Phase transformation and stability studies of the Zr-H system

Abstract

Zirconium alloys are widely used in the nuclear industry because of their high strength, good corrosion resistance and low neutron absorption cross-section. Zirconium has a strong affinity for hydrogen, however, and if hydrogen concentration builds up, the material will gradually degrade. In one class of such hydrogen caused degradation, called hydride induced embrittlement, hydrogen chemically reacts with zirconium forming one, or several, crystal phases of zirconium hydride. These hydrides play a primary, but sometime not fully understood, role in crack initiation and propagation within these materials. Despite the fact that hydride induced embrittlement in zirconium have been studied for several decades, there are still some unresolved issues.



It has been the aim of the research presented in this thesis to provide the research community with new and updated data of the hydrides themselves in order to aid further studies within the field of hydride induced embrittlement in general, and the mechanism of delayed hydride cracking in particular. To that end, the research presented here proceeded, in short, as follows: First, zirconium hydride powder, of well defined hydrogen concentration, was produced from commercial grade zirconium. This powder was subjected to heat treatment and the hydride phases were characterized both in situ and ex situ using neutron, synchrotron X-ray, and conventional laboratory X-ray based diffraction techniques. Next, most of the low-pressure zirconium hydride phases were produced under hydrogen/argon atmosphere from commercial grade zirconium powder. This process was simultaneously monitored and recorded in real time using synchrotron X-ray diffraction.



These experiments have produced new data of the behavior of different hydride phases during thermal treatment and in situ hydrogenation. For the first time all commonly reported zirconium hydride phases and the complete transformation between two different hydride phases were recorded with a single experimental arrangement. The phase transformation between δ and ε zirconium hydride was recorded in detail and presented. Finally, the controversial γ zirconium hydride was observed both in situ and ex situ and the preparation route, its crystal structure, and formation mechanisms were analyzed and presented.

Abstract (Swedish)

Popular Abstract in English

Zirconium alloys are widely used in nuclear power reactors as cladding material for their fuels due to the high strength, good corrosion resistance and low neutron absorption cross-section, even in extreme environment, that is characteristic for zirconium. The element, however, also has strong affinity for hydrogen, and when both are present they readily react and form a family of compounds called hydrides. It is well known that even small amounts of hydrides in zirconium based materials will cause the material to loose ductility, making it brittle, and susceptible to cracking. In the case of nuclear fuel, these zirconium based alloys are not only the cladding of uranium fuels in the reactors but also the first barrier of defense of nuclear waste and radioactive substance in general, and as such the problem of hydrogen absorption and hydrides formation becomes significant and important to study.



Despite the fact that zirconium hydrides have been extensively studied for about half a century, the basic nature and mechanisms of hydride formation, the transformation between various phases of hydride, and their exact crystal structures are not yet fully understood. In order to find answer to some of these problems, the precipitation and dissolution, that is the formation and removal, of hydrides, in pure zirconium powder were carried out and studied. The processes were monitored in real time using high resolution synchrotron and neutron radiations, whereby whole diffraction patterns of crystal structures where recorded of zirconium and the hydrides. These patterns were used in the analysis of the environments and conditions that are important for the formation of hydrides, as well as to answer questions about the mechanisms and speed with which these events occur.



Based on the observations during these experiments, and the results of the analysis, a better understanding of the behavior of different hydride phases has been achieved. In particular during thermal treatment and in-situ hydrogenation compared to those being reported in literature. Some new findings were also reported. The main ones being: 1. All commonly reported zirconium hydride phases were recorded, for the first time, in a single in-situ experiment. 2. The complete reversible transformation between two different zirconium hydride phases was observed. 3. The phase transformation type between two commonly reported zirconium hydrides, called delta and epsilon, was analyzed and defined. 4. The preparation route of a controversial zirconium hydride, known as gamma zirconium hydride, is introduced and its exact crystal structure and formation mechanisms are also discussed in detail.

• author: Maimaitiyili, Tuerdi
• supervisor: Christina Bjerkén; Per Ståhle
• opponent: Hallstadius, Lars, Westinghouse Electric Company
• organization: Materials Engineering
• publishing date: 2015
• type: Thesis
• publication status: published
• subject: Materials Engineering
• keywords: zirconium hydride, γ-ZrH, synchrotron X-ray diffraction, neutron diffraction, hydrogen charging, in situ phase transformation, hydrogen embrittlement, hydrogen induced degradation
• defense location: M:E, M-Building, Ole Römers väg 1, Lund University, Faculty of Engineering, LTH.
• defense date: 2015-12-03 10:15:00
• external identifiers: other:LUTFD2/TFMT--15/1015--SE(1-65)
• ISBN: 978-91-7623-553-9
• ISBN: 978-91-7623-552-2
• language: English
• LU publication?: yes
• id: 4887530f-dfd9-4e6f-b20b-a3c78ecee375 (old id 8167401)
• date added to LUP: 2016-04-04 14:26:43
• date last changed: 2020-04-29 12:48:37