The Formation of A2/L2 1 Microstructure in Fe-Al-Ti-Cr Alloys
- PDF / 400,601 Bytes
- 6 Pages / 432 x 648 pts Page_size
- 45 Downloads / 178 Views
The Formation of A2/L21 Microstructure in Fe-Al-Ti-Cr Alloys Suguru Hotta1, Satoru Kobayashi2 and Takayuki Takasugi1,2 Department of Materials Science, Graduate School of Engineering, Osaka Prefecture University, 1-1Gakuen-cho Naka-ku, Sakai, Osaka 599-8531, JAPAN 2 Osaka Center for Industrial Materials Research, Institute for Materials Research, Tohoku University, 1-1Gakuen-cho Naka-ku, Sakai, Osaka 599-8531, JAPAN
1
ABSTRACT The composition areas of the A2/L21 two-phase field in the Fe-Al-Ti-Cr quaternary system at 800 ºC and 700 ºC were investigated using a diffusion-multiple (DM) technique for which Fe-30Al-7Ti, Fe-30Cr-7Ti and Fe-7Ti alloys were diffusion-coupled and heat-treated. The A2/L21 two-phase field was found to expand slightly toward lower Al content when the Cr content is increased. The field is slightly shifted toward lower Al content when the temperature is decreased. INTRODUCTION Bcc-based ordered structures such as the FeAl (B2) and Fe2AlM (L21) exist in the Fe-AlM (M = Ti, Ni, Nb, Ta) ternary systems. The ordered phases are formed within the Į-Fe (A2) matrix phase during cooling in the alloy systems [1-8]. A small misfit between the phases forms A2/B2 and A2/L21 coherent microstructures similar to Ni-base superalloys. The alloys with such “bcc-type superalloy microstructures” have been reported to show excellent high temperature strength and creep resistance [4-8]. The alloys are, however, bothered by severe brittleness at low temperatures. One of the reasons of the brittleness should be their high Al contents in the A2 matrix phase which is in equilibrium with the B2 and L21 phases. In the present study, we investigated phase equilibria in the Fe-Al-Ti-Cr quaternary system with a close attention to the A2/L21 two-phase field to aim at designing alloys strengthened by coherent L21 particles in a Cr-rich A2 matrix which can be expected to be more ductile than an Al-rich A2 matrix.
EXPERIMENTAL A diffusion-multiple (DM) technique [9-12] was used to determine phase equilibria in the Fe-Al-Ti-Cr quaternary system.
165
Figure 1. Schematic illustrations showing the heat treatment processes in a diffusion-multiple technique used in this study. The nominal compositions of the alloys used for a diffusion-multiple are listed in Table I. These alloys were prepared from iron of 4N purity, aluminum of 4N purity, titanium of 3N purity and chromium of 3N purity by arc melting under an argon atmosphere back-filled after evacuation to 8×10-4 Pa. The alloys will be referred to by their Al, Ti and Cr contents, as shown in the table. The ingots were homogenized at 1200 ºC for 20 h and subsequently cut using an electrical discharge machine. The surfaces were mechanically polished down to 0.05 ȝm Al2O3 powder for diffusion bonding. A diffusion-multiple sample was assembled in the arrangement shown in Figure 1(a), and heat-treated at 1200 ºC for 20 h to introduce composition gradients of Al and Cr in the A2 matrix, as schematically shown in Figure 1(b). The sample was subsequently annealed at 800 ºC and 700 ºC fo
Data Loading...
