John Joy presenterar sitt examensarbete

Program: Materials Engineering

Handledare: Guocai Chai (Alleima)

Examinator: Fang Liu, IMS

Opponent: Ahmadreza Babaahmadi

Abstract:

The purpose of this diploma work is to increase understanding on the deformation and fracture behaviour in Alloy 718 and medium-high entropy alloys, which include the following two parts:

Influence of the precipitate and strain rate on the intermediate temperature embrittlement (ITE) in Alloy 718
Present study focuses on investigating the intermediate temperature embrittlement behaviour of the nickel-based super alloy 718 under various strain rates and heat treatment conditions. Tensile tests were performed at room temperature (RT), 650℃, and 750℃ with strain rates ranging in 10^(-1), 10^(-2), 10^(-3) & 10^(-4) s^(-1). The tensile strength and elongation were compared across different test conditions. Notably, embrittlement phenomena were observed at 650℃ and 750℃ under low strain rates of 10^(-3) & 10^(-4) S^(-1) respectively, resulting in significantly reduced elongation and strength percentages. To investigate the underlying mechanisms, microstructural analysis was conducted to identify precipitates at grain boundaries and examine crack propagation along the grain boundaries. Advanced techniques such as Field Emission Gun Scanning Electron Microscopy (FEG SEM) with Energy Dispersive X-ray Spectroscopy (EDX), Electron Backscatter Diffraction (EBSD), and Electron Channelling Contrast Imaging (ECCI) were employed for detailed microstructure characterization. Influences of Laves phase and strain rates on the elongation of Alloy 718 at RT, 650C and 750C with different strain rate have been studied. The results show that the presence of Laves phase may promote ITE. On the other hand, ITE in Alloy 718 can be avoided with a higher strain rate.

Twinning-induced plasticity (TWIP) behaviour of Fe-Ni-Cr-Mn-Mo high-medium entropy alloys
The deformation behaviours of Fe-Ni-Cr-Mn-Mo high-medium entropy alloys were investigated at three different temperatures (-196°C, room temperature, and 300°C). The study focused on understanding the effects of composition, entropy, and temperature on the deformation twinning behaviour through tensile testing and microstructure analysis using Electron Backscatter Diffraction (EBSD) and Electron Channelling Contrast Imaging (ECCI). The results revealed the presence of multi-order twinning, which explains the TWIP mechanism. This research contributes to a deeper understanding of the deformation mechanisms in Fe-Ni-Cr-Mn-Mo high-medium entropy alloys, providing valuable insights for the development of advanced materials with enhanced mechanical properties.