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Shyam, Amit

Publications and source records attributed to Shyam, Amit.

43 records · Page 3
All sourcesNASA NTRSDOE OSTI
DOE OSTI2021

The role of Si in determining the stability of the θ; precipitate in Al-Cu-Mn-Zr alloys

Precipitate-strengthened Al-Cu-Mn-Zr (ACMZ) alloys demonstrate improved microstructural stability compared to conventional Al-Cu alloys. Si is typically considered an impurity in these alloys, but a systematic analysis of ACMZ alloys with a range of Si levels revealed that there is a Si content range (~0.05 wt. % < Si < ~0.10 wt. %) for achieving optimal high temperature microstructural stability. A combination of hardness testing, scanning electron microscopy, and atom probe tomography has been used to understand the relationship between Si content, aging response, and thermal stability in ACMZ alloys. It is shown that the optimum Si content range corresponds to a reduced as-aged hardness, but a greater hardness value retained by the alloy after thermal exposure to 350 °C for 200 hours. A mechanism is proposed in which optimum Si levels reduce the number density of nucleation sites for θ' precipitates, resulting in larger precipitates that, on average, provide a reduced as-aged strength but are more coarsening resistant. This inherent θ' coarsening resistance allows more time for slow-diffusing Mn and Zr to reach the θ' interface and further stabilize the precipitates. Designing the optimal as-aged microstructure for improved thermal stability of the alloy by controlling impurity levels is a concept that has implications beyond the investigated Al-Cu alloy system.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Atomic structures of interfacial solute gateways to θ' precipitates in Al-Cu alloys

Many materials employed in critical structural applications depend upon metastable strengthening precipitates that transform or dissolve at elevated temperatures. Herein, aberration-corrected scanning transmission electron microscopy and first-principles calculations are used to accurately determine the atomic structure of the highly mobile, semi-coherent precipitate interfaces that control this process in the classic θ' (Al 2 Cu) precipitate in the Al-Cu system. Semi-coherent {110} interfaces are found to be composed of an array of unexpected misfit dislocations that are arranged in two different structural units. Dislocations accommodate nearly all of the misfit between the Al matrix and strengthening phase. Cu is observed to segregate to the compressed edge of the dislocation cores at specific sites in this interface. First-principles calculations revealed the energetic landscape that facilitates these sites to become entry and exit gateways of Cu atoms in this semi-coherent interface. In conclusion, this investigation reveals critical features within semi-coherent interfaces that determine the thermal stability of precipitation-hardened alloys.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Crystallographic orientation-dependent strain hardening in a precipitation-strengthened Al-Cu alloy

While the strengthening of Al-Cu alloys due to precipitation has been extensively studied, the effect of crystallographic orientation of the matrix and precipitates, as well as precipitate morphology, on the strain hardening behavior is not well understood. In this paper, we investigate this effect with in situ neutron diffraction during deformation of an Al-Cu alloy (206) after multiple aging treatments. Precipitate-dislocation interactions were found to change from precipitate shearing for microstructures predominantly containing GPI and θ" precipitates to Orowan looping for microstructures with primarily θ' and θ precipitates. Notably, significant anisotropy in strain hardening behavior was observed when θ' precipitates were present, which was attributed to crystallographic orientation dependent load transfer from the Al matrix to the θ' precipitates. The anisotropic load transfer is hypothesized to be caused by the extent of rotation of high aspect-ratio θ' precipitates, owing to dislocations looping around them during plastic deformation of the matrix. Predictions from an analytical model describing the anisotropic magnitude of load transfer from precipitate rotation agree well with experimental results, successfully validating the precipitate rotation hypothesis and explaining the anisotropic strain hardening behavior. This model allows for the prediction of stresses separately in the precipitate and matrix phases as a function of crystallographic orientation, only given the bulk mechanical properties.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Influence of copper content on the high temperature tensile and low cycle fatigue behavior of cast Al-Cu-Mn-Zr alloys

The relationship between low cycle fatigue (LCF) and monotonic tensile fracture strain was investigated for cast Al-Cu-Mn-Zr alloys containing 6 and 9 wt% Cu at 250 °C. The 9% Cu alloy consisted of larger size and volume fraction of brittle intermetallic grain boundary particles that fractured during tensile tests to reduce the fracture strain by 50% compared to the 6% Cu alloy. We found that LCF life was similar between the two alloys and weakly influenced by the particles. LCF life and fracture strain were poorly correlated due to the dominant influence of particles on monotonic tensile crack but not on fatigue crack.

36 MATERIALS SCIENCE↗