Dilatometric analysis of austenite formation during intercritical annealing

Dilatometry is a useful method to investigate the transformation kinetics of ferrous alloys. In the present study, a dilatometric analysis procedure that considers the effect of non-isotropic volume change was applied to evaluate the kinetics of austenite formation during the intercritical annealing of TRIP steels. Metallographic analysis was conducted to validate the dilatemetric analysis results. The austenite fractions from the dilatometric analysis showed reasonable agreement with those from the metallographic one. This verifies that the kinetics of austenite formation during heat treatment of low carbon TRIP steel can be reliably analyzed with the proposed procedure.     

Development and Trend of Advanced Polymer Composites as Structural Materials in Korea

A brief review of recent research and development of advanced polymeric composite materials in Korea is discussed. This paper also introduces the government research institutes and universities doing active research on composite materials. Aerospace companies are developing many advanced composite materials for defense, aerospace, transportation, and industrial applications. The anticipated applications of polymer composite materials as a structural materials are also discussed.     

Interpretation of viscous deformation of Zr-based bulk metallic glass alloys based on Nabarro-Herring creep model

Superplastic-like viscous deformation of bulk metallic glass alloys around the glass transition temperature (Tg) was analyzed based on the Nabarro-Herring creep model, a classical creep model, where the diffusional motion of atoms or vacancies through the lattice (atomic configuration) is considered. The amorphous matrix of bulk metallic glasses that has a randomly-packed atomic configuration was assumed to behave in a manner similar to the grain boundary in polycrystalline metals so as to approximate the diffusivity of the major constituent element. In spite of rough approximation of the parameters in the Nabarro-Herring creep equation, a reasonable value of the diffusion path (d) could be obtained from the experimentally-obtained metal flow data, including the steady state stress and the strain rate. Due to the absence of vacancy sources such as grain boundaries in homogeneous metallic glasses, the diffusion path, which, in polycrystalline materials, generally is the average distance between vacancy sources such as grain boundaries, was considered in this work as the average distance between tunneling centers in bulk metallic glass alloys. The calculated diffusion path was comparable to the density of tunneling centers around Tg, proposed by M. H. Cohen and G. S. Grest based on free volume theory. The calculated diffusion path showed monotonous decrease with temperature over Tg for Zr-based bulk metallic glass alloys. Based on this analysis, a schematic model for viscous deformation of bulk metallic glass was proposed.     

Electrochemical corrosion of novel beta titanium alloys

Electrochemical corrosion behaviors of novel beta titanium alloys, Ti−30Ta−10Nb−10Zr, Ti−40Ta−10Nb−10Zr and Ti−40Ta−10Nb−4Sn were characterized in naturally aerated Hank's solution at 37°C compared with currently used biomedical titanium alloys. Ti−30Ta−10Nb−10Zr and Ti−40Ta−10Nb−10Zr exhibited comparable corrosion resistance to CP Ti or Ti−6Al−4V, while the corrosion resistance of Ti−40Ta−10Nb−4Sn was greatly inferior to the other alloys. An EIS data analysis confirmed that the resistance of passive film for Ti−30Ta−10Nb−10Zr and Ti−40Ta−10Nb−10Zr was comparable to CP Ti or Ti−6Al−4V. Ti−30Ta−10Nb−10Zr and Ti−40Ta−10Nb−10Zr alloys are promising metallic biomaterials for the future, owing to their very low elastic modulus and good corrosion resistance capabilities.     

Prediction of microstructure evolution during high temperature blade forging of a Ni−Fe based superalloy,Alloy 718

The mechanical properties of the Ni−Fe-based Alloy 718 depend very much on grain size, as well as the strengthening phases, γ’ and γ. The grain structure of the superalloy components is mainly controlled during thermo-mechanical processes by the dynamic, meta-dynamic recrystallization and grain growth. In this investigation, the evolution of the grain structure in the process of two-step blade forging was experimentally and numerically dealt with. The evolution of the grain structure in Alloy 718 during blade forging was predicted using a 2-DFE simulator with implemented constitutive models on dynamic recrystallization and grain growth. The comparison of the simulated microstructure with the actual grain structure of the forged parts validated the prediction of the grain structure evolution. The effect of dynamic recrystallization on the evolution of grain structure is highlighted in this article.     

