo fiber coatings was modeled using the multiple concentric cylinder model (MCCM) from NASA-LeRC. The modeling results were compared to experimental measurements made using both continuous indentation and neutron diffraction. In this study, good agreement between the measured and calculated residual stress distributions in all samples was obtained. The results demonstrated the potential for continuous indentation to measure the local stress variations in composites both in the matrix and in the fiber coatings, and validated the calculations of the models for both local and average residual stresses.

Microstructures and mechanical properties of directionally solidified NiAl–Mo and NiAl–Mo(Re) eutectic alloys

Abstract The room temperature fracture toughness of a directionally-solidified (DS) NiAl–9 at.%Mo eutectic alloy was evaluated. It was found that the toughness of NiAl-refractory metal eutectic composites may be limited by the low ductility of the reinforcing bcc metal phase at room temperature and weak interfaces. The effects of the residual interstitial impurities and solid–solution-strengthening of Mo fibers by Ni and Al which partition to the fibers from the matrix on the mechanical properties were identified. The potential of rhenium (Re) addition, which is known to solid–solution soften and lower the ductile-to-brittle transition temperature of various bcc metals, to enhance the ductility and toughness of DS NiAl–9Mo eutectic alloy was also investigated. Re partitioned to the bcc metal phase and formed a substitutional solid solution. The interface morphology was changed from a faceted to a non-faceted one. Re alloying caused softening of the Mo fibers, and as a result the NiAl–Mo(Re) alloys were softer in compression and had ∼20% higher fracture toughness values as compared to the transverse orientation toughness of NiAl–9Mo alloy. The toughness of the NiAl–Mo(Re) alloys was lower than the longitudinal orientation toughness of the NiAl–9Mo alloy due to the poor alignment of the Mo(Re) phase with the growth direction. Processing conditions were successfully changed to produce plane front solidification resulting in completely eutectic microstructures with the fibers well-aligned with the growth direction in the NiAl–9Mo alloy; similar effort is needed for NiAl–Mo(Re) alloys to achieve higher toughness in the longitudinal orientation. The toughening mechanisms operative in these alloys are discussed.

Interface shear properties and toughness of NiAl/Mo laminates

Mechanical properties at the interface between NiAl intermetallic and Mo ductile phase in sandwich laminates have been studied under a variety of stress states involving primarily shear. A shear/compression test for laminates of NiAl/Mo/NiAl has been used in which a double-notched specimen with cut-through metal interlayer is loaded to failure in compression. Because of the large elastic mismatch between NiAl and Mo, a primarily shear loading generates a significant amount of normal stress in this test. Interface shear response and toughness obtained using this technique are compared for NiAl/Mo laminates with interfacial segregant, Cr, Re and C layers fabricated by high temperature diffusion bonding. A microstructural investigation of the interfacial reaction zone and the shear failure path is conducted to identify failure mechanisms. Failure strength and related interfacial toughness is examined using an FEM model of local stress state close to the notch in the bimaterial shear/compression specimen geometry. A trend of increasing bond strength with reducing notch spacing is identified and analyzed in terms of local stress state, the onset of microyielding and crack shielding. Thus, a new fracture stress locus has been developed for bimaterial interfaces. Residual stress due to CTE thermal expansion mismatch is measured and the contribution of thermal residual stress to the failure behavior of the laminate is explored. The combined effect of stress concentration at crack tip, residual stress and slip discontinuity across interface have been used to explain the apparently brittle loading response and easy debonding of Mo fibers in eutectic NiAl/Mo alloys.

Effects of Rhenium Alloying on the Microstructures and Mechanical Properties of Directionally Solidified NiAl-Mo Eutectic Alloy

ABSTRACTLow ductility of the reinforcing bcc metal phase at room temperature and weak interfaces can limit the intrinsic toughness and ductility of NiAl-bcc metal eutectic composites. The potential of rhenium (Re) addition, which is known to solid solution soften and lower the ductile-to-brittle transition temperature of various bcc metals, to enhance the ductility and toughness of a directionally solidified NiAl-9 at.% Mo eutectic alloy was investigated. Re partitioned to the bcc metal phase and formed a substitutional solid solution. The interface morphology was changed from a faceted to a non-faceted one. Re alloying caused softening of the Mo fibers, and as a result NiAl-Mo(Re) alloys were softer in compression and flexure and had ∼20% higher fracture toughness values as compared to the transverse orientation toughness of NiAl-9Mo alloy. The toughness of the NiAl-Mo(Re) alloys was lower man the longitudinal orientation toughness of the NiAl-9Mo alloy due to the poor alignment of the Mo(Re) phase with the growth direction. The toughening mechanisms have been evaluated and schemes for processing NiAl-Mo(Re) alloys for higher toughness in the longitudinal orientation are suggested. The role of the residual interstitial impurities and partitioning of Ni and Al to Mo fibers on the mechanical properties are highlighted.

On the fatigue response of bridging ductile fibers in an MoSi 2 intermetallic composite

The fatigue response of bridging molybdenum fibers in an MoSi 2 matrix has been investigated. The composite consists of a MoSi 2-40% SiC matrix reinforced with alumina coated Mo fibers. Previous work demonstrated that the ductility and interfacial debonding of coated Mo fibers promoted high monotonic fracture resistance based on a bridging mechanism. The current study shows that debonding ductile fibers have also the potential to give adequate fatigue crack growth resistance. A tensile test was devised to measure the opening of a bridged crack as a function of number of cycles. The results suggest that if the applied stress is below a threshold stress governed by the flow stress of the ductile fibers, then the crack opening remains constant after a large number of cycles. This information can be used, in principle, to predict the crack growth rate in composites.

Fracture Toughness of NiAlIn-SituEutectic Composites.

ABSTRACTMechanical tests were performed on directionally solidified (DS) NiAlin-situeutectic composites in order to evaluate the effect of ductile reinforcements on the fracture resistance of the B2 ordered intermetallic compound NiAl. Reinforcements consisted of i) Mo fibers, ii) Cr fibers, and iii) Cr(Mo) solid solution plates. Near stoichiometric NiAl ingots were prepared by induction melting as reference material to compare with the eutectic composites. Resistance curves were obtained for the NiAl/Mo fibrous eutectic alloy as well as for the NiAl/Cr(Mo) layered material. The initiation fracture toughness of the DS NiAl/Mo and NiAl/Cr(Mo) eutectic composites is larger than that of the stoichiometric NiAl, with the layered material. producing the better properties. The mechanisms for such increase in fracture toughness are discussed.