Design of Multiscalar Metallic Multilayer Composites for High Strength, High Toughness, and Low CTE Mismatch

We propose a new class of multilayer composites that consists of alternating tough and strong layers. Both the tough and the strong layers are metallic, effectively reducing the coefficient of thermal expansion (CTE) mismatch problem that often plagues metal-ceramic composites. The high-strength layers are themselves very fine-scale metallic multilayer composites. The high strengths result from Orowan strengthening of these very fine-scale layers. We present detailed analyses of the flow stress, toughness, and thermal stability of these multiscalar metallic multi-layer composites (M³C) as a guide for microstructural optimization. The dominant term in the flow stress is proportional to the volume fraction of the strong layers and scales inversely with thickness of the very fine-scale layers that make up the strong layer. The toughness is dominated by the plastic flow of the tough layers and is proportional to the volume fraction and flow stress of the tough layers, as modified by plastic constraint. The thermal stability of M³Cs is discussed in the context of solubility, length scales, and interdiffusivity of the two metals. Preliminary results suggest that M³Cs do exhibit an unusual combination of high toughness and strength.