Tae Eung Kim

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%.

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.

Suppression of discontinuous precipitation in AZ91 by addition of Sn

The effect of Sn (5 wt%) addition on the aging behavior of the AZ91 alloy has been investigated in the present study. The addition of Sn effectively suppresses the discontinuous precipitation during aging treatment. The aging response of the Sn containing AZ91 alloy is far better than that of the AZ91 alloy due to much higher density of continuous precipitation in the matrix. The yield strength and total elongation to failure at the peak aged condition of the AZ91 and Sn containing AZ91 alloys are 119.4 and 161.9 MPa and 8.8 and 8.6%, respectively, indicating that 35.6% increase of yield strength can be obtained by the addition of Sn in the AZ91 alloy maintaining almost same level of ductility.

Design of ultrafine eutectic-dendrite composites with enhanced mechanical properties in Fe-based alloy

The effect of Mn content on the evolution of microstructure and the enhancement of mechanical properties in Fe-Nb-Mn hierarchical composites consisted of ultrafine eutectic and primary dendrite has been studied by using X-ray diffractometry, scanning electron microscopy, transmission electron microcopy and compression test. Fe-11Nb-5Mn hierarchical composite consisted of α′-Fe dendrite and urtrafine α′-Fe + Fe2Nb eutectic, and exhibited a reasonably good combination of mechanical properties, i.e. yield strength of 1283 ± 10 MPa and compressive plastic strain of 7.75 ± 5%, while Fe-11Nb-15Mn composite consisted of ɛ-Fe dendrite and ɛ-Fe + Fe2Nb eutectic structure with some retained γ phase, and exhibited a far better combination of mechanical properties, i.e. higher yield strength of 1462 ± 10 MPa and larger compressive plastic strain of 11.28 ± 2%. The origin for the simultaneous enhancement of high strength and large plastic strain is attributed to ɛ-Fe martensite formation and strain-induced martensitic transformation from ɛ to α′ during deformation.