Solid State Phenomena Vols. 124-126

Abstract: The main emphasis of this study is to utilize thixoextrusion process for improving extrudability of 7075 Al wrought alloy. In the present study, the influences of thixoextrusion parameters, such as initial ram speed and die bearing length, on mechanical properties of thixoextruded 7075 Al wrought alloy were investigated. The results of thixoextrusion experiments about microstructures and mechanical properties were compared with conventional extrusion results. The maximum extrusion pressure of thixoextrusion processes was seven times lower than that of conventional hot extrusion processes. The hot extruded bars show a large amount of grains which are stretched along the extrusion direction and asymmetry of grains was formed to extrusion direction, while the microstructures of the thixoextruded bar did not show any asymmetry and grains were homogeneously distributed. The hardness values of thixoextruded bar were higher than that of conventional hot extruded bar.

Abstract: Flexible printed circuit board (PCB), which is used for folder and slide type cellular phones, consists of flexible copper clad laminate (FCCL) and cover layer. Through it an electric current is applied to liquid crystal display (LCD) from the main board of cellular phone. In thin Cu foils of flexible PCB fatigue cracks due to repeated bending motion generate and propagate, and they cause a short circuit. Fatigue behavior of thin Cu foils being used for flexible PCB must be evaluated and confirmed to resolve this problem. It is based on findings by several researches that the mechanical properties of thin film materials differ from those of their bulk counterparts. Thin film properties have been investigated over the last years; however fatigue behavior of thin films has not yet been studied as thoroughly as monotonic behavior. In this study fatigue properties of thin Cu foils for the application in flexible PCB are obtained. Fatigue testing was performed for two kinds of Cu foils that were made by rolling and electrochemical procedures respectively. Differences of fabrications in fatigue behavior of thin foils were distinguished. Especially for rolled Cu foils, effects of rolling directions in fatigue properties were evaluated.

Abstract: The difference in annealing characteristics of oxygen free copper (OFC) and deoxidized low-phosphorous copper (DLP) processed by ARB was studied. The copper alloys processed by eight cycles of the ARB were annealed for 10 minutes at various temperatures ranging from 100 to 400°C. The variation of microstructure and mechanical properties with annealing was significantly different in both copper alloys. In case of OFC, the ultrafine grained (UFG) structure formed by the ARB still remained up to 200°C, and above 200°C it was completely replaced with a coarse grained structure due to an occurrence of the conventional recrystallization. However, in case of DLP, the recrystallization did not occur even at 350°C. The strength of the OFC also decreased significantly at annealing temperatures above 200°C, while the hardness of the DLP did not decrease so largely up to 350°C. These differences in annealing characteristics in both copper alloys were discussed in terms of purity.

Abstract: The effects of oxygen content on microstructures, elastic modulus and tensile properties of quenched Ti-Nb alloys were investigated in order to design a desirable Ti based alloy through casting process. From the microstructural and phase analysis, it is evidently revealed that the volume fraction of β phase increased with increasing content of oxygen, and the occurrence of intermediate ω phase was suppressed in metastable β Ti-Nb based alloys. Martensite transformation temperature decreased with increasing content of oxygen. Therefore, it is suggested that oxygen acts to stabilize β phase rather than α stabilizer in quenched state. Yield strength increased with increasing content of oxygen without a large consumption of ductility in metastable β Ti-Nb based alloys. The variation of mechanical property was explained by the phase stability, phase formation and microstructure in correlation with oxygen and Nb content.

Abstract: Acoustic emission (AE) technique was applied to evaluate both tensile deformation of 316L stainless steel and bending-fatigue damage of socket-welded pipe. AE activity was monitored during tensile deformation of plate-type specimens subjected to various heat treatments. Variation in RMS voltage of AE signal was correlated with the amount of strain-induced martensite phase. Secondly, actual size of socket-welded pipe specimen was bending-fatigued under various stresses. Crack initiation was determined by observing an abrupt increase in AE count, and confirmed by radiographic examination before and after the crack initiation cycle. Potentials of AE technique for monitoring fatigue crack initiation were discussed.

Abstract: Fatigue crack growth behavior of the ultrafine-grained pure Ti produced by ECAP was investigated. The ECAPed sample with a total strain of 4.6 showed nearly equiaxed grains with an average size of about 0.3 μm. After ECAP, the ultimate tensile strength was increased by 60%, while the tensile elongation was decreased by 31%. The ECAPed Ti showed much higher resistance in fatigue crack growth by about a factor of 4 over the whole +K due to plasticity-induced crack closure.

Abstract: The dual phase steel, which consists of hard martensite islands embedded in a ductile ferrite matrix, is known to possess high strength, toughness, and superior wear resistance. However, the detailed wear mechanism of the steel has not yet been understood thoroughly. In the present study, dry sliding friction and wear characteristics of an ultra-fine grained ferrite-martensite dual phase steel has been investigated at room temperature. Wear tests of the steel were carried out using a pin-on-disk wear tester against an AISI 52100 bearing steel ball at loads ranging from 1N to 10N. Normalizing heat treatment was also performed on the steel to produce a ferrite-pearlite microstructure, and the wear characteristics of the normalized specimen were compared with that of the dual phase steel. The dual phase steel exhibited lower wear rates than the normalized steel, but the steady-state friction coefficients of the two steels were similar. The wear of the dual phase steel proceeded with a tribochemical reaction on the wearing surface accompanied with subsurface strain hardening, which explained the lower wear rate of the steel.

Abstract: The activation energy for recovery and recrystallization was calculated using DSC data. The annealing below 250°C resulted in the bimodal grain size distribution, while that above 300°C resulted in the uniform distribution of coarse grains. The formation of a bimodal microstructure would be responsible for the good combination of uniform elongation and tensile strength. Additionally, the little variation of hardness for different annealing time at 300°C also indicated that mechanical properties of deformed and annealed 5052 Al alloy were significantly influenced by the volume fraction of recrystallized grains rather than the coarsening of recrystallized grains.

Abstract: The microstructures and mechanical properties of unidirectional deformation structured Al alloy during ECAP with various deformation routes were investigated. In order to fabricate unidirectional deformation structure for Al alloy, hot extrusion was carried out. It was found that the deformation route A in ECAP routes is the dominant route for the grain refinement and strengthening. In deformation route A, the high strength ultra-fine grained Al alloy with a grain size of ~ 200 nm was obtained due to the accumulation of consecutive strain process. In contrast, the strength of ECAP’ed Al alloy produced via deformation route C was greatly increased after one pass because the grains were strained and cancelled each pass. By contrast, the equiaxed grains were obtained in deformation route BC because the sample was rotated 90 O in the same sense in each pass. The deformation route BC was superior to the deformation route C because the deformation route BC was more favorable than the deformation route C in the accumulation of consecutive strain. It is also found that unidirectional deformation structured Al alloy via hot extrusion shows similar grain refinement tendency with equiaxed structured Al alloy during ECAP processing.