Date of Award

2018

Document Type

Open Access Thesis

Department

Mechanical Engineering

First Advisor

Djamel Kaoumi

Abstract

X-750 is a nickel-chromium based super alloy of usefulness in a wide variety of applications such as gas turbines, rocket engines, nuclear reactors, pressure vessels, tooling, and aircraft structures. Its good mechanical properties are due to the strengthening from precipitation of γ′ particles upon prior ageing heat treatment. In this work, the effect of such heat treatment on the mechanical properties, tensile behavior, and fracture mechanisms of X-750 was studied at various temperatures by comparing it with a non-aged, solution annealed X-750. Tensile tests were conducted from room temperatures up to 900 °C at three separate strain rates (10-3, 10-4, 10-5 s-1); tested samples were analyzed by means of SEM observations. In addition, the microstructure of both aged and solution annealed materials were studied using SEM and TEM, both on as received and on tested specimens. Serrated flow was observed for a range of temperatures referred to as the Portevin Le Chatelier (PLC) regime (interaction of solutes with dislocations causing stress serrations) in both heat treated (HT) and non-heat treated (NHT) samples. There is a different level of prominence in the Normal and Inverse PLC effect between HT and NHT X-750. Sinusoidal stress serrations are observed for both HT and NHT material at high temperatures, and dynamic recrystallization becomes a dominant deformation mechanism. Vacuum effects were observed to be relevant for mechanical properties, flow behavior, and dynamic recrystallization. v When tested between room temperatures and 650 °C, the fracture surface of HT material evolves from purely intergranular to purely transgranular due to the thermal activation of dislocation mobility that relieves the stress at the grain boundaries, while the rupture of the NHT material is due to the coalescence of voids induced by decohesion at the MC (one metallic element with one carbon atom) carbides/matrix interface. At higher temperatures, precipitation of γ’ particles upon testing of the NHT material leads to a temperature-dependent increase in both yield strength and ultimate tensile strength. At the same time, an overall decrease of the HT material mechanical properties is observed. Minimum ductility was observed at 750 °C for both solution annealed and aged specimen, due to the oxidation of grain boundaries leading to an environmentally-induced fracture mechanism. At higher temperatures, dynamic recovery and dynamic recrystallization occur which prevents such a rupture mechanism, but finally leads to rupture by grain boundary slipping at 900 °C.

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