Identification of deformation mechanisms in thermally stable cast Al-Cu alloys via neutro diffraction

Abstract

Al-Cu alloys have long been a subject of study due to their long history, high strength, and ease of manufacture. However, advanced characterization techniques have shown that the deformation mechanisms and precipitate-dislocation interactions are still not understood perfectly. For example, there has been recent studies that have concluded that θ′ precipitates, previously assumed to be unshearable by dislocations, can be sheared during the strain hardening regime of the alloy. This study, therefore, intends to identify the deformation mechanisms and precipitate-dislocation interactions in Al-Cu alloys as a function of microstructure and testing temperature. The primary experimental technique that will be implemented is in-situ neutron diffraction, which allows for the measurement of stresses within individual phases and crystallographic orientations, providing the constitutive response of the matrix and precipitates separately.Three primary research questions were asked: 1. What is the effect of microstructure on the strain hardening mechanisms at room temperature in an Al-4.5%Cu alloy? 2. What is effect of testing temperature on precipitate-dislocation interactions during tension in an Al-4.8%Cu-1.2%Ni-Mn-Zr thermally stabilized alloy? 3. What are the deformation mechanisms in a creep condition at 300°C at high stress in the same Al-4.8%Cu-1.2%Ni-Mn-Zr alloy? It was found that, at room temperature, while the deformation in GP zone and θ precipitate-strengthened conditions are controlled by a single, simple mechanism, the behavior of θ′-strengthened alloys was much more complex. The θ′ precipitates were primarily bypassed by Orowan looping, and the strain hardening behavior was found to be highly influenced by the transfer of load from the matrix to the θ′ precipitates, and this load transfer was a strong function of the orientation of the θ′ precipitates relative to the tensile direction. In the elevated temperature tensile tests, it was found that the θ′ precipitates underwent both Orowan looping (in the initial strain hardening regime), then sheared by dislocations (in the later stages of plastic deformation), and the CRSS of the precipitate was a strong function of temperature. In the creep condition, the deformation mechanisms that were identified were very similar to the mechanisms active in tension. The strain hardening effects related to Orowan looping and load transfer helped explain a very high creep resistance that was observed. Models were developed and implemented in order to quantify the load transfer behavior as a function of precipitate orientation, as well as the CRSS of the θ′ precipitate as a function of temperature. These results and conclusions provide a better understanding of the mechanical behavior of Al-Cu alloys at a precipitate scale and may be used to inform future alloy development efforts.