발간논문

Home > KJMM 논문 > 발간논문

The influence of Nb addition on the precipitation behavior of carbonitride on the austenite grain growth of Timicroalloyed steels during reheating to the austenite region has been studied using optical microscope and transmission electron microscope. An addition of small amount of Nb in a Ti-microalloyed steel is not effective in raising the austenite grain coarsening temperature. This is because the coarsening rate of carbonitride is distinctively high in a Ti-Nb containing steel as compared to that in a Ti containing steel due to the high diffusion rate and equilibrium concentration of Nb in austenite. Addition of large amount of Nb however tends to slightly raise the austenite grain coarsening temperature, despite a similar coarsening rate of carbonitride, because of an increase in the volume fraction of carbonitrides with increasing Nb content. The precipitation of Ti as a carbonitride is so slow that a significant amount of Ti remains supersaturated in as-cast state, whereas Nb reaches near-equilibrium concentration even in the as-cast state because of its high diffusion rate.

keyword :

The phenomenological theory of martensitic transformation has been applied to crystallographic analysis of the B2 to 9R martensite transformation in Cu-Zn-Al shape memory alloys. The formulation procedure for solving the crystallographic relation between B2 and 9R phases is based on the basic concept of the invariant plane strain. Analytical crystallographic solutions have been derived to predict habit plane, direction and size of invariant plane strain, total shapede formation, orientation relationships between parent and martensite phases. Comparison between these derived solutions and experimental data has shown good agreement. It has been demonstrated that these analytical solutions are useful to understand various crystallographic features of the B2 to 9R martensite transformation.

keyword :

A study has been made to investigate the effects of colony size and lamellar spacing on the fatigue crack propagation behavior of the fully lamellar γ-TiAl alloys. The results show that the overall crack growth rates of the fully lamellar microstructure are not largely affected by the variation of the colony size, though its respective roles on the intrinsic and extrinsic nature are quite different. On the other hand, the lamellar spacing is proved to be the more important factor to control the fatigue resistance. The fine lamellar spacing microstructure represents superior fatigue crack growth resistance as compared to the coarse lamellar spacing microstructure, which is mainly attributed to the higher intrinsic nature of the microstructure as well as the higher extrinsic closure effects. The colony boundaries and the lamellar interface play an important role in retarding the advancing crack, serving as barriers for the dislocation movement and as sinks for dislocation pile-ups.

keyword :

Cavitation during superplastic flow was investigated for 7075 Al alloy strained under uni-axial tension at 516℃. The 7075 Al alloy was thermomechanically processed to develop fine grain sizes of Bum and 12㎛, respectively. Cavitation and superplastic elongations between specimens with 8㎛ and 12㎛ were observed to be different. In the 8㎛ microstructure, the cavity stringers were formed along the tensile direction, whereas in the 12㎛ microstructure, the stringers were always oriented along the rolling direction, regardless of the tensile direction. The difference in the formation of cavity stringers seems to be mainly due to the distribution of the second phase particles in the matrix.

keyword :

The dependency of the ductility of Al-Mg-Mn alloy containing Al_6Mn particles on temperature and strain rate was examined by using standard tensile test. The ductility minimum was found to be at 773K for 2.7×10^(-3), 5.5×10^(-3) and 1.1×10^(-2) strain rates, and the reduction of area decreased with increasing temperature `irrespective of strain rates. The rate of ductility loss increased significantly at lower strain rate and at higher temperature. However, the maximum ductility drop occurred at the same temperature, 773K, for various strain rates. For 2.7×10^(-3) and 5.5×10^(-3) strain rates, ductility was determined uniquely by the parameter Z=ε˙ exp(Q/RT) at Z= ≥10^6s^(-1). Brittle fracture showing the ductility loss at higher temperature was associated with cavity growth as well as interlinkage of cavities, while a number of dimples were observed on the fracture surface at lower temperatures.

keyword :

