Papers by Author: Il Ho Kim

Abstract: The formation behavior of γ″(Ni3Nb) precipitates in IN 718 alloy before and after ECAP(equal channel angular pressing) was investigated by microstructural observation and the hardness test. For the alloy examined before ECAP, the γ″ precipitates were formed only after aging treatment at 720
, whereas after ECAP, the γ″ precipitates were formed at the aging temperatures of both 600
and 720
. 600
is normally too low a temperature for γ″ precipitates to be formed in commercial IN 718 alloy, however, they were able to be formed due to severe plastic deformation by ECAP. It was found that the ECAP process changed the formation behavior of the γ″ precipitates in IN 718 alloy.

Abstract: For measuring deformation of electronic packages, various techniques have been used. Each technique has some merits and demerits. Some techniques have very high resolution but small measuring area. On the contrary, other techniques have large measuring area and low resolution. So the judicious selection of techniques with a trade-off between the resolution and available measuring area is important. In this research, a new laser profiler was developed by integrating high resolution laser displacement sensor into the x-y scanner. This system has 10nm vertical resolution and 100nm horizontal resolution with a measurement area up to 25mm by 25mm. The residual deformation of a lead-contained and lead-free PBGA package after reflow process was measured by the newly developed system. And the effect of aging was evaluated.

Abstract: This study investigated the microstructure and mechanical properties of a wide-gap region brazed with various powders mixing ratios of additive powder to filler metal powder. The IN738 and BNi-3 alloy powders were used as additive and filler metal powder for the brazing process. The wide-gap brazing process was carried out in a vacuum of 2×10-5 torr. It was observed that the region brazed with only filler metal had a microstructure consisting of fully eutectic and dendrite structures. However, the region brazed with 60wt.% IN738 additive powder consisted of IN738 additive, Ni3B-Ni eutectic and (Cr, W)B. The fracture strength of the wide-gap region (70 wt.% additive and 30 wt.% filler metal powder) was as high as 687 MPa at room temperature. The Cracks in the wide-gap brazed region initiated at the Ni3B-Ni eutectic and (Cr, W)B, and then propagated through them. It was found that the Ni3B-Ni eutectic and (Cr, W)B in the brazed region are important microstructural factors affecting the mechanical properties of the wide-gap brazed region.

Abstract: The effects of adding Y2O3, and the precipitation of Ni3Nb by heat treatment, on the mechanical properties of mechanically alloyed Ni20Cr20Fe5Nb alloy were studied. The addition of Y2O3 caused an increase in the tensile strength at room temperature, 400°C and 600°C. The difference in the tensile strength between the Ni20Cr20Fe5Nb and Ni20Cr20Fe5Nb1Y2O3 alloys decreased gradually with increasing test temperature. The tensile strength of the Ni20Cr20Fe5Nb1Y2O3 alloy at relatively low temperature was increased by the addition of Y2O3, but decreased abruptly at temperature above 600°C. This seems to result from a change in the deformation mechanism due to the ultra-fine grain size, that is, grain boundary sliding is predominant at temperatures above 600°C while internal dislocation movement is predominant at temperatures below 600°C. Following the conventional heat treatment of the solution and subsequent aging, only a small amount of δ(Ni3Nb) phase was formed in the Ni20Cr20Fe5Nb alloy, whereas in a previous report it was indicated that a large amount of γ″(Ni3Nb) was formed in IN 718 alloy. The small amount of δ(Ni3Nb) phase formed in the present case is due to the exhaustion of the Nb content resulting from the formation of NbC during consolidation.

Abstract: The cyclic deformation in Cu and Cu-35Zn alloy were studied using ultrasonic nondestructive evaluation (NDE) technique to measure the ultrasonic velocity, attenuation coefficient and a nonlinear parameter. These materials were cyclically deformed under total strain amplitude control to investigate their ultrasonic reaction to different dislocation substructures, depending on the stacking fault energy difference. The microstructure evolution was observed using a transmission electron microscope, and the ultrasonic NDE parameter was measured after several cycles of fatigue deformation, in order to clarify the relationship between them. In both materials, the ultrasonic velocity was observed to decrease as the fatigue life fraction increased, which was attributed to the increasing dislocation density caused by the cyclic deformation. In the case of Cu, with its cell structure evolving during cyclic deformation, the rate of increase of the ultrasonic attenuation coefficient and the ultrasonic nonlinear parameter was higher than that observed in the case of the Cu-35Zn alloy with its planar array structure. This result implies that the dislocation cell structure is more sensitive to the ultrasonic parameter changes than the planar array structure formed during cyclic deformation.

