Jae-Hyeok Shim

Non-metallic inclusion and intragranular nucleation of ferrite in Ti-killed C–Mn steel

Abstract The influence of Ti addition on the development of acicular ferrite microstructure during the γ/α transformation in C–Mn steels has been studied. The optical microstructures of the heat-treated specimens with different Ti concentrations were characterized. Transmission electron microscopy analysis with thin foil specimens was carried out to investigate the phase composition of non-metallic inclusions and the local variation of chemical composition around the inclusions. It has been found that an acicular ferrite dominant microstructure could be produced when the Ti concentration is higher than about 50 ppm. The transition from the conventional bainitic microstructure to the interlocking acicular ferrite microstructure occurs in response to the change in the main-component of the non-metallic inclusions from Mn–Si oxide to Ti 2 O 3 . The Mn depleted zones around Ti 2 O 3 particles were detected, which could explain how the intragranular nucleation is facilitated on Ti 2 O 3 particles.

Nucleation of intragranular ferrite at Ti2O3 particle in low carbon steel

Abstract Systematic experiments have been performed to study the intragranular nucleation phenomena of ferrite in Ti-bearing low carbon steels. The fine intragranular acicular ferrite structure in Ti-bearing low carbon steels is induced by heterogeneous nucleation of ferrite plates at fine dispersed Ti2O3 particles, which are thermodynamically stable in the steels. From a steel–Ti2O3 diffusion bonding experiment, it is clearly observed that a local Mn-depleted zone (MDZ) has developed in the vicinity of the steel–Ti2O3 interface. The MDZ formation around Ti2O3 particles is believed to be a dominant driving force for the heterogeneous nucleation of intragranular ferrite. It is also confirmed that the development of the MDZ is associated with the absorption of Mn into the Ti2O3 phase at high temperatures. The width of the MDZ decreases with decreasing bonding (austenitizing) temperature and influences greatly the intragranular nucleation ability of acicular ferrite at Ti2O3 particles.

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.

A thermodynamic study on the AgSbSn system

Abstract A thermodynamic assessment for the AgSbSn solder alloy system has been attempted through the least squares optimization procedure taking available thermodynamic and phase equilibria information into account. The low-order AgSb and AgSn binary systems have been re-evaluated and thermodynamic properties of the ternary alloys have been extrapolated from these of the three binary systems using the Muggianu model. The calculated phase diagrams and thermodynamic quantities of the AgSb and AgSn binary systems show very good agreement with the experimental data, and liquidus projection and isoenthalpy curves in the ternary liquid alloys are well reproduced with the assessed thermodynamic model parameters.

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.

Mechanochemical synthesis of nanocrystalline TiN/TiB2 composite powder

Abstract TiN/TiB2 composite powder has been synthesized from a mixture of Ti and BN powders by high-energy ball milling. An abrupt displacement reaction between Ti and BN occurs in about 2 h. After the reaction, the crystallite sizes of TiN and TiB2 decrease continuously and become about 10 nm after 16 h of milling.

Screw dislocation assisted martensitic transformation of a bcc Cu precipitate in bcc Fe

Molecular dynamics simulations of the interaction between a screw dislocation and a coherent bcc Cu precipitate in bcc Fe indicate that the screw dislocation stress field assists a martensitic transformation into a close-packed structure for precipitate diameters larger than 1.8nm, resulting in a stronger obstacle to dislocation glide. The observed martensitic transformation mechanism agrees with the Nishiyama-Kajiwara [Jpn. J. Appl. Phys. 2, 478 (1963)] model. For coherent bcc Cu precipitates with diameter larger than 2.5nm, the screw dislocation bypass mechanism becomes Orowan looping due to the coherency loss of the precipitates during the transformation.

Molecular dynamics simulation of screw dislocation interaction with stacking fault tetrahedron in face-centered cubic Cu

The interaction of a gliding screw dislocation with stacking fault tetrahedron (SFT) in face-centered cubic (fcc) copper (Cu) was studied using molecular dynamics simulations. Upon intersection, the screw dislocation spontaneously cross slips on the SFT face. One of the cross-slipped Shockley partials glides toward the SFT base, partially absorbing the SFT. At low applied stress, partial absorption produces a superjog, with detachment of the trailing Shockley partial via an Orowan process. This leaves a small perfect SFT and a truncated base behind, which subsequently form a sheared SFT with a pair of opposite sense ledges. At higher applied shear stress, the ledges can self-heal by gliding toward an SFT apex and transform the sheared SFT into a perfect SFT. However, complete absorption or collapse of an SFT (or sheared SFT) by a moving screw dislocation is not observed. These observations provide insights into defect-free channel formation in deformed irradiated Cu.

Influence of Mn on microstructural evolution in Ti-killed C–Mn steel

Abstract The influence of manganese on the microstructure in C–Mn steels containing a small amount of titanium oxide particles as inoculants for acicular ferrite during the austenite–ferrite transformation has been studied. An acicular ferrite dominant microstructure could be produced when the Mn concentration is larger than about 2 wt.%.

Influence of stearic acid on mechanochemical reaction between Ti and BN powders

Abstract The influence of stearic acid as a process control agent on the mechanochemical reaction between Ti and BN to form TiN/TiB 2 nanocomposite powder by high energy ball milling has been investigated. A powder mixture of pure Ti and hexagonal BN powders with a molar ratio of 3:2 was milled for up to 40 h with up to 1.75 wt.% of stearic acid. The XRD analysis shows that an intimate mixture of TiN and TiB 2 crystalline powders was formed during milling by a displacement reaction. It has been observed by monitoring the temperature of the vial surface during the milling process that the addition of over 1.5 wt.% of stearic acid changed the reaction mode from a mechanically induced self-propagating reaction (MSR) to a gradual reaction. It has also been found by XRD and TEM analyses that the MSR produces TiN and TiB 2 particles initially of sub-micron size, which are reduced gradually by further milling. However, there still remain some coarse TiB 2 particles larger than a few hundred nanometers even after 16-h milling due to their extremely high hardness. On the other hand, the gradual reaction induced by stearic acid successfully inhibits the formation of coarse TiB 2 particles and eventually decreases the crystallite size of the products below 40 nm after 16 h of milling.