鋳物用Ａｌ‐Ｓｉ系過共晶合金の熱衝撃性における初晶Ｓｉの挙動

The hypereutectic aluminium-silicon alloys AlSi25Cu4Cr and AlSi25Cu5Cr are heat treated T5, T6 and T7. The quenching of the alloys was carried out under the same conditions. Artificial aging at T5 was carried out at a temperature of 180º C for 2, 4 and 6h, and at T6 at 180º C for 8, 10, 12 and 14h. The artificial aging of the studied compositions at T7 was carried out at a temperature of 370º C for 20, 40 and 60 min. The influence of heat treatment on the size, shape and distribution of primary and eutectic silicon crystals in the alloy structure was investigated. It was established that the proposed heat treatment regimes do not lead to a significant change in the shape and size of the primary silicon crystals, but contribute to their uniform distribution in the structure of the studied alloys. Rounding of the silicon crystals in the composition of the eutectic of the investigated alloys was observed.

Study on thermal shock behavior of YSZ abradable sealing coating prepared by mixed solution precursor plasma spraying

Thermal shock behavior of EB-PVD thermal barrier coatings

A new method of decantation during centrifugal casting has been used to study crystal growth. The shape of a primary silicon crystal grown from a hypereutectic Al–Si alloy was obtained by means of this method and a growth mechanism depending on the use of dislocation steps was found to apply for primary silicon crystals. Furthermore a mechanism for the branching of primary silicon crystals and the development of a structure of α-Al–Si eutectic encapsulated in primary silicon crystal was successfully explained by the growth mechanism of the primary silicon crystal. A growth process leading to the α-Al–Si eutectic surrounding the primary silicon crystal was also observed.

Thermal shock behaviors and failure mechanisms of EB-PVD thermal barrier coatings using radiant heating

Sulphur addition was known to have a refining effect on primary silicon in hypereutectic Al-Si alloys. The purpose of this study is to clarify effects of an amount of S addition, holding time and superheating temperature on refinement of primary silicon crystals in an Al-20%Si alloy. Grain size of primary silicon and microstructure were observed by an optical microscope and an X-ray microprobe analyzer.When a melt was held at 900°C for 10min, addition of 0.3-0.4%S resulted in refinement of primary silicon. Ten minute holding at 900°C after a modification treatment increased a refinining effect significantly. However, longer holding did not introduce improvement of a refining effect. The finest silicon crystals were obtained when a melt, modified by 0.3%S, was superheated at 1100°C for 10min. In the S-treated specimens, grey colored particles were seen at the center and the boundary of the primary silicon and in the eutectic structure. A microanalysis showed these particles to contain two components, Al and S. On a cooling curve of a modified specimen, a clear break was observed and was identified with a point, at which the primary silicon was nucleated. These results were interpreted in a manner similar to the AlP nucleation theory of primary silicon crystals.

This study describes the formation of a primary silicon network during separation of an Al‐30% Si melt and the process conditions to make bigger primary silicon crystals. As the crucible rotates in the centrifugal separation, the unfrozen aluminum‐rich phase and small silicon particles are pushed outside through the openings in the silicon network. As a result, primary silicon crystals are separated in the form of a foam after centrifugation. The recovery of the silicon ranged from 13 to 18% depending on the location in the crucible. The size of the primary silicon achieved by changing the cooling rate and quenching temperature during solidification is also measured using a quenching furnace. Primary silicon particles exhibit a coarse, plate‐like morphology, although small star‐like silicon particles are also found in the aluminum‐rich matrix. The fraction of plate silicon decreases, while the fraction of small globular silicon increases with an increasing cooling rate. The thickness of the primary silicon plate also decreases with an increasing cooling rate in the samples quenched at various temperatures during solidification.

The (TiCr)N hard reactive films with 4 different Ti/Cr atomic ratios are prepared by using multi-arc ion plating. The Ti and Cr targets are used as cathodes in the co-deposition process. The high speed steel (HSS) specimens are adopted as substrate. The thermal shock cycle tests in air cooling (AC) way for the (TiCr)N films are carried out. The test temperatures are chosen as 600 °C and 700 °C, respectively. The changes of the surface compositions and morphology in the thermal shock cycle tests are investigated. The effects of Cr content in the (TiCr)N films, the temperature at which the thermal shock cycle tests are performed and the cycling times on the thermal shock resistance of the (TiCr)N films are discussed and analyzed in detail. The thermal shock behaviors of the (TiCr)N films show that the thermal shock cycling process of the (TiCr)N films consists of two stages, the steady-state oxidation stage with the increase of oxygen content at the early stage of thermal cycles, and, the instable oxidation stage with the oxidation aggravation and the occurrence of the surface cracks. The simple increase of Cr content in the (TiCr)N film does not improve the thermal shock resistance of the (TiCr)N films but easy to aggravate the surface oxidation process during the thermal shock cycles.