Abstract: This study investigated the microstructure and mechanical properties of a wide-gap region brazed with various process variables. The IN738 and DF 4B alloy powders were used as additive and filler metal powder for the brazing process. The wide-gap brazing process was carried out in a vacuum of 2×10-5 torr. The wide-gap region brazed with 60wt.% IN738 additive powder had a microstructure consisting of IN738 additive and (Cr, W)2B. The fracture strength of the wide-gap region (60 wt.% additive and 40 wt.% filler metal powder) brazed at 1230°C for 30hr was as high as 862MPa (93% of base material strength). It was observed that the brazing temperature was the main process variable affecting the mechanical properties of the wide-gap brazed region. The creep rupture life of the region brazed with 60wt.% additive and 40 wt.% was longer than that of other brazed samples. The Cracks in the wide-gap brazed region initiated at the (Cr, W)2B and propagated through them. It was found that the (Cr, W)2B and the pore in the brazed region are important microstructural factors affecting the mechanical properties of the wide-gap brazed region.

Abstract: The effects of adding Al, Y2O3 and the use of H2O as a PCA (process control agent), on the mechanical properties of mechanically alloyed Ni20Cr20Fe5Nb alloy were studied. The addition of Y2O3 and Al caused an increase in the tensile strength at room temperature, 400°C and 600°C. However, it was confirmed that the increase of tensile strength at room temperature and 400°C was predominantly caused by addition of Y2O3, while that at 600°C was mainly due to addition of Al. These results can be attributed to the dispersion strengthening of Y2O3, preventing the formation of Cr2O3 and the change of fracture mode at 600°C by the addition of Al. Therefore, the Ni20Cr20Fe5Nb2Al alloy using H2O as a PCA showed superior tensile strength at room temperature, 400°C and 600°C. The increase in the tensile strength at room temperature and 400°C can be attributed to the strengthening of the solid solution induced by the increase in the amount of Nb solid solution, resulting from the prevention of NbC formation, while the increase in the tensile strength at 600°C can be attributed to the strengthening of the grain boundary afforded by the presence of Al1.54Cr0.46O3 formed by the addition of Al. After aging treatment for 10 hours at 600°C, g²(Ni3Nb) precipitates were formed in the Ni20Cr20Fe5Nb2Al alloy in which H2O used as the PCA, and the formation of these precipitates caused an increase in hardness.

Abstract: Fatigue behaviors of 63Sn37Pb and two types of lead-free solder joints were compared using pseudo-power cycling testing method, which provides more realistic load cycling to solder joints than chamber cycling method does. Pseudo-power cycling test was performed in various temperature ranges to evaluate the shear strain effect. A nonlinear finite element model was used to simulate the thermally induced visco-plastic deformation of solder joint in BGA packages. The results revealed that lead free solder joints have a good fatigue property in the low temperature condition, where a small strain was induced. In the high temperature condition where a large strain was induced, however, lead contained solder joints have a longer fatigue life.

Abstract: The oxidation and tensile properties of a Ni20Cr20Fe5Nb alloy and a Ni20Cr20Fe 5Nb1Y2O3 alloy with nano-sized grains were compared with those of the comercial IN718 alloy. The oxidation resistance of the Ni20Cr20Fe5Nb1Y2O3 alloy was superior to that of the Ni20Cr20Fe5Nb and IN 718 alloys. This superior oxidation resistance was the result of both the formation of dense oxides on the surface of the alloy and the interruption of Cr migration in the alloy by the addition of Y2O3. Moreover, the tensile property of the Ni20Cr20Fe5Nb1Y2O3 alloy at room temperature and 400oC was higher than that of the Ni20Cr20Fe5Nb and IN718 alloys by more than 300MPa (30%). This result can be attributed to the dispersion strengthening of Y2O3. The relatively low tensile strength at 600°C and 800°C of the alloys fabricated by mechanical alloying was attributed to grain refinement showing intergranular fracture at high temperatures.

Abstract: The oxidation characteristics of a Ni-20Cr-20Fe-5Nb-1Y2O3 alloy and a Ni-20Cr- 20Fe-5Nb alloy both with ultra-fine grains were compared with those of commercial Inconel 718 and PM1000 alloys. The oxidation resistance of the Ni-20Cr-20Fe-5Nb-1Y2O3 alloy was superior to that of both the Ni-20Cr-20Fe-5Nb and commercial Inconel 718 alloys. This was attributed to the suppression of successive oxidation via the oxide formed during the oxidation of the ODS (oxide dispersion strengthening) alloy. It was confirmed that the prior powder boundary is the site where the oxide grows continuously during ODS alloy oxidation.