The metastable beta titanium alloy, Ti-39Nb-6Zr (TNZ40), is a promising implant material due to its excellent biocompatibility and low elastic modulus, effectively mitigating stress shielding compared to conventional Ti-6Al-4V. Its mechanical properties and surface reactivity can be precisely controlled by managing microstructure through heat treatment. This study investigated how microstructural changes, induced by various heat treatment conditions, affect single-acid etching behavior. were controlled by solution treatment (ST) at 950℃, with some specimens undergoing additional aging treatment (STA) at 550℃. These were then subjected to single-acid etching using HCl, H₂SO₄, and HF, evaluating surface roughness (Sa) and morphology. Due to TNZ40's high acid resistance, HCl and H₂SO₄ failed to achieve the target roughness (Sa 1.0~2.0 ㎛). However, HF etching successfully met this range. Surface roughness increased in the order of ST (10% HF) < ST (30% HF) < STA (10% HF) < STA (30% HF). For STA, the Nb-depleted secondary α phase was selectively dissolved, creating surface irregularities. The most appropriate surface roughness and morphology were achieved under STA (10% HF/7 min) and STA (30% HF/2 min) conditions. This study demonstrates that optimizing the Ti-39Nb-6Zr alloy surface for biomedical implants is achievable through combined control of microstructure via heat treatment and adjustment of HF concentration.

In this study, a heat-transfer-based analytical approach was developed to predict the cooling behavior and associated mass effect in large S20C forgings using experimentally measured cooling data from small-scale specimens. Time-temperature cooling curves were obtained from thermocouples installed at the surface, 1/4D, and center positions of 100 × 100 × 100 mm S20C samples during air cooling from 900· to room temperature, and the air-cooling heat transfer coefficient (12~27 W/m²·K) was identified through inverse analysis. The derived heat transfer coefficient, together with experimentally determined temperature-dependent thermal conductivity, specific heat, and density of S20C, was implemented in a transient thermal simulation to predict the cooling behavior of 20-ton cylindrical and cubic forgings. The analysis revealed that the center temperature of the cylindrical specimen required approximately 55 h (200,000 s) to reach ~45℃, evidencing a pronounced mass-effect-induced cooling delay, while the cubic geometry exhibited faster cooling at corners and surfaces due to shape-dependent thermal dissipation characteristics. These results confirm that the proposed methodology successfully links small-scale measurements to large-scale cooling predictions and provides a physically grounded framework for estimating cooling-time deviations associated with the mass effect, thereby supporting the engineering design and optimization of heat treatment processes for large forgings.

This study investigates the effects of material cleanliness on the microstructure and mechanical properties of high-cleanliness and air-melted 3.5NiCrMoV steels for gas turbine compressor applications. While both steels exhibited similar major alloying element contents, the high-cleanliness steel showed significantly lower impurity levels: S (50% reduction), P (62.5% reduction), and Al (94.6% reduction). After quenching at 850℃ and tempering at 580℃ for 1 h, the high-cleanliness steel exhibited refined, homogeneous martensitic packets with uniform grains, whereas the air-melted steel showed pronounced band structures and non-uniform grain distribution due to chemical segregation. Following long-term thermal exposure at 400℃ and 500℃ for 1,000 h, both steels showed moderate strength degradation with limited ductility changes. However, Charpy impact tests revealed a critical difference: the high-cleanliness steel maintained significantly higher impact energy and minimal ductile-to-brittle transition temperature (DBTT) shift, while the air-melted steel exhibited substantial impact energy loss and marked DBTT elevation, indicating higher susceptibility to temper embrittlement. These results demonstrate that improved cleanliness enhances microstructural homogeneity, toughness stability, and resistance to temper embrittlement, confirming the superior suitability of high-cleanliness 3.5NiCrMoV steel for long-term high-temperature service in gas turbine compressor components.

In this study, Fe-36 wt.% Ni Invar alloy sheets were cold-rolled to various reduction ratios in order to investigate the evolution of mechanical properties, microstructure, and residual stress behavior. With increasing reduction ratio, tensile strength increased while elongation decreased, indicating typical work-hardening behavior. Electron backscatter diffraction (EBSD) analysis showed that grain boundary fractions, kernel average misorientation (KAM), and grain orientation spread (GOS) increased with increasing reduction ratio and reached maximum values at approximately 52.4%, followed by a gradual decrease at higher reduction ratios. Residual stresses measured by X-ray diffraction exhibited a similar trend, showing a peak at the same reduction ratio. Subsequent annealing and stress relief heat treatments effectively reduced residual stresses depending on temperature, holding time, and sheet thickness. These results indicate the presence of a critical reduction ratio at which the dominant deformation mechanism in cold-rolled Invar alloy sheets changes, providing important insights for residual stress control and dimensional stability of Invar alloy sheets.

Waste plastic gasification is an important technology in the circular economy, converting plastic waste into synthesis gas. However, chlorine (Cl) and hydrogen chloride (HCl) released from PVC-containing plastics cause severe corrosion of refractory linings during high-temperature syngas cleaning processes above 1,200℃. This study investigates the chemical stability of refractory materials under chlorine-rich environments through thermodynamic analysis and experimental validation. Thermodynamic simulations were performed using HSC Chemistry to calculate Gibbs free energy changes for reactions between chlorine gas and refractory materials over a temperature range of 1,000~1,500℃. Experimental validation involved exposing alumina-based refractory composites to a 99.9% Cl₂ atmosphere at 1,500℃ for 24 h. Microstructural changes and phase evolution were analyzed using SEM/EDS and XRD, and mechanical degradation was evaluated by Vickers hardness test. The thermodynamic results showed that Al₂O₃ maintained positive Gibbs free energy values across the entire temperature range, indicating high resistance to chlorine attack. In contrast, SiC exhibited thermodynamic instability, promoting the formation of volatile SiCl₄. Experimental results confirmed these predictions. Pure alumina samples showed minimal weight loss and hardness reduction, while Al₂O₃-SiC composites suffered severe degradation due to pore formation from SiC volatilization. Overall, high-purity alumina refractories demonstrated excellent chemical stability and are suitable for chlorine-containing waste plastic gasification environments.

Ti-6Al-4V rotor components experience sustained vibration at cruise-altitude temperatures, yet microstructure-controlled damage accumulation under low-temperature vibration remains unclear. Low-temperature vibration tests (DMA, 3-point bending) and Vickers hardness measurements were conducted on Ti-6Al-4V with an α + β lamellar (as-received) microstructure and a bimodal microstructure produced by solution treatment and aging (STA). The STA specimen showed the highest storage modulus (E') of 84,533 MPa. EBSD-based kernel average misorientation maps revealed high-KAM zones concentrated near micro α + β lamellar boundaries in the STA condition, indicating localized plastic strain and dislocation storage at these interfaces. After low-temperature vibration, the STA hardness increased from 394 HV to 451 HV, whereas the as-received condition showed no comparable hardening. The hardening is attributed to micro α+β lamellar regions acting as local stress concentrators that promote repeated local deformation and dislocation accumulation. Overall, the low-temperature vibration fatigue response and hardening of Ti-6Al-4V are strongly microstructure dependent, providing guidance for microstructure design of aircraft rotor components.