Designing 2D nanostructured metals for age hardenability

Purdue University, West Lafayette IN

Investigators

Abstract

Non-technical description Exceptional materials will, almost by definition, be used in exceptional applications, which can include exposure to elevated temperatures, and therefore the design of ultra-strong metals requires consideration of the strength of materials in extreme conditions. Bulk metals often become mechanically softer after exposure to high temperatures; one exception to this rule is materials that are designed to age harden. Aluminum alloys are the classical example of age hardening, where after heating the strength of the aluminum alloy increases due to local re-arrangements of impurity atoms that lead to an internal nanoscale network of strengthening particles. Stronger materials benefit society on several levels; in particular they allow us to develop lighter weight structures that carry the same loads (i.e. more fuel efficient cars, lower materials costs in bridges, or higher performance in aerospace applications). Ultra-strong metals often use many closely spaced nanoscale particles within the metal to increase strength, but often these particles grow upon exposure to high temperatures and their relative spacing increases, which decreases strength. A new method is proposed in this study to strengthen metals that relies upon forming layers of alternating two different metals which creates a material with many internal interfaces. By confining the strengthening particles to nanoscale slabs the maximum particle growth will be constrained, and at the same time that the particle-to-particle spacing increases the interface-to-particle spacing decreases, resulting in retained strength and thermal stability. The students working on this project will also design classroom materials and accompanying lesson plans for middle school science classrooms to help teachers provide students engaging activities that fulfill Indiana learning standards while teaching college students how to effectively support local educational outreach. Technical description Annealing precipitate-strengthened metals increases the spacing between second phase particles, and the structure will soften. By adding second phase precipitates within layers, annealing will change both the particle-particle and particle-interface spacing. This work will examine the effects of two dimensional confinement on precipitation of Cr particles within a metastable Cr-Cu system. As precipitation occurs within the FCC layer, the nanolaminates with precipitates are expected to have a higher strength than single phase laminates. During annealing the spacing between the particles may increase, but the distance between the particles and the Cu-Cr interfaces will decrease as the precipitates grow. This provides a strengthening mechanism that should lead to increases in strength after annealing, rather than decreasing strength after annealing, and is possible only due to the two dimensional architecture of these materials. The system should also be resistant to over-aging, adding a level of thermal stability not possible in 3D architectures. However, the confined layer slip mechanism of strengthening relies on dislocation core spreading and shear at the FCC/BCC interface; it must also be determined if precipitates at interfaces, rather than in the layer, accentuate or attenuate strength enhancements. The goal of this proposal is to test and verify the possible strengthening mechanisms, determine if the nanoscale features of planar systems provides additive synergistic strengthening with other hardening mechanisms (i.e. are precipitation hardening, confined layer slip, and solid solution hardening additive in 2D confined systems?), and develop strengthening models for this 2D architecture that will predict the strength of new metallic multilayer alloy structures that strengthen when annealed and exhibit smaller decreases in strength at elevated temperatures than other nano-featured metals. This work is motivated in part from recent molecular dynamics simulations that suggested nanolaminate strength increases as precipitate size increases, providing a new direction in strengthening metallic systems for use under elevated temperatures and or subjected to thermal conditions that would traditionally degrade their strength.