발간논문

Home > KJMM 논문 > 발간논문

Thermodynamic and phase equilibrium data of the Fe-P system have been critically assessed by employing the CALPHAD method. The regular type solution model was applied to α(bcc), γ(fcc) and liquid Fe-P phases. The stoichiometric compound model was applied to the intermediate phases such as Fe3P, Fe2P and FeP. The parameters of the thermodynamic models have been evaluated from the thermodynamic and phase equilibrium data in the literature. Stable and metastable phase equilibria, To curves and thermodynamic properties have been calculated using the evaluated parameters. The calculated results are in fairly good agreement with the experimentally measured data.

keyword :

In order to avoid the confusion due to inconsistency between thermodynamic calculations(free energy difference between γ and ε, △G γ→ε of γ→ε martensitic transformation for Fe-Mn binary alloys reported by previous workers, the thermodynamic data of Fe-Mn alloys, △G(γ→ε)Fe and △G(γ→ε)Mn, were newly formulated, and the thermodynamic calculations of γ→ε martensitic transformation for the Fe-Mn binary alloys were performed with a regular solution model. The calculation results were well consistent with experimental results in wide range of 15 to 28wt%Mn, and were closely approached to Takaki`s calculation results rather than Murakami`s calculation results.

keyword :

The effects of ε martensite content and strain amplitude on damping capacity of Fe-(17~23)%Mn martensitic alloys have been investigated to Clarify reson that Fe-17%Mn alloy shows the maximum damping capacity among Fe-Mn binary system. It was observed that the damping capacity of Fe-(17~23)%Mn alloys were decreased with increasing manganese content when the specimens had similar amount of ε martensite. The deterioration of the damping capacity with increasing manganess content is attributted to the following two factors: (l)The decrease in density of stacking faults in ε martensite and γ/ε interface areas. (2) The decrease in mobility of γ/ε interface. Below 3×10-4 strain amplitude, the damping capacity of Fe-17%Mn alloy was linearly Proportional to the ε martensite content, which suggests that stacking faults and variant boundaries in ε martensite plates are the principal damping sources. Above 4×10-4 strain amplitude, however, a maximum damping capacity was observed around 60~70vol.% ε. This means that γ/ε interface acts as the damping source in addition to the stacking faults and variant boundaries in Fe-17%Mn alloy.

keyword :

Effects of carbon and nitrogen additions on microstructure and mechanical properties of Ti-48.5Al-1.5Mo (at.%) alloys were investigated. The alloys added with 0.3%, 1.0% and 2.0%C or N were prepared by vacuum arc melting, and heat-treated at 1390℃ for lamellar microstructure and then aged at 900℃. Tensile tests were performed in air with the strain rate of 2×10-4S-1 at room temperature and 800℃. It was found that C (or N) addition decreased the grain size of lamellar structure. Especially, 1.0%C(or N) and 2.0%C(or N) addition refined the grain size to 1/10 of that of the mother alloy. Only fine Ti3AlN precipitates were formed at the lamellar boundaries for the alloy containing 0.3% N, but Ti2AlN precipitates of a few micrometers in size were also formed in the matrix in addition to Ti3AlN precipitates for the alloys with 1.0 and 2.0% N. Tensile elongation at room temperature decreased markedly for the alloys with 0.3%C or N but recovered for the alloys with 1.0% and 2.0% addition compared to the value of the mother alloy. Yield strength at room temperature for the alloys containing 1.0% and 2.0% C or N increased up to two times of the value of the mother alloy, and the high strength was maintained up to 800℃. The aged alloys showed much higher tensile elongation at room temperature than the alloys only heat-treated to obtain the lamellar microstructure.

keyword :

The critical loads in scratch test are used for the comparison of adhesion in film/substrate systems. But the critical loads are strongly affected by various parameters such as scratching speed, indenter tip radius and film thickness. Therefore, to assess the reliable interfacial adhesion strength, the stress generated by sliding circular indenter was analyzed by considering the static indentation stress, frictional stress and residual stress. Static indentation stress consists of the elastic stress and blister stress generated by plastic deformation of substrate. On the basis of the stress analysis, the elastic deformation energy of film was evaluated under the condition of strain matching criterion at the interface. When the elastic deformation energy stored in film reaches the critical value, this energy will be released by delamination and spallation of film. Thus the interfacial adhesion strength in terms of work of adhesion can be evaluated by extracting the surface energy generated by spallation from the elastic deformation energy. The work of adhesions of PECVO-prepared DLC film on WC-Co substrate were evaluated. In spite of the distinctly different critical loads measured with the change of scratching speed and film thickness, the comparatively constant work of adhesions were obtained.

