Effects of ordering type and degree on monoclinic distortion of 18R-type martensite in Cu-Zn-Al alloys
- PDF / 736,557 Bytes
- 8 Pages / 597 x 774 pts Page_size
- 82 Downloads / 144 Views
I.
INTRODUCTION
THE internally faulted martensites in Cu-Zn-A1 alloys are characterized by a long period stacking order, such as the 9R- or 18R-type structures, depending on the number of close-packed layers in the unit cell. Actually, the 9R and 18R structures are quite similar, except for a doubling of the unit cell in the b- and c-axis directions in the 18R case because of a change in ordering from the B2 to D 0 3 ( o r L21) type. m A number of electron microscopy and X-ray studies t1'2'3] have shown that such structures can exist in the normal or modified conditions, depending on the relative positions of the close-packed layers. The normal structure (N9R or N 18R) is orthorhombic, whereas the modified version (M9R or M18R) is monoclinic, m The difference in the structures of N18R and M18R martensites lies in the stacking positions of the close-packed layers corresponding to the basal plane. In the M18R structure, the relative displacement between two adjacent close-packed planes is deviated from their ideal positions (1/3)a or (2/3)a. m This shift along the a-axis direction may be attributed to a size difference between constituent atoms in the close-packed plane. When this plane consists of atoms with different sizes taking an ordered arrangement such as B2 or L2I, it may be distorted from an exact hexagon in order to have a more closepacked layered structure. The monoclinicity of the martensite unit cell due to this basal plane distortion has been referred to as the monoclinic distortion, t4,51 and it was suggested 16,7'13! that the extent of the monoclinic distortion may depend on the type and degree of long-range order or the alloy composition. Delaey et a1.17] suggested that this distortion decreases with increasing degree of D03 order or with decreasing degree of B2 order, but it needs further consideration in theoretical calculation. In Cu-Zn-A1 alloys, the disordered /3 parent phase DONG W. ROH, Graduate Student, and YOUNG G. KIM, Professor, are with the Department of Materials Science and Engineering, Korea Advanced Institute of Science and Technology, Seoul, Korea. EON-SIK LEE, formerly Graduate Student, Department of Materials Science and Engineering, Korea Advanced Institute of Science and Technology, is with the Metallic Materials Division, Research Institute of Industrial Science and Technology, Pohang, Korea. Manuscript submitted January 2, 1992. METALLURGICAL TRANSACTIONS A
[body-centered cubic (bcc)] present at high temperature goes through two ordering transitions upon cooling. The first transition occurring on cooling is a nearest-neighbor (nn) ordering reaction which leads to a B2 (CsCI superlattice). Further cooling induces the next-nearest-neighbor (nnn) ordering and formation of a D03 (Fe3A1 superlattice), t8'9] However, Wu et al. I~~ have recently proposed that the perfect D03 superlattice with binary superlattice sites would not be expected in the offstoichiometric Cu-Zn-A1 alloy, and the resulting structure due to the nnn ordering could be L2~ (Cu2MnAI superlattice). Since the B2-t
Data Loading...
