Key Engineering Materials Vol. 741

Abstract: Porous aluminum can potentially satisfy both the lightweight and high-energy-absorption properties required for automotive components. In this study, functionally graded porous aluminum consisting of pure aluminum and Al-Mg-Si A6061 aluminum alloy was fabricated by a sintering and dissolution process. It was found that functionally graded porous aluminum with the same pore structures but different types of aluminum alloy can be fabricated. By performing compression tests on the fabricated functionally graded porous aluminum, it was found that its stress-strain curve initially exhibited a relatively low plateau stress similar to that of uniform porous pure aluminum. Thereafter, the stress-strain curves exhibited a relatively high plateau stress similar to that of the uniform porous A6061 aluminum alloy. Namely, it was found that the compression properties of porous aluminum can be adjusted and optimized by selecting the appropriate type of aluminum alloy.

Abstract: Recently, sintering joint using Ag-nanohas been attracting attention as a new joint method to replace the solder. However, the joint layer would contain a lot of voids after sintering processes. Since the voids affect mechanical property, the proper sintering conditions have to be selected in order to reduce these voids. In this research, the authors focus on the effect of pressure conditions at sintering process. Then, by creating FEM models including voids from cross section image of the joint layer and carrying out tensile analysis, the mechanical property of the joint layer has been acquired. Using this approach, the influence of pressure conditions on the mechanical properties is revealed.

Abstract: The paper is focused on the analysis of the role of lamellar microstructure in fracture performance of model TiAl intermetallic compound. Coarse lamellar colonies and, at the same time, fine lamellar morphology were prepared by compressive deformation at 1553 K (region of stable α phase in TiAl equilibrium diagram) followed by controlled cooling to 1473 K (region of α+g phase) with delay on this temperature and then cooling down. The fracture toughness was evaluated by means of chevron notch technique. In addition, because of enhanced toughness, crack resistance curves were obtained by load - unload technique of pre-racked beams, namely in two directions of crack propagation relative to lamellar structure. Extensive development of shear ligament toughening mechanism was observed in fracture surfaces leading to quite good fracture toughness thanks to the heat treatment applied.

Abstract: This paper describes a new evaluation method of resin delamination strength at the adhesive interface. Power module consists of different materials. Difference of thermal expansion between the dissimilar materials causes the delamination at their interface. The new pudding-cup test method has been used to evaluate the delamination stress, where singular stress or stress intensity factors are used to evaluate the delamination strength. But it is difficult to consider the effect of micro interface structure or bonded compound. In this study, an interface structure was considered to analyze the delamination deformation at the resin interface to develop a new approach to evaluate the delamination behavior. However, it was difficult to recreate the details in the analytical model. In this study, a method to simplify the interface structure as a simple interface layer was proposed. Deformation value of the interface structure is evaluated in this study. As a result, it was confirmed that the analytical method to simplify the interface structure can be used to simulate the delamination behavior. Deformation absolute value in the delamination points was confirmed to agree with various load points in same material. These results suggested the deformation of the interface layer can be used as a simple parameter to estimate the delamination of a resin structure boundary.

Abstract: The effects of massive transformation and subsequent heat treatments on the microstructure of Ti-46Al-7Nb-0.7Cr-0.2Ni-0.1Si (mol%) intermetallic compounds are studied. Massive transformation occurs at the center region of the specimen by cooling from α single phase state. At the surface side of the specimen, α phase has remained. Fine convoluted microstructure with α₂, γ phases and lamellar structure has formed by heating at (α+γ) two phase state after massive transformation. Colony size or grain size is about 25 μm. Fine fully lamellar structure is obtained after heat treatment of convoluted microstructure at α phase for 60 s. Fracture toughness seems to be increasing with the increase in lamellar colony size. However, some massively transformed specimens show lower toughness due to the formation of microdamage present in samples before the test.

Abstract: In this study, the effect of metal salt coating processing of aluminum surface on the bond strength of the solid-state diffusion bonded interface of titanium and aluminum has been investigated by SEM observation of the interfacial microstructures and fractured surfaces after tensile test. Aluminum surfaces were coated by boiling in 5% aqueous solution of NaOH for 90 s and 98% formic acid for 60 s. Bonding process was performed at a bonding temperature of 713 ~ 773 K under a load of 12 MPa (for a bonding time of 900 s). As a result of the metal salt coating processing, high strength joint can be achieved with lower bonding temperature compared with unmodified joints. From this study, it is found out that metal salt coating processing is effective at removing oxide film and substitution to metal salt on the aluminum bonding surface.

Abstract: The tensile properties and the stability of retained austenite in TRIP steels with different volume fraction of retained austenite have been studied at low temperature. The steels showed a good valance of strength and ductility at 193 K. Their work-hardening rates were decreased linearly and kept a high value in the high strain regime at 193 K. The retained austenite was mostly transformed into martensite less than 10% strain at 193 K.

Abstract: The effects of chloride ion concentration on SCC susceptibility of 15Cr and 13Cr martensitic stainless steels were investigated at 180°C by SSRT. Transgranular SCC occurred in the environment containing CO₂ and chloride ion. The increasing chloride ion concentration was significantly affected SCC susceptibility of 15Cr SS. In addition, the contribution of hydrogen to SCC was examined at high temperature by SSRT with electrochemically polarization. The cathodically charged specimens showed hydrogen embrittlement. The fracture surface was similar to that of high temperature SCC. On the other hand, the SCC was accelerated by anodic polarization and not by cathodic polarization. The SCC behavior of martensitic stainless steel at high temperature is affect by evolved hydrogen atom. It is concluded that hydrogen plays a key role in the crack propagation.

Abstract: This paper is focused on prediction of nitinol stents lifetime. Stents have been tested before and after mechanical loading. Corrosion tests of mechanically loaded specimens indicated impairment of corrosion resistance and SEM observation confirmed presence of micro cracks in surface oxide layer created during heat treatment. Electrochemical impedance spectroscopy and potentiodynamic polarization in physiological solution was used for these particular experiments. Corrosion rate and other parameters have been used for calculation of released nickel ions weight and prediction of application lifetime.