NiCoCrAlY coatings strengthened by different content of nano-Al2O3p, using crosscurrent CO2 laser, were prepared on Ni-based superalloy substrates, and thermal shock behaviors of these coatings were investigated by cycling between 1050°C and room temperature (forced water quenching).The results show that the thermal shock resistances of nano-Al2O3p-added coatings are definitely better than that without adding nano-Al2O3p. Among the nano-Al2O3p-added coatings, the coating added with 0.5wt% nano-Al2O3p performs best. After 10 thermal shock cycles, the spalled and spalling area in the oxide scale of the no-nano-Al2O3p coating reaches up to 27.7%, and the main failure form of the oxide scale is an expanding unit-spalling type in thermal shock cycles. While the area in the oxide scale of nano-Al2O3p-added coatings is only 10%~60% to the no-nano-Al2O3p one, and the failure of these oxide scales is mainly in the form of unit-spalling type. After 100 thermal shock cycles, many cracks turn up in the cross-section of the no-nano-Al2O3p coating, including propagating crack and internal crack. While no internal crack appeared in nano-Al2O3p-added coatings, and the propagating cracks are smaller than no-nano-Al2O3p one.

Effects of the high-energy compression plasma flows on the structure, elemental composition, and phase state of Al-44 at.% Si hypereutectic silumin alloy have been investigated. Using scanning electron and optical microscopy it was found decreasing in grain size of both primary silicon particles and Al-Si eutectic parts with increase of absorbed energy density of compression plasma flows. The primary silicon crystals were dispersed down to 300 nm in the result of high cooling rate of the melted layer after its homogenization by means of hydrodynamic mixing. It was found that increase in the absorbed energy density, homogenization of elemental composition in the modifies layer occurs due to increase in lifetime of the melted state and more efficiency mixing process.

An investigation on the deterioration of physical and mechanical properties of granite after cyclic thermal shock

Addition of Se was reported to have a stronger effect on refinement of primary silicon in hypereutectic Al-Si alloys than that of P. The present study was conducted to clarify effects of Se addition, holding time and superheating temperature on refinement of primary silicon in an Al-20%Si alloy. Grain size of primary silicon was determined. Changes of microstructures were observed with an optical microscope and an X-ray microanalyzer. When a melt was heated at 900°C for 15min, addition of 0.15% Se resulted in refinement of primary silicon. However, increase of Se up to 0.5% did not show a significant improvement of refining effect. Holding time at 900°C did not affect refining effect of Se. The finest silicon was obtained when 0.5%Se was added to a melt at 1, 200°C for 15min. Grey colored particles were present in primary silicon, at the interface of primary silicon with eutectic structures and in eutectic structures. These grey particles were identified to have Al and Se through a microanalysis by the X-ray microanalyzer. A definite break was observed in a cooling curve of a Se treated specimen when primary silicon started to crystallize. These results were interpreted similarly as the AlP nucleation theory in primary silicon refinement.

Marble is a common rock used in many buildings for structural or ornamental purposes and is widely distributed in underground engineering projects. The rocks are exposed to high temperatures when a tunnel fire occurs, and they will be rapidly cooled during the rescue process, which has a great impact on the rock performance and the underground engineering stability. Therefore, the role of cyclic thermal shocks on the physical and mechanical properties of marble specimens was systematically investigated. Different cyclic thermal shock treatments (T = 25, 200, 400, 600, 800 °C; N = 1, 3, 5, 7, 9) were applied to marble specimens and the changes in mass, volume, density and P-wave velocity were recorded in turn. Then, the thermal conductivity, optical microscopy and uniaxial compression tests were carried out. The results showed that both the cyclic thermal shock numbers (N) and the temperature level (T) weaken the rock properties. When the temperature of a thermal shock exceeds 600 °C, the mass loss coefficient and porosity of the marble will increase significantly. The most noticeable change in P-wave velocity occurs between 200 and 400 °C, with a 52.98% attenuation. After three thermal shocks, the cyclic thermal shock numbers have little influence on the uniaxial compressive strength and Young’s modulus of marble specimens. Shear failure is the principal failure mode in marble specimens that have experienced severe thermal damage (high N or T). The optical microscopic pictures are beneficial for illustrating the thermal cracking mechanism of marble specimens after cyclic thermal shocks.

Ag sintering has emerged as a reliable high-temperature die-attach material owing to its high thermal conductivity and mechanical stability. This study investigated the effect of bond-line thickness (BLT) on the thermal shock reliability of Ag-sintered joints through experiments and finite element analysis (FEA). Ag-sintered layers with BLTs of 50, 100, and 200 ㎛ were fabricated, and their microstructural evolution and shear strength were evaluated after thermal shock cycling between -45 ℃ and 150 ℃. Thermomechanical stress and strain distributions at 150 ℃ were analyzed using FEA. Cross-sectional SEM observations showed that cracks and delamination occurred near the Ni/Ag and Ag/AlN interface edges in the 50 ㎛ sample after thermal shock, whereas no significant interfacial damage was observed in thicker joints. The shear strength decreased with the number of cycles, dropping to about 3 MPa for the 50 ㎛ sample after 1000 cycles, while the 200 ㎛ joint maintained approximately 24 MPa. FEA showed that increasing BLT reduced interfacial stress concentration. The maximum equivalent stress decreased from 251 MPa to 141 MPa (?44%) as the BLT increased from 50 to 200 ㎛, and the equivalent strain also decreased. The experimentally observed crack locations corresponded to the high-stress regions predicted by FEA, showing consistent trends between experiment and simulation. Overall, increasing the BLT alleviated interfacial stress concentration and improved thermal shock reliability. These results provide insight into optimizing BLT for reliable high-temperature applications.

Hertzian stress analysis and characterization of thermal barrier coatings containing unidirectional vertical cracks