The power electronics operation environment is characteristically non-isothermal, due joule heating of active devices and their repeated heating and cooling during switching, among other factors. Through an analysis of strain-hardening behaviour under quasi-cyclical uniaxial tensile tests, this work aims to help understand the interactions of multiple microstructural processes (e.g., dislocation multiplication, recovery and recrystallisation) occurring in tandem under such loading profiles. These processes either exacerbate or offset one another and thus cannot easily be separated out. Yet clear delineation and quantification of their contributions to the degradation process is necessary for damage modelling and prediction [1,2].

375μm diameter high purity Al wires are subjected to uniaxial tensile tests on a DMA-Q800 TA Instruments platform. Two temperature cycles (40°C-120°C; 90°C-120°C) are mimicked through discontinuous, non-isothermal uniaxial tests performed under alternating and temporarily isothermal states: i.e. the ‘miminum’/low and ‘maximum’/high temperatures. Each ‘cycle’ is performed on the same specimen at a constant strain rate of 0.0001/s and terminated at 2% strain. This process is repeated at least 10 times (five full cycles) or until tensile fracture occurs. Results are benchmarked against isothermal tests at the same strain rate.Dislocation density evolution under the various loading states is visualised using Kocks-Mecking plots, extracted strain-hardening exponents (n) and plastic strain energy densities via numerical integration of the post-yield part of flow curves.

The temperature and ΔT-dependence of strain hardening rate and activation energy for plastic flow are read from the benchmarking tests. Under non-isothermal testing, all flow curves exhibit Stage III strain hardening post-yield, with temperature sensitivity consistent for high SFE materials. As multiple slip occurs right from the outset, an athermal work hardening phase may be concealed. There is also evidence of Stage IV strain hardening during the highT segments of ‘cycling’. The ‘low T’ test for both temperature range experiments show higher strain hardening exponents; however, the high T(120°C) segments for both cycling ranges give similar n_values at the same value of strain. The latter are remarkably low, and at times negative, indicating equal recrystallisation and recovery rates, with recrystallisation rate exceed recovery at points where negative strain hardening is observed. The appearance of Stage IV characteristics at such low cumulative strain values is remarkable, and clearly brought on by the repeated low temperature segments. These observations and emerging themes from plastic strain energy density visualisations will be discussed within the context of aluminium wire interconnects in power electronics modules.

References

[1] Agyakwa, P.A., et al.; IEEE TDMR, 2010 10 (2) 254-262.

[2] Yang, Li, Agyakwa, P, Johnson, CM; IEEE TDMR, 2014 14 (4) 989-994.