On Mo-9Si-18B Alloys with T 2 -Mo ss eutectic microstructure: Mechanical Properties and Protective Silicide Coating
- PDF / 2,275,701 Bytes
- 6 Pages / 612 x 792 pts (letter) Page_size
- 40 Downloads / 230 Views
BB5.26.1
On Mo-9Si-18B Alloys with T2-Moss eutectic microstructure: Mechanical Properties and Protective Silicide Coating Michiaki Kumagai, Kazuhiro Ito and Masaharu Yamaguchi Department of Materials Science and Engineering, Kyoto University, Sakyo-ku, Kyoto 606-8501, Japan
ABSTRACT Direct access to a T2-Moss eutectic microstructure with a nominal composition of Mo-9Si-18B (at.%) was obtained using an optical floating zone method at a growth rate of less than 5mm/h. The Mo-9Si-18B two-phase alloy has relatively high 0.2% proof stress at 1500°C of about 700 MPa and fracture toughness at room temperature (RT) of ~11 MPa√m. The (Mo,Nb)-19.5Si-B alloys with (Mo,Nb)ss-T1-T2 three-phase eutectic microstructure have lower yield stress at 1500°C and fracture toughness at RT lower than the Mo-9Si-18B two-phase alloy. The dense short rod-type morphology of Moss in the Mo-9Si-18B alloy may be pulled out of T2 matrix and crack bridging may play a crucial role in toughening. Due to the limited ductility of Mo at RT, its morphology is more important rather than its volume fraction in improving the fracture toughness of multi-phase alloys. Silicide coating on the Mo-9Si-18B alloys with the T2-Moss two-phase microstructure using a halide-activated pack cementation was also examined. The B-doped Mo5Si3 layer with the D8m structure was formed on the substrate after oxidation at 1500°C for 24 hours. The substrate is the source of B atoms. A steady-state weight gain was observed at 1300-1500°C and their steady-state constants are almost equal to those of MoSi2. The weight loss was saturated in 16-26 cycles at 1500°C. It was about –0.4mg/cm2, suggesting thermal stress resistance of the coated Mo-9Si-18B two-phase alloys is sufficient in the present condition.
INTRODUCTION Multi-phase alloys based on the Mo5SiB2(T2) phase in the Mo-Si-B system have recently been attracting much attention for use at ultra-high temperatures. Among the alloys based on the T2 phase, Mo-rich alloys containing the Mo solid solution phase (Moss) have been the most intensively investigated [1-7]. This stems mainly from the facts that the T2-Moss two-phase microstructure is stable up to 2100°C and the existence of Moss is beneficial to increasing toughness of the alloys. However, basic information on the mechanical properties of the T2-Moss two-phase eutectic microstructure is still very much lacking, due to difficulty in its synthesis. We succeeded in growing Mo-9Si-18B (at.%) alloys with the T2-Moss two-phase eutectic microstructure and its mechanical properties were measured. When some of Mo atoms are substituted by Nb atoms in Mo-Si-B alloys, (Mo,Nb)ss-(Mo,Nb)5Si3(T1)-T2 three-phase eutectic microstructure was reported to be obtained [4]. The three-phase microstructure can be obtained by arc-melting and its microstructure can be controlled by growth rate in a directional solidification method. In this study, mechanical properties of the (Mo,Nb)-19.5Si-B alloys with (Mo,Nb)ss-T1-T2 three-phase eutectic microstructure were also investigated to obtain systema
Data Loading...
