Ki Buem Kim

Heterogeneity of a Cu47.5Zr47.5Al5 bulk metallic glass

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 10–20nm 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 5nm 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.

High-strength Ti-base ultrafine eutectic with enhanced ductility

(Ti0.705Fe0.295)100−xSnx (x=0 and 3.85) ultrafine eutectics were prepared by slow cooling from the melt through cold crucible casting. The addition of 3.85 at. % Sn to the binary Ti–Fe eutectic decreases the strength slightly but considerably improves the plastic deformability under uniaxial compressive loading from ef=2.1% to 9.6% strain to failure. The change in the morphology of the eutectic and the distribution of the FeTi phase are suggested as origin of the improvement of the mechanical properties.

Modeling deformation behavior of Cu–Zr–Al bulk metallic glass matrix composites

In the present work we prepared an in situ Cu47.5Zr47.5Al5 bulk metallic glass matrix composite derived from the shape memory alloy CuZr. We use a strength model, which considers percolation and a three-microstructural-element body approach, to understand the effect of the crystalline phase on the yield stress and the fracture strain under compressive loading, respectively. The intrinsic work-hardenability due to the martensitic transformation of the crystalline phase causes significant work hardening also of the composite material.

Deformation-induced rotational eutectic colonies containing length-scale heterogeneity in an ultrafine eutectic Fe83Ti7Zr6B4 alloy

Microstructural investigations of an ultrafine eutectic Fe83Ti7Zr6B4 alloy reveal that spherical eutectic colonies are composed of nanoeutectic areas which are encapsulated by submicron eutectic areas indicating length-scale heterogeneity of lamellar structure. Furthermore, formation of the wavy and discontinuous shear bands indicative of dissipation of the shear stress are possibly caused by a rotation of the eutectic colonies along the submicron eutectic areas during deformation, leading to developing typical dimples on the fracture surface. The rotation of the eutectic colonies containing the length-scale heterogeneity is proposed to be responsible for macroscopic plasticity of the ultrafine eutectic Fe83Ti7Zr6B4 alloy.

Formation of a bimodal eutectic structure in Ti–Fe–Sn alloys with enhanced plasticity

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%.

Work hardening ability of ductile Ti45Cu40Ni7.5Zr5Sn2.5 and Cu47.5Zr47.5Al5 bulk metallic glasses

Ductile Ti45Cu40Ni7.5Zr5Sn2.5 and Cu47.5Zr47.5Al5 bulk metallic glasses (BMGs) present different work hardening abilities under compression. Microstructural investigations reveal that nanoscale chemical heterogeneities occur throughout the samples. The morphology of the chemically heterogeneous domains in the as-cast Ti45Cu40Ni7.5Zr5Sn2.5 BMG is irregular and significantly interconnected. In contrast, the as-cast Cu47.5Zr47.5Al5 BMG exhibits a spherical morphology of the chemically heterogeneous regions. Furthermore, the distribution of the nanoscale chemical heterogeneity is macroscopically inhomogeneous throughout the material. These findings suggest that the different work hardening abilities of the Ti45Cu40Ni7.5Zr5Sn2.5 and Cu47.5Zr47.5Al5 BMGs possibly originate from the different morphologies and distributions of the chemically heterogeneous regions.

Propagation of shear bands and accommodation of shear strain in the Fe56Nb4Al40 ultrafine eutectic-dendrite composite

Microstructural investigations of Fe56Nb4Al40 ultrafine eutectic-dendrite composite reveal that its high room temperature plasticity mainly originates from the evolution of slip in the dendrites and multiple shear banding in the ultrafine eutectic matrix, respectively. Here, we sequentially describe that the shear bands in the ultrafine eutectic matrix are propagated with generation of strain field in the soft alpha-Fe(Al) layers rather than passing through by sharp shear banding in the hard (Fe,Al)2Nb intermetallic layers. Consequently, step morphologies at the interfaces of the alternating lamellae and the dendrite/eutectic matrix strongly support the effective accommodation of shear strain during shear band propagation.Microstructural investigations of Fe56Nb4Al40 ultrafine eutectic-dendrite composite reveal that its high room temperature plasticity mainly originates from the evolution of slip in the dendrites and multiple shear banding in the ultrafine eutectic matrix, respectively. Here, we sequentially describe that the shear bands in the ultrafine eutectic matrix are propagated with generation of strain field in the soft alpha-Fe(Al) layers rather than passing through by sharp shear banding in the hard (Fe,Al)2Nb intermetallic layers. Consequently, step morphologies at the interfaces of the alternating lamellae and the dendrite/eutectic matrix strongly support the effective accommodation of shear strain during shear band propagation.

Propagation of shear bands in Ti66.1Cu8Ni4.8Sn7.2Nb13.9 nanostructure-dendrite composite during deformation

During deformation of Ti66.1Cu8Ni4.8Sn7.2Nb13.9 nanostructure-dendrite composite, primary and secondary shear bands form under perpendicular orientation. Detailed investigation of the microstructure of deformed specimens reveals deformed body-centered-cubic (bcc) β-Ti dendrites forming a stepped morphology at the interfaces between the bcc β-Ti dendrites and the nanostructured matrix, consisting of hexagonal close packed (hcp) α-Ti and body-centered-tetragonal (bct) Ti2Cu phases. In the nanostructured matrix, the primary shear bands pass through coherent grain boundaries between the hcp α-Ti and the bct Ti2Cu phases. In contrast, the secondary shear bands in the nanostructured matrix are arrested by sandwiched nanoscale grains of the hcp α-Ti and bct Ti2Cu phases.

Deformation-induced nanostructuring in a Ti–Nb–Ta–In β alloy

Easy deformation-induced nanostructuring was found in a Ti–Nb–Ta–In β alloy with low stability against α″ martensitic transformation. Upon severe plastic deformation at the sample center, the reversible β→α″ martensitic transformation plays a significant role for grain refinement. A possible mechanism is proposed, in which the formation of fine martensite, the interaction among slip dislocations, martensite and twins, and the reversible transition from α″ back to β phase are considered as the main causes leading to pronounced grain refinement to the nanoscale.

High strength Ni–Zr binary ultrafine eutectic-dendrite composite with large plastic deformability

A Ni–8Zr high strength ultrafine eutectic-dendrite composite with large plasticity has been developed in the Ni–Zr binary eutectic system. The excellent mechanical properties are attributed to the specific heterogeneous microstructure with distinctly different length scales, i.e., micrometer-size ductile dendrites combined with an ultrafine eutectic matrix. The plastic deformation mainly proceeds through a shear banding mechanism. However, there is no significant shear localization due to the constraint effect of ductile solid solution Ni phases including dendrites and/or alternating lamellar layers. Furthermore, excessive shear stress and accumulated shear strain can be effectively released and accommodated by delocalization and multiplication of shear bands.