keyword :

We estimate the "true" hardness value of a thin-film from composite hardness obtained by a conventinol microhardness tester. To separate the substrate and film contributions to the composite hardness, we apply the plastic-zone volume-law-of mixtures theory through the stress analysis of indentation. At this time, the geometrical deformation types are considered: spherical for substrate and cylindrical for film to consider the surface effect When an interface has a strong bonding, namely, no interface delamination occurs during loading, the deformation in the soft substrate is assumed to be restricted by the brittle film on condition that the strains of both film and substrate match at interface. And, in calculating the plastic-zone radii, the virtual pressures which the substrate and film actually support are considered. On the other hand, when an interface is weak, an interface parameter is introduced to reflect the partial debonding at interface during loading, resulting in the partial reease of plastic-zone confinement in the soft substrate.

keyword :

In order to investigate the effect of grain size on the overall elastic and plastic properties of nanocrystalline materials in which the grain size ranges typically from a few nanometers to a few tens of nanometers, the elasto-plastic finite element method was used. The tensile deformation behaviors of the unit cell composite materials composed of crystalline of various sizes and grain boundary region of 1 nm width have been analyzed. The grain boundary region had a lower elastic modulus and a higher yield strength than the crystalline and the materials were assumed to be elastic-perfect-plastic because of the small grain size. According to the calculated results, a drop in the grain size from 100 nm to 2 nm increased the yield stress and decreased the elastic modulus. The yield stress-(grain size) -1/2 shape is S curve which fitted well with the mixture rule based on the volume fraction. Therefore, the exponent of Hall-Petch equation can be - 1 or -1/2, and the gradient of the curve can undergo a change according to the range of the grain size.

keyword :

A new approach to structural superplasticity is attempted in this study based on the internal deformation variables. The basic concept of this theory is that the high temperature deformation, especially superplasticity, of crystalline materials consists mainly of grain boundary sliding (GBS) and grain matrix deformation by dislocation glide process to accommodate the incompatibility caused by GBS. The high temperature deformation behavior of 8090 Al-Li alloy has been studied experimentally within the framework of the new theory proposed. A series of load relaxation and tensile tests at various temperatures ranging from 200℃ to 530℃ have been conducted to obtain the flow curves and the results were systematically analyzed based on the internal variable theory. The flow curve of log σ vs. log ε has been found to be a composite curve consisting of GBS and plastic deformation confirming the kinematic relation of the steady state form, ε=α+g at sufficiently high temperature with α and g representing the strain rates due to plastic deformation and GBS, respectively. The GBS in 8090 Al-Li alloy examined in this study appears to be a Newtonian viscous flow process characterized by the power index value of Mg= 1.0. The internal strength variable (σ*) is found to be inversely proportional to the square root of grain size and the activation energy for grain matrix deformation is obtained as 124.9 kJ/mol in the temperature range from 470℃ to 530℃, which is very similar to the activation energy for self diffusion in pure Al.

keyword :

The residual stress distributions in a brass tube drawn by a fixed back-tapered plug method have been measured by a material stripping method. The measured data are comparable with the results calculated by the elasto-plastic finite element method. The residual stress distributions in brass tubes made by sinking, and fixed tapered, fixed cylindrical and fixed back-tapered plug methods have been calculated using the finite element method. The FEM results show the development of a compressive residual stress in the inner walls and a tensile stress in the other walls of the tubes produced by the sinking and fixed tapered plug method, whereas the nature of the stresses between the inner and outer walls were switched in case of the tube made by the fixed back-tapered plug method. The lowest residual stresses develop in the tube made by the fixed cylindrical plug method.

keyword :

Morphological characteristics of Al-4.3wt.%Cu and Al-15wt.%Cu ribbons spun by planar flow casting were analyzed using an optical microscope, SEM and TEM. The local solidification velocity within the Al-4.3wt.% Cu ribbon was found to be a function of ribbon thickness as V = 34.78 S-1. The absolute stability velocity of an Al-4.3wt.%Cu alloy was estimated as 2,460mm/s. Segregation free zone with the planar interface was developed from the bottom layer of the ribbon, where relevant solidification velocity was higher than the critical value of 2, 460mm/s. Following this zone, cellular/dendritic transitions occurred as the velocity decreased across the ribbon thickness. In the Al-15wt.%Cu ribbon, however, no segregation free zone was found and only cellular/dendritic region was developed due to its lower velocity compared to the absolute velocity for an Al-15wt.%Cu alloy throughout the entire ribbon thickness. Solidification sequences during ribbon formation were proposed for Al-Cu hypoeutetic alloy systems.

keyword :