The relationship between pouring temperature and carburized microstructure of 1％Nb-added SCH13A heat-resistant cast steel manufactured by the self-hardening mold casting process at pouring temperatures of 1753 K, 1823 K and 1883 K was investigated. The creep properties of these materials subjected to repeated carburizing and quenching treatment were also examined. As a result, with increasing pouring temperature, the carburized depth decreases and the carbon concentration distribution in the carburized layer increases. A relative comparison of the creep properties revealed that the volume of primary carbides and the number of voids and micro-cracks in carbides generated due to repeated vacuum carburizing quenching treatment affected the creep rupture time. Creep rupture time of the carburized specimens was shorter than that of the as-cast specimens. Additionally, pouring at 1823 K exhibits the longest rupture time among the samples poured at three different temperatures.

This paper focuses on effect of rhodium addition on degradation behavior of Ir-10 mass％Rh alloy for a spark plug during oxidation at elevated temperatures. Oxidation tests were conducted at the temperature range from 1173 K to 1473 K in Ar-21％O₂ gas mixture to clarify the effect of rhodium addition on degradation behavior of the alloy. As a result, there are three effects of rhodium addition to prevent the alloy from the degradation: (1) dissolution of rhodium into iridium oxide scale at below 1273 K, which decreases activity of iridium oxide in the oxide scale, (2) dissolution of rhodium into the alloy at above 1273 K, which decreases activity of iridium in the alloy and (3) formation of rhodium oxide, which decreases surface area of the alloy. All effects contribute to decrease vapor pressure of volatile iridium oxide, resulting in suppression of the degradation.

To reduce the cost of solution heat treatment process Ni-base single crystal superalloy TMS-238 containing Re and Ru, quantitative analysis of dendrite-interdendrite segregation of alloying elements under various solution heat treatment conditions were conducted, and influence on high-temperature creep strength were investigated. In this study, we defined the solution rate R_sol (= (1 − V_f,e) × 100％, where V_f,e is volume fraction of eutectic γ′ that precipitates in the final solidification zone during casting) as a parameter to reveal the microstructure homogeneity. The R_sol values were 71％, 97％, 99％, 100％ for solutioning at 1250℃/20 h, 1320℃/5 h, 1320℃/20 h and 1335℃/20 h, respectively. Furthermore, it was confirmed that Re and W segregated in the dendrite core area and γ′ formers whereas Ta and Al segregated in the interdendrite. The magnitude of these segregations decreased as the solution temperature and time increased, and eventually the structure became almost homogeneous for solutioning at 1335℃ for 20 h. Additionally, creep test results indicate that Larson-Miller parameter (LMP) at 800℃-735 MPa, 900℃-392 MPa and 1000℃-245 MPa creep conditions show the same values for R_sol ≥ 97％. On the other hand, under 1100℃-137 MPa creep condition, LMP decreased as the R_sol decreased. A factor analysis of creep rupture properties suggested that the degradation of LMP under 1100℃-137 MPa was caused by the decrease of Re content and γ/γ′ lattice misfit in the interdendritic region.

Flame-retardant magnesium alloys prepared by adding calcium have been used as lightweight structural material. Thermomechanical treatments using hot rolling or hot extrusion are effective to improve the mechanical properties of flame-retardant magnesium alloys. This study focuses on the uniaxial hot pressing process for improving the mechanical properties of cast Mg-9Al-1Zn-2Ca alloy. The hot pressing tests were carried out with different temperatures up to 673 K and maximum compression strains of 65％. Microstructural characterization was carried out using a field-emission scanning electron microscope equipped with an electron back scattered diffraction system. 0.2％ proof stress, tensile strength and elongation to fracture were measured by a universal testing machine at room temperature. The Mg-9Al-1Zn-2Ca alloy showed the highest strength after the 65％ compression at 573 K, and the highest ductility after the 65％ compression at 673 K. The enhanced mechanical properties are attributed to grain refinement caused by dynamic recrystallization and reduced cavities due to the high compression strain.