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DUCTILITY ENHANCEMENT VIA MICROSTRUCTURAL CONTROL IN Cr MODIFIED Ni3 A1 SINGLE CRYSTAL S.E. HSU, Y.P. WU, T.S. LEE AND S.C. YANG Materials Research and Development Center, Chung Shan Institute of Science and Technology, Lung-Tan, Taiwan,

R.O.C.

ABSTRACT An attempt to improve the ductility in Cr modified Ni 3Al alloys is made by means of microstructural control. In order to select an alloy composition such that it locates in the 71 single phase region at 1000 °C and in the y + y' two-phase region at 1200 0C of the Ni-Al-Cr ternary phase diagram, we first calculated 7 and 7' phase boundaries, using the Cluster Variation Method. Single crystal specimens with desired composition of Ni-19.5A1-7.5Cr at% were grown and homogenized at the above two temperatures for 72 hours and the mechanical properties were tested at room temperature. It is found that the elongation of specimen homogenized at 1200 OC is almost twice as large as that of one homogenized at 1000 *C. The theoretically predicted microstructures and the 7 precipitates are also confirmed by optical, SEM micrographs and EDS analysis. It is concluded that a small amount of disordered 7 phase precipitates in the y' matrix can drastically enhance the tensile ductility of Ni 3Al (L1 2 )-base alloys INTRODUCTION Aluminide intermetallic alloys are currently of considerable interest as a basis for a new generation of high temperature structural materials. In particular, alloys based on Ni 3Al are especially attractive because of their positive temperature dependence of yield strength and excellent high temperature oxidation resistance. However, most ordered intermetallics are, in nature, brittle, which provides one reason why they have had limited applications as structural materials yet. In this regard, a large amount of research studies have been directing at the ductility improvement in ordered intermetallics. Based on our past experience in the study of a number of different ternary additions to Ni 3Al, we have empirically found that, in addition to Ni-rich alloys, specimens with good ductility were always obtained if there are small amount of disordered 7 phase precipitates in the 7' matrix (1]. However, since the alloy compositions were not fixed, it is difficult to determine whether the ductilization is due to different chemistry or microstructure. The purpose of this work is to test the idea that a new method to ductilize an L1 2 type ordered intermetallics can be accomplished by means of a proper microstructural control (i.e. control the thermal treatment).

Mat. Res. Soc. Symp. Proc. Vol. 288. 01993 Materials Research Society

618

Al 0.0

0.3 0-0

1200C 1AIO000

0.1

0.2

Cr 0.7

Figure 1:

..

0.2

.

",

0.8 Atomic Concentration

0.1

0.9 Ni

Calculated isothermal section of 7/7' phase boundaries in the Ni-Al-Cr system at 1000 °C (triangle symbol) and 1200 °C (circle symbol). The composition of Ni-19.5AI-7.5Cr alloy is labelled "x" within the shaded area.

ALLOY DESIGN As can be seen from the binary Ni-Al phase diagram that the solubility of the 7' phase