Search

Arc-melted multiphase intermetallic alloys, Mo76Si14B10, Mo74Si14B10Zr2, Mo79.5Si12B8.5 and Mo77.5Si12B8.5Zr2 with microstructures containing Moss, Mo3Si and Mo5SiB2 were isothermally exposed for 1 h or 24 h at 1000 °C, 1150 °C and 1300 °C in dry or moist air. The residual area fraction was found to be the highest on exposure at 1150 °C in dry air, but the least at this temperature in moist air. Examination of dry air formed oxide scales by X-ray diffraction, and scanning electron microscopy with energy-dispersive spectroscopy showed the presence of Mo, and MoO2 with a continuous, protective glassy B2O3–SiO2 top layer. Formation of Zr(MoO4)2 in the Zr-containing alloys had lowered mass loss by arresting MoO3 vaporization, and had enhanced the B2O3–SiO2 scale formation kinetics. Further, Si(OH)4, cristobalite, H3BO3 and MoO3·(H2O)2 appeared to be constituting the oxidation products formed in moist air. Alloying with Zr enhanced the kinetics of protective scale formation by lowering the Moss content, and microstructural refinement. The deterioration of oxidation resistance in moist air was conjectured to be due to longer duration required for protective scale to form, formation of β-cristobalite permitting grain boundary diffusion of oxygen anions, as well as volatile hydrated oxidation products as mentioned above. Formation of ZrSiO4 and dissolved ZrO2 in B2O3–SiO2 scale of the Mo-Si-B-Zr alloys in both dry air and moist air is considered to have contributed to the enhanced protective character of oxide scale by stabilizing the glassy scale. [ABSTRACT FROM AUTHOR]

A mechanical mixture of alumina and pulverized zircon sand in 3:2 M ratio has been plasma sprayed to obtain mullite coating. Thereafter, the top layer of the coating has been remelted using laser. The presence of a mullite phase in the as-sprayed and laser remelted coatings has been confirmed qualitatively using X-ray diffraction. Both as-sprayed and laser remelted coatings have been characterized for their microstructure, hardness and porosity. The ultrafine grain structure of the coating produced by rapid quenching has been analyzed using transmission electron microscope. Presence of a mullite phase in the coatings has also been confirmed using small angle electron diffraction. Laser remelting has resulted in an appreciable reduction in porosity and increase in hardness in the coatings. [ABSTRACT FROM AUTHOR]

Highlights: [•] Effect of Sn on the indentation size effect in Ti–Fe–(Sn) has been explored. [•] Fitting through ISE models point the existence of strain gradient beneath indenter. [•] Correlation has been established between dislocation density, grain size and ISE. [•] ISE in Ti–Fe–Sn ultrafine composites requires a dislocation density of ∼1014–1016/m2. [Copyright &y& Elsevier]

Abstract: Mn-doped ZnO samples having composition Zn1−x Mn x O (x=0.02, 0.04 and 0.05) were synthesized by solid state reaction technique with varying concentration of Mn from 0.02 to 0.05. Evidence of room temperature ferromagnetism was observed only in the composition Zn0.98Mn0.02O sintered at 500°C. Our XRD pattern confirms the presence of Mn3O4 impurity phase in all the Zn1−x Mn x O samples with the exception of Zn0.98Mn0.02O. We emphasize that the appearance of Mn3O4 phase in the system forbids the exchange type of interaction between the Mn ions and suppresses the ferromagnetism in all the Mn over-doped Zn1−x Mn x O (x>0.02) system. SEM microstructure study also supports the interruption of exchange type of interaction inside the system with the increase in Mn concentration in the sample. Interestingly, for this particular composition, Zn0.98Mn0.02O sintered at 500°C, glassy ferromagnetism type of transition is observed at low temperature. This type of transition is attributed to the formation of the oxides of Mn clusters at low temperature. [ABSTRACT FROM AUTHOR]

