Search

Abstract: A series of Ti–Nb–Ta–(In/Cr) and Ti–Fe–(Sn) alloys containing a bcc β-Ti phase with a grain size ranging from the nanometer/ultrafine regime to the micrometer scale has been prepared by slow cooling from the melt. The plastic deformation behavior has been investigated under compression and the deformation-induced microstructure evolution was checked by X-ray diffraction, scanning and transmission electron microscopy. The data reveal that the underlying mechanisms related to plasticity or brittleness of the β-phase depends significantly on the supersaturation, the local lattice strain, the ease of dislocation slip, as well as twinning and diffusionless transformation triggering grain refinement. The intrinsic structure, the short-range order and the stability of the β-Ti phase have a strong influence on the mode of plastic deformation of the investigated Ti-alloys. [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: A Ti67.79Fe28.36Sn3.85 (at.%) alloy has been prepared through arc-melting and solidification at a cooling rate of 40–10K/s. The as-prepared microstructure exhibits a two-phase ultrafine eutectic consisting of FeTi (B2) and β-Ti (A2) phases. The mechanical properties (maximum strength, σ max =1939MPa; strain to fracture, ɛ f =13.5%) are considerably improved if compared with the model Ti70.5Fe29.5 eutectic alloy (σ max =1733MPa, ɛ f =3.4%). The improvement of plastic deformability is assessed in terms of a detailed investigation of the microstructure and fractographic studies. The presence of Sn is believed to boost the ease of slip transfer across the interface between the A2/B2 phases due a higher lattice mismatch between the structures. [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]

The present study demonstrate the effect of stacking fault energy (SFE) on the evolution of crystallite size, dislocation density, twin density, stacking faults, and their mutual interactions during cryorolling in pure copper (SFE 78 mJ/m 2 ) and copper–zinc alloys with varying Zn content of 10 wt% (SFE 35 mJ/m 2 ), 20 wt% (SFE 19 mJ/m 2 ), and 30 wt% (SFE 14 mJ/m 2 ). The dislocation density, twin density and extrinsic fault probability increase whereas crystallite size decreases upon cryorolling and with the decrease of SFE. Accumulation of dislocations at the twin boundaries causes dynamic recrystallization, promotes grain refinement by forming subgrains inside the twin lamellae. Detail investigation on the activity of structural defects during cryorolling of Cu/Cu-Zn alloys has been done through resistivity measurements. Nanoindentation studies have performed to understand the deformation kinetics of the cryorolled Cu/Cu-Zn alloys with high concentration of structural defects in term of strain rate sensitivity and activation volume. A micromechanical model has been used to demonstrate the effect of SFE and the role of structural defects during nanostructuring at cryogenic temperature. [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]

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: Phase formation in the Cu47Ti33Zr11Ni8Si1 alloy synthesized by various solidification techniques has been studied using X-ray diffraction (XRD), differential scanning calorimetry (DSC) and transmission electron microscopy (TEM) paying, in particular, attention to the TEM sample preparation. While a reduced cooling rate results in a duplex microstructure, consisting of nanocrystals in an amorphous matrix, glass formation is achieved upon employing a faster quenching rate. The difference in composition and quench temperature influence the phase transition upon heating. The essential differences observed in TEM, notwithstanding the ion-milling effects and the different pre-alloy ingots used for metallic glass synthesis, can be linked to differences in the nucleation and growth mechanism which, in turn, is influenced by the chemical short range order (CSRO) obtained upon quenching. [Copyright &y& Elsevier]

Abstract: Ni60Nb40 metallic glass powders were prepared by mechanical alloying and subsequently combined with Al to form an Al–30wt.% Ni60Nb40 metal matrix composite (MMC). The crystallization of the Ni60Nb40 metallic glass reinforcements and their reaction with the Al matrix during preparation of the MMC were investigated in a differential scanning calorimeter (DSC) upon isothermal annealing. The comparison of the DSC curves of the Ni60Nb40 metallic glass and of the Al–30wt.% Ni60Nb40 composite shows that the amorphous Ni60Nb40 particles have a very strong resistance against reaction with Al but reaction occurs after crystallization of the metallic glass. The analysis of the isothermal annealing DSC curves gives an activation energy of crystallization of the Ni60Nb40 metallic glass of 724kJ/mol. The interfacial reaction between Al and the Ni60Nb40 reinforcements is mainly controlled by diffusion. X-ray diffraction (XRD) and scanning electron microscopy (SEM) results indicate that during the interfacial reaction, Ni diffuses from the Ni60Nb40 particles to react with Al and form Al3Ni. At the same time, Al diffuses into the reinforcement particles to react with remaining Nb and form Al3Nb. [Copyright &y& Elsevier]