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|Title: ||Microstructure control of AZ91 magnesium alloy by high density electropulsing treatment : mechanisms and application to AZ91 alloy strip manufacturing|
|Other Titles: ||Gao neng dian mai chong chu li AZ91 mei he jin wei guan zu zhi kong zhi ji qi zuo yong ji li yan jiu|
高能電脈沖處理 AZ91 鎂合金微觀組織控制及其作用機理研究
|Authors: ||Jiang, Yanbin ( 姜雁斌)|
|Department: ||Department of Physics and Materials Science|
|Degree: ||Doctor of Philosophy|
|Issue Date: ||2011|
|Publisher: ||City University of Hong Kong|
|Subjects: ||Magnesium alloys -- Microstructure.|
|Notes: ||CityU Call Number: TN799.M2 J43 2011|
220 leaves : ill. (some col.) 30 cm.
Thesis (Ph.D.)--City University of Hong Kong, 2011.
Includes bibliographical references.
|Abstract: ||On the basis of the self-made electroplastic-rolling mill in the research group,
the effects of the high-density electropulsing treatment (EPT) on the
microstructure evolution and the mechanical properties of the aged AZ91
magnesium alloy and the cold-rolled AZ91 magnesium alloy strip are
systematically investigated, respectively. Some pertinent mechanisms on the
effects observed are proposed.
An appropriate temperature field model of EPT applied to metals and alloys is
established, and the temperature calculated using the model is in good agreement
with that measured by the thermoscope. This temperature field model can be used
to effectively estimate the temperature field of the different metals and alloys
during EPT, and one can effectively control the target temperature field by
adjusting the appropriate electropulsing parameters. This job will give an
important guidance to engineering application of EPT.
The research discovers that compared with conventional heat treatment, EPT
rapidly completes the spheroidization and dissolution of the β-Mg17Al12 phase in
the aged AZ91 alloy strip at relatively low temperature, and such EPT-induced
microstructural improvement results in a remarkably increase in elongation to
failure without loss of their tensile strength. According to the experimental
observations and theoretical deductions, EPT dramatically decreases the apparent solid
solution temperature of the β-Mg17Al12 phase due to a reduction in the nucleation thermodynamic barrier. It also accelerates the kinetics of dissolution of β-Mg17Al12
phase significantly due to an enhancement of atomic diffusion resulting from the
coupling of thermal and athermal effect, on the basis of the model for the
dissolution kinetics of the β-Mg17Al12 phase under EPT.
The research also reveals that compared with conventional heat treatment, EPT
with optimized parameter of pulses rapidly completes the recrystallization process of
the AZ91 alloy strip at relatively low temperature, suppressing the precipitation of the β-
Mg17Al12 phase. EPT succeeds in obtaining fine microstructure of quasi-single-phaserecrystallized
materials, and such EPT-induced microstructure changes improve the
mechanical properties remarkably and weaken the intensity of the texture as well.
According to the experimental observations and theoretical deductions, a model has been
proposed to explain that EPT dramatically accelerates recrystallization kinetics. EPT can
both increase dislocation climbing velocity and subgrain growth rate substantially due to
an enhancement of atomic diffusion resulting from the coupling of thermal and
athermal effects, which decreases the incubation period for recrystallization and increases
the nucleation rate of recrystallization. Such effect of EPT results in fine microstructure of
recrystallized materials at relatively low temperature in a very short time.|
|Online Catalog Link: ||http://lib.cityu.edu.hk/record=b4086288|
|Appears in Collections:||AP - Doctor of Philosophy |
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