Abstract: A comparative study has been carried out to investigate the effects of Al and/or Ce additions on microstructure of Mo–Si–B alloys and their isothermal oxidation behaviour at 500 and 700 °C in laboratory air for 24 h. Microstructure of arc melted Mo–Si–B–(Al, Ce) alloys consists of bcc α-Mo dendrites embedded in ultrafine lamellar Mo3Si and Mo5SiB2 eutectic matrix. Isothermal oxidation kinetics of ultrafine structured Mo–Si–B alloy at 500 °C has been found to show hardly any mass change during 24 h exposure. Addition of Al to Mo–Si–B alloy refines the microstructure, decreases the net mass loss at 700 °C by ∼43%, whereas Ce does not bring about any significant change. The enhanced oxidation resistance of Mo–Si–B–Al alloy is due to the formation of Al–O rich inter-layer at the alloy/oxide interface along with the formation of a protective and dense Al2(MoO4)3 outer layer, which reduces the sublimation of MoO3 at 700 °C. Various transient/complex oxides formed on the alloys during their high temperature exposure have been examined to determine the oxidation mechanisms. [ABSTRACT FROM AUTHOR]

Abstract: Mechanical prehistory is an important factor on the stability of the structure, and consideration of this issue is also useful for determining the mechanisms of post-deformation and fracture processes in bulk metallic glasses. We present investigations to reveal the effect of prestraining on the compressive deformation, morphology of the fracture surface and the evolution of microstructural inhomogeneity in Zr44Ti11Cu9.8Ni10.2Be25 (Vitreloy 1b) “brittle” bulk metallic glass (BMG). The intrinsic plasticity of the BMG increases linearly from 0.7% up to 15% plastic strain upon cold rolling up to 30% prestrain at room temperature due to the change of the short-range order of the glassy phase. Regions of river-like patterns appeared on the fracture surface of prestrained BMGs after compression test, which is similar to that appear during tensile deformation indicating the development of a deformation-induced inhomogeneous residual stress distribution. The effect of uniaxial cyclic compression on the flow stress of the as-cast BMG is also investigated. [ABSTRACT FROM AUTHOR]

Abstract: Ultrafine structured Ti70.5Fe29.5 and Ti67.79Fe28.36Sn3.85 eutectic rods comprising of β-Ti and TiFe phases were prepared by copper mould casting. The corrosion and passivation behaviour were investigated in halide free aqueous media (pH 1–13) by means of potentiodynamic polarisation and potentiostatic current transient measurements. Both eutectic alloys exhibit excellent corrosion resistance due to spontaneous passivation. The passive film compositions grown several nanometres on single phase polycrystalline specimens were analyzed with Auger electron spectroscopy. They consist of oxidized Ti and Fe species. Layers grown in strongly acidic environment are depleted in Fe. Sn was detected only on layers grown on the constituent β-Ti phase and only in the metallic state. The pitting susceptibility was investigated in strongly acidic media (pH≤1) containing 0.1–3M of chloride ions. The Sn-containing eutectic alloy was found to be more susceptible to pitting corrosion. [Copyright &y& Elsevier]

Abstract: To circumvent the limited plasticity of bulk metallic glasses (BMGs), heterogeneous materials with glassy matrix and different type and length-scale of heterogeneities (micrometer-sized second phase particles or fibers, nanocrystals in a glassy matrix, phase separated regions, variations in short-range order by clustering) have been reported. We developed bulk metallic glasses and glass matrix composites in martensitic Zr–Cu-base alloys. Large plasticity can be obtained from microstructure consisting of either a glassy structure, or for alloys with martensitic second phase embedded in a glassy matrix. This type of glasses and glass–matrix composites are able to achieve high strength together with pronounced work-hardening. We explore the possibilities to synthesize such in situ composite microstructures based on shape memory alloys (“M-Glasses”) through metal mold casting. [Copyright &y& Elsevier]

Abstract: Investigation of the microstructure of a compressed Ti66.1Cu8Ni4.8Sn7.2Nb13.9 nanostructure–dendrite composite reveals that the deformed bcc β-Ti dendrites form a stepped morphology at the interfaces between the individual dendrites and the nanostructured matrix of hcp α-Ti and bct Ti2Cu phases. Furthermore, there is a unique stained contrast during the propagation and the interaction of the shear bands in the dendrites indicating a local structural change near to the apex of the shear bands in the dendrite. In contrast, the shear bands pass through coherent grain boundaries between the hcp α-Ti and the bct Ti2Cu phases of the nanostructured matrix. [Copyright &y& Elsevier]