Effect of be addition on the oxidation behavior of Mg−Ca alloys at elevated temperature

Beryllium was added to Mg−Ca alloys to study their ignition-proof properties. The ignition temperatures of Mg−2Ca alloys were increased dramatically with increasing Be addition. Thermogravimetric measurement revealed that the oxidation of Mg−2Ca alloys was slowed down by Be addition. After elevated temperature exposure to air, the Mg−2Ca alloy was partially ignited, while the surface of Be-containing alloys was smooth without any partial ignition. SEM, low-angle XRD, and AES observations indicated that the surface of Becontaining alloys became compact and dense, and the oxide film formed at elevated temperature mainly consisted of CaO together with MgO and BeO. It was found that the CaO enriched oxide layer acted as an impermeable barrier to the inward diffusion of oxygen and thus further oxidation was prevented.     

Effects of phosphorous addition on mechanical properties and retained austenite stability of 0.15C−1.5Mn−1.5Al TRIP-aided cold rolled steels

The effects of a P addition on the mechanical properties and austenite stability are investigated for 0.15C−1.5Mn−1.5Al TRIP-aided cold-rolled steels containing 0.05 and 0.1 wt.% of P. The strength and retained austenite fraction are increased by an increment of the P content. The strengthening of P-added TRIP-aided steel partially comes from the solid-solution hardening effect of P, and a higher fraction of strain-induced martensite plays an important role as well. The elongation of steel containing 0.1 wt.% P is diminished compared with that containing 0.05 wt.% P. This is attributed to the lower mechanical stability of retained austenite in TRIP-aided steel containing 0.1 wt.% of P, which inhibits persistent work hardening during deformation.     

Micro-forming characteristics of a Zr-based amorphous alloy with a gear-like mold

The amorphous alloy Zr₆₂Cu₁₇Ni₁₃Al₈ exhibits a supercooled liquid state over a wide temperature range under high temperatures. In this present paper, the authors studied the micro-forming ability of the alloy by compressing specimens under a miniature pattern mold with a micro-gear shape. Micro-compressive formation is a formation method in which the surface of specimens of the Zr₆₂Cu₁₇Ni₁₃Al₈ amorphous alloy is compressed under conditions of dead loads and high temperatures and maintained for a given period for the fabrication of miniature patterns with a micro-gear shape. All the experiments were carried out under a vacuum environment to prevent the specimens from suffering deleterious effects, such as air traps in the miniature patterns and oxidation at the surface of the Zr₆₂Cu₁₇Ni₁₃Al₈ amorphous alloy. The characteristics of micro-compressive formation were investigated at temperatures higher than the glass transition temperature with different experimental parameters of loads and times. In addition, the micro-forming characteristics of the gear-like shape were investigated by means of scanning electron microscopes and a 3-D surface profiler system.     

High density plasma assisted sputtering system for various coating processes

High density plasma assisted sputtering source (HiPASS) has been introduced to develop a remote plasma sputtering. In HiPASS the remote plasma source (RPS) was an electron beam excited plasma using a direct current discharge supplying Ar plasma (10^10–11 cm^− 3). Ar plasma was transported from the RPS to a sputtering cathode by external magnetic fields. The transported Ar ions generated a physical sputtering at the negatively biased sputtering cathode. HiPASS has an advantage that sputtering current and voltage were controllable independently. The sputtering current was dominantly controllable by the discharge power of the RPS. And sputtering voltage was applied to the sputtering cathode freely with the fixed sputtering current modulated by the discharge power of the RPS. As the discharge power of the RPS is increased from 2.4 to 8.4 kW, the sputtering current of sputtering cathode (78.5 cm²) was varied from 1.2 ± 0.1 to 3.6 ± 0.2 A. This was an improved sputtering current comparing with practical sputtering current (1 A at 78.5 cm²) of radio frequency excited remote plasma sputtering. The remote plasma sputtering with the improved sputtering current could be applied to low voltage high current sputtering for damage-free transparent conductive oxide and barrier film depositions as well as high voltage high current sputtering for hard coating applications.