Mechanical Properties and Strengthening Mechanisms of Al-15 Pct B 4 C Composites with Sc and Zr at Elevated Temperatures
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I.
INTRODUCTION
AL-B4C metal matrix composites (MMCs) have been widely used as neutron absorber materials in the transport and storage of spent nuclear fuels in the nuclear industry because of the special capacity of B4C for excellent neutron absorption.[1,2] In service, the composites can be exposed at elevated temperatures [523 K to 623 K (250 C to 350 C)] for extended periods of time, owing to the heat generation and accumulation by absorbing thermal neutrons from the spent fuels.[1,3] To improve the overall performance of the neutron absorber materials, it is desirable to maximize the operating temperature and thermal stability of materials at such high temperature. As the matrix of MMCs, most commercial precipitation-strengthened 2xxx, 6xxx, and 7xxx aluminum alloys are limited to be used below 473 K (200 C). The mechanical properties of these matrices can be seriously deteriorated at higher temperature because of rapid coarsening of their precipitates (overaging effect).[4] For precipitation-strengthened aluminum alloys, Al-Sc alloy is a rare exception, which can be used up to 573 K
JIAN QIN, Ph.D. Candidate, ZHAN ZHANG, Research Professor, and X.-GRANT CHEN, Professor, are with the Department of Applied Science University of Quebec at Chicoutimi, Saguenay QC G7H 2B1, Canada. Contact e-mail: [email protected] Manuscript submitted October 26, 2015. METALLURGICAL AND MATERIALS TRANSACTIONS A
(300 C) because it can form nanoscale coherent Al3Sc precipitates with a low coarsening rate.[5] Above this temperature, Al3Sc precipitates may coarsen and lose coherency, which results in the degradation of mechanical properties of materials.[6,7] It was reported that Zr could partially substitute Sc to form Al3(Sc1xZrx) precipitates with better coarsening resistance,[8,9] which improves the thermal stability of precipitates up to 623 K (350 C)[9] and increases the strength and recrystallization resistance.[10–12] Compared to Sc, Zr has lower diffusivity, and its substitution forms a shell at the a-Al/Al3(Sc1xZrx) interfaces, which inhibits Sc diffusion.[13–15] Moreover, Zr can decrease the lattice parameter of the Al3Sc precipitates, which decreases the lattice parameter misfit.[16] Thus, Zr decreases the coarsening rate of precipitates and benefits the thermal stability of materials.[17] In our previous works,[18,19] the mechanical properties of Al-B4C (15 to 30 vol pct B4C) MMCs without alloying at elevated temperatures have been investigated. The reinforcement of B4C particles and their interfaces with Al matrix were very stable at high temperature. The presence of B4C particles can provide an additional strengthening to Al matrix at both ambient and elevated temperatures. The strengthening mechanisms of reinforcements in MMCs can be divided into two categories, direct and indirect strengthening. The former refers to the load transfer from the weak matrix, across the matrix/reinforcement interface, to the higher stiffness reinforcement. And the latter involves
dislocations accumulation around the reinforce parti
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