Jin-Yoo Suh

Designing metallic glass matrix composites with high toughness and tensile ductility

The selection and design of modern high-performance structural engineering materials is driven by optimizing combinations of mechanical properties such as strength, ductility, toughness, elasticity and requirements for predictable and graceful (non-catastrophic) failure in service. Highly processable bulk metallic glasses (BMGs) are a new class of engineering materials and have attracted significant technological interest. Although many BMGs exhibit high strength and show substantial fracture toughness, they lack ductility and fail in an apparently brittle manner in unconstrained loading geometries. For instance, some BMGs exhibit significant plastic deformation in compression or bending tests, but all exhibit negligible plasticity (<0.5% strain) in uniaxial tension. To overcome brittle failure in tension, BMG–matrix composites have been introduced. The inhomogeneous microstructure with isolated dendrites in a BMG matrix stabilizes the glass against the catastrophic failure associated with unlimited extension of a shear band and results in enhanced global plasticity and more graceful failure. Tensile strengths of ∼1 GPa, tensile ductility of ∼2–3 per cent, and an enhanced mode I fracture toughness of K1C ≈ 40 MPa m1/2 were reported. Building on this approach, we have developed ‘designed composites’ by matching fundamental mechanical and microstructural length scales. Here, we report titanium–zirconium-based BMG composites with room-temperature tensile ductility exceeding 10 per cent, yield strengths of 1.2–1.5 GPa, K1C up to ∼170 MPa m1/2, and fracture energies for crack propagation as high as G1C ≈ 340 kJ m-2. The K1C and G1C values equal or surpass those achievable in the toughest titanium or steel alloys, placing BMG composites among the toughest known materials.

Development of tough, low-density titanium-based bulk metallic glass matrix composites with tensile ductility

The mechanical properties of bulk metallic glasses (BMGs) and their composites have been under intense investigation for many years, owing to their unique combination of high strength and elastic limit. However, because of their highly localized deformation mechanism, BMGs are typically considered to be brittle materials and are not suitable for structural applications. Recently, highly-toughened BMG composites have been created in a Zr–Ti-based system with mechanical properties comparable with high-performance crystalline alloys. In this work, we present a series of low-density, Ti-based BMG composites with combinations of high strength, tensile ductility, and excellent fracture toughness.

Microstructural evolutions of the Al strip prepared by cold rolling and continuous equal channel angular pressing

Abstract A new metal forming technique termed the continuous confined strip shearing (C2S2) process based on equal channel angular pressing (ECAP) is introduced. Unlike the conventional batch-type ECAP process dealt in the earlier studies, the developed technique enables sheet forming in a continuous mode and in a repeatable manner. Likewise rolling, the developed process is able to control the mechanical properties of the material through the multipass operation. Both hardening and softening mechanisms in ECAP were investigated by correlating the measured hardness with microstructural evolutions of the Al specimens using transmission electron microscopy. The process conditions required to produce the fine-grained structure were suggested in terms of the strain per passage and the total strain imparted to the specimens. The effect of the temperature on the accelerated grain refining during the continuous ECAP operation was discussed.

Ferrite nucleation potency of non-metallic inclusions in medium carbon steels

Abstract In recent years, there has been renewed interest in the role of non-metallic inclusions in controlling the microstructure of steels. The potency of various inclusions and precipitates such as SiO 2 , MnO·SiO 2 , MnS, Al 2 O 3 , Ti 2 O 3 and VN for the nucleation of intragranular ferrite has been examined in the present study. Among them, single SiO 2 , MnO·SiO 2 , Al 2 O 3 , TiN and MnS particles seem to be inert to the nucleation of intragranular ferrite under the present experimental condition. Ti 2 O 3 particles in a Mn-containing steel are very effective for the nucleation of intragranular ferrite, being (Ti,Mn) 2 O 3 particles by absorbing Mn atoms from a steel matrix. On the other hand, Ti 2 O 3 particles in a Mn-free steel are not effective. MnS and Al 2 O 3 particles in high nitrogen steels containing vanadium also appear to be potent for the nucleation of intragranular ferrite. The decrease in transformation temperature causes a change in the morphology of intragranular ferrite from idiomorphic ferrite to acicular ferrite.

Inoculated acicular ferrite microstructure and mechanical properties

Abstract Low carbon–manganese wrought steels with three different microstructures were prepared by different thermomechanical treatments without changing the chemical composition and their mechanical properties were compared. The microstructure of fine interlocking acicular ferrite nucleated intragranularly at Ti 2 O 3 inclusions shows higher strength and good toughness at low temperatures than those of coarser polygonal ferrite/pearlite and aligned ferrite with second phase.

Correlation between fracture surface morphology and toughness in Zr-based bulk metallic glasses

Fracture surfaces of Zr-based bulk metallic glasses of various compositions tested in the as-cast and annealed conditions were analyzed using scanning electron microscopy. The tougher samples have shown highly jagged patterns at the beginning stage of crack propagation, and the length and roughness of this jagged pattern correlate well with the measured fracture toughness values. These jagged patterns, the main source of energy dissipation in the sample, are attributed to the formation of shear bands inside the sample. This observation provides strong evidence of significant “plastic zone” screening at the crack tip.

Glassy steel optimized for glass-forming ability and toughness

An alloy development strategy coupled with toughness assessments and ultrasonic measurements is implemented to design a series of iron-based glass-forming alloys that demonstrate improved glass-forming ability and toughness. The combination of good glass-forming ability and high toughness demonstrated by the present alloys is uncommon in Fe-based systems, and is attributed to the ability of these compositions to form stable glass configurations associated with low activation barriers for shear flow, which tend to promote plastic flow and give rise to a toughness higher than other known Fe-based bulk-glass-forming systems.

Effect of deformation histories on texture evolution during equal- and dissimilar-channel angular pressing

Abstract Finite element analyses and full constraint Taylor analyses based on the rate sensitivity model were performed to analyze deformation history and corresponding texture evolution during equal- and dissimilar-channel angular pressing, respectively.

Micro-to-nano-scale deformation mechanisms of a bimodal ultrafine eutectic composite

The outstading mechanical properties of bimodal ultrafine eutectic composites (BUECs) containing length scale hierarchy in eutectic structure were demonstrated by using AFM observation of surface topography with quantitative height measurements and were interpreted in light of the details of the deformation mechanisms by three different interface modes. It is possible to develop a novel strain accommodated eutectic structure for triggering three different interface-controlled deformation modes; (I) rotational boundary mode, (II) accumulated interface mode and (III) individual interface mode. A strain accommodated microstructure characterized by the surface topology gives a hint to design a novel ultrafine eutectic alloys with excellent mechanical properties.

Combinatorial influence of bimodal size of B2 TiCu compounds on plasticity of Ti-Cu-Ni-Zr-Sn-Si bulk metallic glass composites

We report on the formation of Ti-Cu-Ni-Zr-Sn-Si bulk metallic glass composites containing bimodal size of B2 TiCu compounds. The small B2 TiCu compound with a size of 1 to 10 μm has a strong influence on the oscillation of the shear stress, thus causing wavy propagation of the shear bands. In contrast, the large B2 TiCu compound with a size of 70 to 150 μm dissipates the shear stress by branching and multiplication of the shear bands. By forming the bimodal size of B2 TiCu compound, it is possible to determine the harmonic influence to further enhance the plasticity of the Ti-Cu-Ni-Zr-Sn-Si bulk metallic glass composites.