Nonequilibrium synthesis of NbAl 3 and Nb-Al-V
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INTRODUCTION
E A R L I E R articles tl,2j in this journal reported the microstructure and oxidation properties of NiToA120CrTHf3 alloys produced by laser cladding, a nonequilibrium synthesis technique. In this article, we will discuss the microstructure of laser-clad NbA13 and Nb-A1-V alloys. A13X aluminides, where X represents Ti, Zr, Fe, V, Nb, etc., are presently materials of interest for hightemperature structural applications that require a high strength-to-weight ratio and superior oxidation resistance. However, the room-temperature ductility of these materials is very poor, and efforts are underway to improve this via alloying additions and rapid solidification processing techniques, t3-13] In materials which solidify under equilibrium conditions to the D022 crystal structure (tI8), as in the case of A13Ti, A13Nb, and AI3V , deformation has been found to proceed mainly by twinning of the {111} (112) type, which does not disturb the D022 symmetry of the lattice. This means that there are only four independent ordered twinning systems. Therefore, the improvement of ductility of A13X compounds by partial replacement of the X by other elements of the group IVB, VB, and VIB arises from the fact that the activity of ordered twinning is increased substantially from that for the binary compounds. However, it is also important to know the effect of this alloying addition on the oxidation response of the system at high temperatures and oxidizing atmospheres for which these alloys are being conceived. The oxidation behavior of Nb alloys was the subject S. SIRCAR, Principal Scientist, is with the Lockheed Engineering and Sciences Company, NASA-White Sands Test Facility, Johnson Space Center, Las Cruces, NM 88004. K. CHATTOPADHYAY, Associate Professor, is on leave from the Department of Metallurgy, Indian Institute of Science, Bangalore, India. J. MAZUMDER, Professor, is with the Department of Mechanical and Industrial Engineering, Laser Aided Materials Processing Laboratory, University of Illinois at Urbana-Champaign, Urbana, IL 61801. Manuscript submitted February 28, 1991. METALLURGICAL TRANSACTIONS A
of considerable research during the period of 1955 through 1970 and has been reviewed extensively by Stringer) ~4J Although this work does not make any recommendations on the type of alloy to be selected for various uses, it gives a very comprehensive data base for various Nb alloys produced by conventional (equilibrium) procedures. Investigations were also carried out by Svedburg t~SJ in 1975 and 1976 to produce high-temperature oxidationresistant Nb-base alloys. Of all the binary alloys he studied, NbA13 was the one which had the slowest rate of oxidation at 1200 ~ and also formed a continuous protective layer of A1203. Sophisticated techniques have emerged over the years to study the surface layers of oxidation products. These techniques have been utilized to study surface oxides in Nb-alloys, as shown in Part II. [311Recently, Perkins et al. 116'17A81have been conducting studies to develop a model to understand
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