CenterforAdvancedAerospaceMaterials,PohangUniversityofScienceand Technology,Pohang790-784,RepublicofKorea bPlateResearchGroup,TechnicalResearchLaboratories,POSCO,Pohang790-785, RepublicofKorea Received11February2005;receivedinrevisedform11May2006;accepted11May 2006