Fatigue crack paths and properties in A356-T6 aluminum alloy microstructurally modified by friction stir processing under different conditions

A356-T6 cast aluminum alloy is a light weight structural material, but fatigue crack initiates and propagates from a casting defect leading to final fracture. Thus it is important to eliminate casting defects. In this study, friction stir processing (FSP) was applied to A356-T6, in which rotating tool with probe and shoulder was plunged into the material and travels along the longitudinal direction to induce severe plastic deformation,resulting in the modification of microstructure. Two different processing conditions with low and high toolrotational speeds were tried and subsequently fully reversed fatigue tests were performed to investigate theeffect of processing conditions on the crack initiation and propagation behavior. The fatigue strengths weresuccessfully improved by both conditions due to the elimination of casting defects. But the lower tool rotationalspeed could further improve fatigue strength than the higher speed. EBSD analyses revealed that the higher tool rotational speed resulted in the severer texture having detrimental effects on fatigue crack initiation andpropagation resistances.&nbsp

EBSD-assisted fractographic analysis of crack paths in magnesium alloy

 Magnesium (Mg) alloys are attractive as structural materials due to their light weight and high specific strength. It is well known that Mg alloy has hexagonal close-packed (HCP) structure and only basal slipor twinning can operate during plastic deformation because critical resolved shear stresses of the other slipsystems such as pyramidal or prismatic slips are much higher than the basal slip. Thus sometimes characteristicfracture surfaces are formed during stress corrosion cracking (SCC) or fatigue crack propagation (FCP) in Mgalloys, where many parallel lines are formed. These lines are different from so-called fatigue striations, becausethey are formed even under sustained load condition of SCC. Consequently, electron back scattered diffraction(EBSD) technique was applied on the fracture surface, and the formation mechanism of parallel lines wasinvestigated. EBSD-assisted fractography had revealed that the characteristic parallel lines were formed due tothe operation of basal slips, not twining. It is considered that hydrogen-enhanced localized plasticity (HELP)mechanism had been activated under corrosive environment

Charge Ordering in alpha-(BEDT-TTF)2I3 by synchrotron x-ray diffraction

The spatial charge arrangement of a typical quasi-two-dimensional organic conductor alpha-(BEDT-TTF)2I3 is revealed by single crystal structure analysis using synchrotron radiation. The results show that the horizontal stripe type structure, which was suggested by mean field theory, is established. We also find the charge disproportion above the metal-insulator transition temperature and a significant change in transfer integrals caused by the phase transition. Our result elucidates the insulating phase of this material as a 2k_F charge density localization.Comment: 8 pages, 5 figures, 1 tabl