The interaction between particle reinforcement and matrix was evaluated from the characteristic change at the interfacial microstructure during underaging(UA) and overaging(OA) heat-treatment of SiC reinforced 6061 Al composites. The objective of this study is to correlate the change of the interfacial microstructure with the failure mode of the composites. The underaged material exhibited a pronounced enrichment of Mg at the interface. The width of the Mg-enriched layer was wider than the overaged material, owing to equilibrium and non-equilibrium segregation. The failure was initiated at the matrix alloy which contained lower Mg contents. The failure mode was interface decohesion, resulting in the increased fracture toughness value. In the overaged case, the Mg-enriched layer was narrowed by the migration of non-equilibrium(vacancy drag) segregation to the interface, forming precipitates at the interface. The precipitation at the interface increases the interface strength, which causes the increased load transfer to the SiC particles. The failure mode was particle fracture, resulting in the lower fracture toughness value.

keyword :

Formability, thermal stability, electrical resistance and mechanical properties of amorphous Al_(86)Ni_(10-x)M_xLn₂ (M=transition metal, Ln=rare-earth metal, 0≤X≤10 at%) alloys were investigated. The effect of additional elements on the formability of amorphous phase was decreased in sequence of (Fe, Co)>(Mn, Cu, Zr)>(Ti, Cr, Nb), indicating the effectiveness of the attractive bonding nature between Al or Ce and M The crystallization temperature(T_x) and room-temperature electrical resistance(ρ_(R. T.)) of Al_(88)Ni_(10-x)M_xCe₂ amorphous alloys increased linearly with Fe of Co content but decreased for other ones. The high tensile fracture strengths above l000㎫ obtained in most of Al_(88)Ni_(10-x)M_xLn₂ amorphous alloys. The maxima of the σ₁ in Al_(88)Ni_(10-x)(Fe, Co)_xLn₂ amorphous alloys were obtained at X=5%, e.g., 1420㎫ for Al_(88)Ni_5Fe_5Nd₂ alloy. The high tensile strength and thermal stabilities in a wide compositional range are thought to result from the combination effect of alloying elements which has a large negative interaction(i.e., a large negative mixing enthalpy) with Al or Ce.

keyword :

Microscopic aspects of superplastic deformation behavior in an α₂+β_0 Ti₃Al-Nb alloy have been studied by using transmission electron microscopy. Ti-24A1-11Nb specimen with grain size of 3.6㎛ was superplastically deformed at 970℃ and at an initial strain rate of 10^(-3)/sec. Localized dislocation activities along α₂/β_0 phase boundaries and α₂/α₂ grain boundaries with almost no dislocations inside the a₂ grains were observed in the superplastically deformed specimen. This result shows that phase boundary sliding (PBS) and grain boundary sliding (GBS) played a major role in superplastic deformation of this alloy. Severe deformation of soft β_0 phase and dislocation pile-ups inside the α₂ grains at/toward triple-points were also observed. From these results, it is concluded that the local stress concentration produced by PBS and GBS is accommodated by dislocation motion inside β_0 and α₂ phases.

keyword :

The purpose of this study is to investigate the effect of barrier structure on the dislocation motion, which is in turn believed to determine the permeability parameter p of plastic equation of state. It has also been intended to verify whether there exists a mechanical equation of state for Al single and poly crystals. A series of load relaxation tests was conducted on A1 single and poly crystals. In addition, tests at various temperatures were performed to investigate the temperature effect on the flow behavior. The test results are then analyzed using the recently proposed internal variable theory which has been successfully applied to many other materials. It has been confirmed in this study that the mechanical equation of state does exist in both single and poly crystals of Al. The permeability parameter p was obtained as a constant, p = 0.15 for polycrystalline Al as in the other studies on cubic materials, while that of single crystal was obtained as p = 0.23. As the test temperature increased, the flow curves appeared to shift into the lower stress and faster strain rates region.

keyword :

Deformation behavior of austenitic super stainless steels(21Ni-22Cr-6Mo-N) was investigated in terms of elevated temperature compression test. Hot rolled plates were machined into cylindrical specimens and the isothermal compression test was carried out in a thermo-mechanical simulator in the temperature range between 900 and 1250℃. The effect of nitrogen contents on deformation resistance was also examined. Dynamic recrystallization occurred at deformation temperatures higher than 1200℃ and dynamic recovery mechanism was dominant at lower temperatures. Recrystallized grain size was decreased with higher strain rate and lower temperature. Higher nitrogen resulted in higher resistance to deformation as well as higher peak stress. Empirical constitutive equations for the deformation were obtained after testing at various strain rates and temperatures. Relations between microstructure and hot deformation behavior were also explained.

keyword :