Abstract: The deformation behavior of multicomponent Ti66Cu8Ni4.8Sn7.2Nb14 composite containing primary dendrites embedded in a nanoeutectic matrix was studied under uniaxial compression. The evolution of slip and shear bands has been investigated at various stages of deformation. This type of advanced composite not only displays a high compressive plasticity of 31.8%, but also exhibits high fracture strength of ≈2.02GPa (true fracture strength 1.38GPa). Pronounced work hardening was observed after yielding. The surface deformation morphology reveals that the work hardening behavior of the composite is related to the plastic deformation of the dendritic phase through slip bands and the interaction of shear bands in the nanostructured matrix with the hardened dendrites. The nucleation of shear band occurs at a certain amount (8%) of plastic deformation at the dendrite–nanoeutectic matrix interface. The final stage of compression proceeds through passing of the shear bands through the work-hardened dendrites into the nanoeutectic matrix, whereby the propagation of the shear bands in the matrix is retarded by the nanoscale phases. [Copyright &y& Elsevier]

Abstract: Zr73.5Nb9Cu7Ni1Al9.5 (numbers indicate at.%) nanostructure-dendrite composites were fabricated using three different casting techniques: suction casting, centrifugal casting and arc-melting. The microstructure of the suction casting sample consists of micrometer-scale dendrites in a nanostructured matrix. However, some areas in the matrix contain nano-scale crystals together with an amorphous phase revealing an inhomogeneity of the specimen. On the contrary, the microstructures of the centrifugally-cast and the arc-melted samples are overall homogeneous, consisting of micrometer-scale dendrites that are homogeneously distributed in a nanostructured matrix. The dendrites correspond to a body centered cubic (bcc) β-Zr phase, whereas the nanostructured matrix consists of body centered tetragonal (bct) Zr2Cu-type and hexagonal close-packed (hcp) MgZn2-type phases. The comparison of the microstructures of these two alloys reveals that the formation of nano-scale twins and a disordered ω-phase in the β-Zr dendrites only happens in the centrifugally-cast sample. The differences in the phases and the microstructures between the differently prepared samples significantly influence the corresponding mechanical properties of the specimens. [Copyright &y& Elsevier]

Abstract: Bulk nanostructured/glassy Ti-base multicomponent alloys are attractive advanced high-strength materials for structural applications or as functional materials, e.g. for medical applications. Typically, the strength and elastic strain limits of such alloys are significantly higher than for conventional microcrystalline alloys. It is of strong interest to develop a one-step process that can directly produce such alloys in bulk form, e.g. through casting, with an in situ formed composite microstructure that combines phases with different desirable properties. Along this line, a series of (Ti–Zr–Ta)–Cu–Ni alloys designed from glass-forming alloy compositions with different volume fractions of β-Ti phase have been investigated. The introduction of the β-Ti dendritic solid solution phase enhances the ductility of the material. A further improvement can be achieved by adding isomorphous β-phase stabilizers in (Ti–Nb–Ta)–Si/In alloys. These alloys exhibit low elastic modulus (50GPa) and very large deformability (>100% plastic strain) at room temperature without appearance of any microcracks. The microstructure of these alloys consists of a high volume fraction of bcc β-Ti phase coexisting with a nanostructured interdendritic matrix. The superplastic-like deformability of the near beta alloys is correlated with copious twinning in the micrometer-size β-Ti-dendrites leading to a nano/ultrafine filamentary structure. [Copyright &y& Elsevier]

Abstract: Systematic investigations of the microstructural changes upon compression have been performed in order to elucidate the microscopic deformation mechanisms of the high-strength and ductile Ti66.1Nb13.9Ni4.8Cu8Sn7.2 nanostructure–dendrite composite. After 8% deformation, a rotation of β-Ti dendrites is observed during the interaction of slip and shear bands. This rotation leads to the formation of new slip bands in the dendrites. The β-Ti dendrites locally transform into ω phase during the interaction between the newly and the previously formed slip bands. The rotation of the dendrites causes local volume changes at the interfacial areas between the dendrites and the matrix, consisting of a mixture of hexagonal close-packed α-Ti and body-centered tetragonal Ti2Cu phases. This induces a rotational stress into the nanostructured matrix. After further deformation up to 25%, the shear bands penetrate the dendrite/matrix interfaces, producing extra interfaces. The nanostructured matrix exhibits a sandwiched microstructure in order to accommodate effectively the shear strains. These results suggest that structural coherency of the dendrite/matrix interfaces is important for controlling both the strength and the ductility of the nanostructure–dendrite composite. [Copyright &y& Elsevier]

