Microstructure and Room-Temperature Ductility of Unidirectionally Grown Ni 3 Al
- PDF / 4,270,170 Bytes
- 6 Pages / 420.48 x 639 pts Page_size
- 73 Downloads / 201 Views
MICROSTRUCTURE AND ROOM-TEMPERATURE DUCTILITY OF UNIDIRECTIONALLY GROWN Ni 3 Al TOSHIYUKI HIRANO AND TOSHIO MAWARI National Research Institute for Metals, Ibaraki 305, Japan
1-2-1,
Sengen,
Tsukuba,
ABSTRACT The formation mechanism of the columnar-grained stoichiometric Ni 3Al by FZ-UDS (unidirectional solidification using a floating zone method) was investigated. The quenched solidification interface showed that Ni 3A1 and I-NiAl formed simultaneously in the form of lamellar structure at the interface. The I-NiAl dissolved into the Ni 3Al matrix during cooling, resulting in the columnar-grained structure of single phase Ni 3 Al at room temperature. The columnar-grained Ni3 AI had a large number of low angle boundaries and exhibited a large tensile ductility. The characteristics of the deformation behavior were also investigated. Slip lines easily transferred acros5 grain boundaries without crack initiation. Deformed bands with many stacking faults were produced by deformation. INTRODUCTION Polycrystalline Ni3 Al is brittle at room temperature owing to intrinsic grain-boundary brittleness, and unless doped with a small amount of boron [I], it easily suffers from intergranular fracture, showing much limited ductility. We recently found that unidirectional solidification (UDS) using a floating zone method (FZ) overcomes this problem without boron doping. We call this method FZ-UDS [2,3]. The stoichiometric Ni3Al grown by FZ-UDS has a columnar-grained structure and shows about a 60% larger tensile elongation at room temperature. The fracture mode is a transgranular type. Of particular interest is that even Al-rich Ni 3Al can be ductilized [4], which is quite different from the boron-doping method [5]. These results indicate the FZ-UDS Is a promising way to improve the ductility of intermetallic compounds. It is Important to examine the formation mechanism of the columnar-grained structure since the ductilization by FZ-UDS has a close relation with this structure. In this study we present the formation mechanism of the columnar-grained Ni 3Al in the FZ-UDS process and some features of the deformation behavior at room temperature. EXPERIMENTAL The feed rod preparation and the FZ-UDS procedure were the same as previously described [2]. Polycrystalline Ni 3Al rods (Ni25.2 at%Al) were grown at a growth rate of 25 mm/h by FZ-UDS in a flowing argon atmosphere. When a steady-rate was attained the process was rapidly stopped by deenergizing the halogen lamps which were used as a heating source in the FZ furnace. The molten zone, the grown alloy, and the solidification interface were quenched. The grown rods were cut along the growth direction and the longitudinal sections were subjected to metallographic observations. The crystal structure was examined by X-ray diffraction (XRD) and transmission electron diffraction (TED). The grain orientation and grain-boundary character were examined by electron channeling pattern using scanning electron microscope. Sheet tensile specimens with a gauge section of 0.7x4.8xlO Mat. Res. Soc
Data Loading...
