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Title

SYNTHESIS OF NANOPLATE STRUCTURES IN NI-BASED ALLOYS VIA DISCONTINUOUS PRECIPITATION

Creator

Zhou, Yang

Date

2017, 2017-07

Description

Nanostructured materials are of increasing interest due to their potential for use in numerous applications including catalysis and filtration...
Show moreNanostructured materials are of increasing interest due to their potential for use in numerous applications including catalysis and filtration. Methods of synthesizing nanomaterials often include chemical techniques producing individual particles, which gives rise to issues of agglomeration and containment, and the processes are difficult to scale-up. This work is directed at addressing issues of the occurrence and kinetics of self-assembly and structural alignment of nanostructures, which involves the discontinuous precipitation transformation followed by selective dissolution. This synthesis method has been demonstrated using a Ni-49at.%Co-12at.%Al alloy. A complete discontinuous transformation in the Ni 49-12 alloy can be achieved in times as short as 30min. Thus, the large-scale production of such structures is feasible using conventional heat treatment facilities. The synthesis technique is generally applicable to any alloy system in which DP goes to completion and one phase can be selectively removed. The nanoplates are self-assembled, self-supported and well aligned if the precipitate is coherent with the matrix. In order to control the process it is necessary to understand the phase equilibria involving γ, γ′ and β phases in the Al-Co-Ni system. This has been investigated both experimentally and computationally. The isothermal sections at 1100°C and 800°C as well as a partial liquidus projection were determined which result in modifications to previously published work. Comparing the experimental results with the calculated results using Thermo-Calc (TCNI8), there is reasonable agreement. The predicted separation of the γ phase into a two-phase ferromagnetic and paramagnetic region has never been observed experimentally in this or other published work. Future work should explore the ferromagnetic and paramagnetic phase separation as predicted using Thermo-Calc to verify its occurrence in this system. The nanostructured material produced by complete DP transformation may have interesting mechanical properties and these have been studied to a limited extent in this work. The hardness tests show that with decreasing annealing temperature to 600°C, the hardness of certain samples increases significantly due to the precipitation of second phase. The tensile and creep properties of alloy samples with DP phase present were also investigated. The results imply that the occurrence of DP will have a favorable effect on the tensile strength of the sample while lowering the ductility at the same time. A method to generate serrated grain boundaries based on the DP transformation is proposed. Such structures are expected to increase the creep resistance. This was found to be the case in a limited temperature and loading range. Higher temperature or stress levels lead to DP occurrence and cause negative effects on the creep resistance compared to conventionally processed material. The Curie temperature in the Al-Co-Ni alloy system was also studied and determined using a combined magnetic TG and DSC method. A ternary contour map of the Curie temperature has been constructed. From the contour map, the Curie temperature was seen to decrease from high Co, low Al content samples to low Co, high Al samples bypassing a platform at mid Co content. A nonlinear surface fitting was made through a Exponential2D model, the function is helpful for the prediction of Curie temperature of γ phase in Al-Co-Ni alloy system. This method also provides a novel idea of detecting phase transformation and precipitation through thermo-magnetically analyzing the magnetic behavior of the alloys.
Ph.D. in Materials Science and Engineering, July 2017
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