Koutarou Hayashi

Decarburization of 0.21C-1.3Mn-0.2Si Steel for Hot Stamping at Various Heating Temperatures

In order to examine the decarburization behavior in the hot stamping (HS) method, the dependence of the microstructure evolution on the annealing temperature was experimentally studied using a Fe-0.21 mass% C-1.3 mass% Mn-0.2 mass% Si steel. The steel was isothermally annealed in the temperature range of T = 773-1173 K for various times of t = 100-12800 s in an ambient atmosphere. Here, the steel possesses the ferrite (α) + cementite (θ) two-phase microstructure at T = 773-923 K, the α + austenite (γ) two-phase microstructure at T = 1013-1073 K, and the γ single-phase microstructure at T = 1093-1173 K. During annealing at T = 1013-1073 K for t = 1600 s, however, the α layer with a uniform thickness is formed at the surface of the steel due to decarburization and gradually grows into the inside. Such formation of the a layer was not clearly observed at T 973 K and T 1093 K. Thus, the formation of the α layer hardly occurs under the HS annealing conditions. At T = 1033 K, the thickness of the α layer is mostly proportional to the square root of the annealing time. Such a relationship is called the parabolic relationship. Furthermore, the grain size of the α layer monotonically increases with increasing annealing time. Hence, the parabolic relationship guarantees that the growth of the α layer is controlled by volume diffusion.

Observation of Fast Positron Diffraction from a Si(111)7x7 Surface

In this article we present the first observation of high-energy positron diffraction from a clean Si(111)7x7 surface. A 20 keV positron beam with a sufficient coherence length was reflected at the surface and a diffraction pattern was observed. The fractional order Laue zones, which were assigned to be the 1/7-th, 2/7-th and 3/7-th orders from the kinematical consideration were well seen. The rocking curve (diffraction intensity versus glancing angle plot) was also measured. In the total reflection region of the rocking curve, the intensity distribution was hardly reproduced when adopting the distance between the adatom and stacking fault layers and also the damping potential due to the electronic excitation determined previously. Possible reasons are discussed.