Structure and mechanical properties of boron-doped cubic zirconium trialuminides
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I.
INTRODUCTION
H E R E D I A and Pope E~Jhave recently reported quite a substantial enhancement of ductility in a single crystal of the boron-doped L12 NisA1 intermetallic. This finding strongly suggests that not only does boron doping improve the grain-boundary cohesion in polycrystalline Ni3A1, but also somehow improves plasticity of the L12 matrix. Cubic A13Ti-base and A13Zr-base intermetallics have the same L12 crystal structure as that of Ni3A1, and they typically exhibit a severe lack of lattice plasticity resulting in a cleavage fracture, t2,3,4] Therefore, by analogy to Ni3A1, it is very interesting to see if there is any systematic effect of boron doping on their ductility and other mechanical properties. The effect of boron on the properties of the L12 AlsCuTi2 alloys has already been published, [5] and the present article repots the effect of boron doping on the mechanical properties of L12 A15CuZr2, A166Mn9Zr25, and A167Cr8Zr25 alloys. II.
EXPERIMENTAL PROCEDURE
The intermetallic alloys were prepared from pure elements: aluminum (99.99999 pct), zirconium (99.8 pct), copper (99.99 pet), manganese (99.9 pct), chromium (99.99 pct), and A1 + 3 wt pct B master alloy which served as a boron source. The amount of pure elements and A1 + 3 wt pct B alloy were calculated accordingly to yield the target boron dopings of 50 and 100 ppm (by I.S. VIRK, Graduate Student, and R.A. VARIN, Associate Professor of Materials Science and Engineering, are with the Department of Mechanical Engineering, University of Waterloo, Waterloo, ON, Canada N2L 3G1. Manuscript submitted July 15, 1991. METALLURGICAL TRANSACTIONS A
weight). The boron-doped L12 A15CuZr2, A166Mn9Zr25, and m167CrsZr25 (atomic percent) alloys were prepared by direct-induction melting using method 3. [6'71 Optical microscopy and quantitative energy dispersive spectroscopy (EDS) were used to observe the microstructure and chemical composition of alloys, respectively. Porosity was measured using an automated Java image analysis package by Jandel Scientific[S[ as described in Reference 6. Due to the small difference in contrast between the matrix and second phase, the volume fraction of second phase was measured using linear analysis. [91 Vickers microhardness tests were performed using loads from 25 to 2000 g at 15 seconds dwell time for all alloys. Prior to hardness tests, the specimens were first polished using a grit 600 emery paper followed by 0.3 /xm of alumina powder. A minimum of five indentations were made at each load. Both diagonals were measured with an optical microscope using an automated Java image analysis system. Is[ All Vickers indentations were only made on the matrix. Compression testing was carried out on as-cast cuboidal specimens (4 • 4 • 5 mm) cut from the ingots with an electrodischarge machine (EDM). Specimens were tested in air at room temperature (RT), peak temperatures (500 ~ to 600 ~ as observed in pure alloytt~ and 900 ~ with an Instron machine at the applied strain rate 3.3 x l 0 - 4 s - l . Always three specimens were tested
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