Microstructural investigation of an as-cast Cu47.5Zr47.5Al5 bulk metallic glass (BMG) reveals two amorphous phases formed by liquid phase separation. The morphology of the phase separated amorphous regions is spherical with 10–20 nm in size. These areas are homogeneously distributed throughout the sample. Moreover, a macroscopic heterogeneity also occurs along with the nano-scale liquid phase separation. The macroscopic heterogeneity can be distinguished from the different degree of the chemical fluctuations in the sample, and the existence of nano-scale crystals of less than 5 nm in size. Presumably, both the macroscopic heterogeneity and the nano-scale phase separation enhance branching of the shear bands during deformation in the Cu47.5Zr47.5Al5 BMG. [ABSTRACT FROM AUTHOR]

We report the microscopic mechanism of lamellae deformation in a series of (Ti 0.705 Fe 0.295 ) 100− x Sn x (0 ⩽ x ⩽ 4) eutectic comprising of ultrafine FeTi and β-Ti phases with varying lamellae thickness. Furthermore, the microstructural refinement and phase rearrangement upon severe forging have been explored using X-ray diffraction and transmission electron microscopy to reveal the dislocation activity inside the lamellae interior and at the β-Ti/FeTi interface. Nanoindentation tests have been performed at control loading rate and control strain rate modes to evaluate the strain rate sensitivity, activation volume, and to compare them with that of the constituent single phases. The strain rate sensitivity slightly decreases upon Sn addition and with the decrease of the lamellar spacing. However, severe forging increases the hardness, reduces the activation volume without any change of the strain rate sensitivity. The role of dislocation accumulation in the ultrafine lamellae, inter-lamellar sliding, formation of sub-boundaries, evolution of shear bands, cutting, and phase rearrangement towards the deformation-induced nanostructuring of the ultrafine lamellar composites, have been discussed using a micromechanical model. [ABSTRACT FROM AUTHOR]

The intrinsic plasticity of Zr44Ti11Cu9.8Ni10.2Be25 and Zr55Ti5Al10Cu20 Ni10 bulk metallic glasses (BMGs) are improved from 0.5% up to 15% plastic strain due to the introduction of microstructural inhomogeneities upon cold rolling at room temperature. This approach shows an easy way to overcome the intrinsic brittleness of the BMGs by modifying their physical properties, which enables easy nucleation and branching of multiple shear bands upon unconstrained loading during the compression test. [Copyright &y& Elsevier]

The effect of Fe addition and the moist environment on the high temperature isothermal oxidation behaviors of multiphase Mo 76- x Si 14 B 10 Fe x (x = 0, 0.5, 1 at.%) composites in the range of 1000–1300 °C have been investigated. The microstructure of all the composites with α–Mo solid solution (SS) and eutectic mixture of Mo 3 Si + Mo 5 SiB 2 phases, has been refined upon Fe addition, which improves the oxidation resistance and reduces the mass loss up to 55% in dry air and 31% in moist air for 24 h exposure. Whereas, the oxidation resistance in moist air improves due to the formation of a protective Fe-rich glassy borosilicate scale, which reduces the vaporization of MoO 3 by forming Fe 2 (MoO 4) 3. The cross-sectional area of the residual alloy for x = 1 were estimated to be 97% in dry air and 85% in moist air that of unoxidized coupons. The bulk hardness of the as-cast composite is 1031 Hv for x = 1, which reduced only by 6.7% (962 Hv) upon oxidation at 1300 °C in both dry and moist air, showing superior stability of the silicide matrix composites for high temperature application. X-ray diffraction, scanning electron microscopy and energy dispersive x-ray spectroscopy studies were performed to identify the various products of oxidation and to explore the mechanism of protection. • Oxidation behaviour of Mo 76- x Si 14 B 10 Fe x at 1000–1300 °C has been studied. • Moist air improves the mass loss by 31% during isothermal oxidation. • Fe addition improves the oxidation resistance in both dry and moist air. [ABSTRACT FROM AUTHOR]

