Fracture of Multi-principal Element Metallic Composites Based on Chemically Complex Intermetallics and FCC


Student thesis: Doctoral Thesis

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Award date28 Aug 2020


Strength and toughness are two important metrics for the design of structural materials. However, these two properties are generally mutually exclusive, perceived as strength-toughness paradox. In recent years, it has been conjectured that this paradox might be overcome in the new class of multi-principal element alloys, such as medium entropy alloys (MEAs) and high entropy alloys (HEAs). Unlike conventional alloys, these multi-principal element alloys generally include more than four principal elements mixed in an equimolar or near-equimolar ratio and were thought to possess a high configurational entropy, severe lattice distortion, sluggish diffusion and a cocktail effect. As a result, it is easy to form multi-component solid solution phases in MEAs and HEAs with a combination of attractive mechanical properties, such as excellent toughness and superb strength. Furthermore, it was hypothesized that, because of the "cocktail" effect, multicomponent or high entropy metallic compounds/intermetallics might not be as brittle as their low entropy counterparts.

In this work, we designed a series of multi-principal element alloys (ternary, quaternary and quinary) and characterized the fracture toughness of these high or medium entropy intermetallic-based composites based on the J integral approach. Firstly, we designed a CoCrFeNiNb0.5 eutectic high entropy alloy (EHEA), which exhibited a hierarchical eutectic lamellar structure consisting of Laves and face centered cubic (FCC) phases mixed in a nearly equal volume fraction. Compared to the conventional Laves phase matrix composites, this EHEAs possessed a unique combination of superb hardness (~9.2 GPa) and superior fracture toughness (~15 MPa*m0.5) at room temperature, where fine lamellas enhanced strength while coarse lamellas increased crack resistance.

Second, we developed a series of dual phase CoCrNiMox (x=0.4, 0.6, 0.7, 0.83 and 1.0) medium entropy alloys (MEAs) containing sigma and face centered cubic (FCC) phases and systematically studied their fracture behavior at room temperature. The experimental results clearly show that, as the volume fraction of the sigma phase increases from 4% up to 72%, the fracture toughness (KJIc) of the dual phase MEA reduces from 72 to 8 MPa*m0.5. Through a quantitative analysis, it is shown that the fracture toughness of the sigma phase containing MEAs is derived from a combination of intrinsic (crack tip blunting, crack-tip deflection and crack-tip micro-crack nucleation) and extrinsic toughening mechanisms (distributed micro cracking and crack bridging).

Last, we developed a CoNiNb0.37 fully eutectic dual phase alloy, which consisted of ~50 vol% intermetallic phase and ~50 vol% FCC phase, and studied its fracture behavior in the as cast state and after thermal annealing in 873K and 1473K. Interestingly, our results revealed that the multi-component intermetallic phase was meta-stable and able to undergo a thermal-induced phase transformation from one type of Laves phase--newly discovered in our research and having a defective orthorhombic lattice--- to another type of Laves phase, i.e. the conventional C14. Along with the phase transformation, the room-temperature fracture toughness (KJc) of the eutectic alloy increased by three folds, from 20 to 62 MPa*m0.5. Through the combined in-situ and ex-situ experiments and modeling, we are able to show quantitatively this high fracture toughness is mainly derived from the significant intrinsic crack-tip plasticity owing to the inherently plastic intermetallic phase. The outcome of our research provides the compelling evidence for strong-yet-ductile high entropy intermetallics, which can lead to the development of strong yet tough HEA/MEA based composites that can have important structural applications.