The evolution of microstructure and the mechanical properties of a series of (Ni 0.912 Zr 0.088 ) 100- x Al x (0≤ x ≤4) eutectic composites, constitute of γ-Ni and Ni 5 Zr nanolamellar phases, have been presented. Al dissolves in γ-Ni phase preferentially, decreases its hardness and refines the microstructure. Strain rate jump test was performed in order to investigate the rate sensitivity. It has been found that activation volume increases from 39 b 3 to 46 b 3 upon Al addition. The strain rate sensitivity of the composites has been estimated to be ~0.008. The scanning and transmission electron microscopic studies have confirmed that dislocation meditated flow in nano-lamellar phases dominates the plastic deformation mechanism. Analysis based on Stroh's pile-up model suggests that the required shear stress for slip decreases and that for cleavage crack nucleation increases around a dislocation pile-up at the lamellae interface, upon Al addition. The nano-lamellar Ni 5 Zr strengthen the composite, whereas, dislocation slip endorses the global plasticity of high strength Ni-Zr-(Al) nanoeutectic composites. [ABSTRACT FROM AUTHOR]

Addition of Fe refines the microstructure of Mo 76-x Si 14 B 10 Fe x ( x = 0, 0.5, 1 at.%) composites containing α-Mo, Mo 3 Si and Mo 5 SiB 2 phases, increases the hardness from 950 Hv ( x = 0) to 1031 Hv ( x = 1), and improves the oxidation resistance at temperature in the range of 800–1300 °C. The hardness of the base alloy substrate decreases only by <7% than that of as-solidified ingots, indicating good microstructural stability of the composite for high temperature application. [ABSTRACT FROM AUTHOR]

Abstract: Due to their exceptionally high yield strength and yield strain as compared to conventional metallic materials, bulk metallic glasses (BMGs) represent a class of promising engineering materials for structural applications. However, inhomogeneous deformation and severe shear localization at ambient temperature often lead to early failure and limit their reliability as structural materials. Heat treatments around the glass transition temperature (T g) generally aggravate the intrinsic brittleness of BMGs. In this paper, we report on the evolution of a nanoscale inhomogeneous microstructure upon low-temperature annealing in a Zr55Cu30Al10Ni5 BMG. This important outcome is explained by the experimentally observed tendency for chemical decomposition between Cu and Zr of the investigated amorphous system and is in accordance with literature data on various Zr–Cu-based amorphous alloys. Finally, these local fluctuations influence the plasticity of BMGs beneficially, in contrast of the generally accepted embrittlement upon annealing. [Copyright &y& Elsevier]

Abstract: Systematic investigations of the microstructure of Ni50Ti33Zr16Si1, Ni45Cu5Ti33Zr16Si1 and Ni40Cu10Ti33Zr16Si1 slowly cooled bulk specimens reveal the local occurrence of an amorphous phase in these alloys. The sites and the morphology of the amorphous phase strongly depend on the presence of Cu. The formation of a nanoscale amorphous phase in the Cu-free Ni50Ti33Zr16Si1 alloy is observed within the primary solidified Ni50(Ti,Zr)50 intermetallic compound with an isolated morphology. In contrast, in the Cu-containing alloys, Ni45Cu5Ti33Zr16Si1 and Ni40Cu10Ti33Zr16Si1, local amorphization occurs at negatively curved interfacial areas between the primary Ni50(Ti,Zr)50 grains and the Ni3Ti and NiTiZr matrix phases. Chemical analysis of the different phases in these alloys reveals that the amorphous phase is more Zr-rich compared to the primary Ni50(Ti,Zr)50 phase, indicating that the Zr content is very crucial for enhancing the glass-forming ability. However, Cu is decisive in controlling the volume fraction, the morphology and the sites of the amorphous phase in these alloys. [Copyright &y& Elsevier]

Microstructural investigations on a series of (Ti70.5Fe29.5)100-xSnx alloys with x=5, 7, and 9 reveal that Sn addition is effective in introducing both structural and spatial heterogeneities in ultrafine eutectic composites stemming from a large temperature difference between two eutectic temperatures upon solidification. The microstructural heterogeneities in these ultrafine eutectic composites strongly enhance the room temperature compressive plasticity up to ∼15.7%. [ABSTRACT FROM AUTHOR]