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GA-BASED ROOM TEMPERATURE LIQUID ALLOYS: FUNDAMENTAL UNDERSTANDING AND USE IN THERMAL MANAGEMENTYifan Wu (18419562) 24 April 2024 (has links)
<p dir="ltr">This work investigates four aspects of Ga-based low melting temperature alloys in their role as TIMs: the interaction between Ga and metal substrates, the change in the thermodynamic behavior of the liquid metal alloy, the evolution of the thermal performance, and mitigation strategies against Ga corrosion.</p>
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Radiation Response of Nanostructured CuCuncai Fan (7036280) 02 August 2019 (has links)
Irradiation of metals with energetic particles causes heavy damage effects in microstructure and
mechanical properties, which is closely associated with irradiation conditions, presence of
impurities, and microstructural features. It has been proposed that the radiation tolerance of a
certain material can be enhanced by introducing a high density of interfaces, acting as ‘sinks’ that
can frequently involve in attracting, absorbing and annihilating defects. Nanostructured materials
with large volume fraction of interfaces, therefore, are assumed to be more radiation tolerant than
conventional materials. This thesis focuses on the radiation damage effects in nanostructured Cu
via the methods of in-situ TEM (transmission electron microscope) radiation experiments, postirradiation TEM analyses, small-mechanical tests (nanoindentation and micro-pillar compression),
and computer simulations (molecular dynamics and phase-field modeling).
We design and fabricate nanostructured Cu using direct current (DC) magnetron sputtering
deposition technique, a typica physical vapor deposition (PVD) method and a bottom-up way to
construct various nanostructured metals. High-density twin boundaries (TBs) and nanovoids (NVs)
are introduced into two distinct nanostructured Cu films, including nanovoid-nanotwinned (NVNT) Cu (111) and nanovoid (NV) Cu (110). The in-situ high-energy Kr<sup>++</sup> (1 MeV) and ex-situ
low energy He<sup>+</sup>
(< 200 keV) irradiations are subsequently preformed on the as-deposited Cu
samples. On the one hand, the in-situ TEM observations suggest that TBs and NVs can influence
the formation, distribution and stability of radiation-induced defects. Meanwhile, the preexisting
microstructures also undergo structural change through void shrinkage and twin boundary
migration. On the other hand, the ex-situ micro-pillar compression tests reveal that the Heirradiated NV-NT Cu contains less defect clusters but experiences more radiation-induced
hardening. The underlying mechanisms of void shrinkage, twin boundary migration, and radiationinduced hardening are fully discussed based on post-irradiation analyses and computer simulations.
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MECHANICAL PROPERTY AND DEFORMATION MECHANISMS OF NANOTWINNED ALUMINUM ALLOYS AND MULTILAYERSYifan Zhang (9127289) 10 September 2022 (has links)
<p><a>Aluminum (Al) alloys have been widely
used in </a>industry as
light-weight structural materials. However, the mechanical strength of
the strongest Al alloys is still much lower than most high-strength steels.
This thesis aims to investigate the fabrication and mechanical behaviors of
nanotwinned high-strength Al alloys and multilayers.</p>
<p>Twin boundaries are
special grain boundaries with mirror symmetry. Twin boundaries can generate
slip discontinuity and block the transmission of dislocations, and serve as
dislocation sources to accommodate plasticity. However, twinning in Al is rare
due to its high stacking fault energy and low unstable stacking fault energy.
In this thesis, we used multiple methods to introduce high-density twins into
Al and achieve outstanding mechanical properties and thermal stability. </p>
<p>Certain type of solutes can greatly increase the twin
density in Al by decreasing the stacking fault energy of Al and retarding the
detwinning process. Nanotwinned Al-Ni and Al-Ti binary alloys fabricated by
magnetron sputtering show high strength, good deformability, and unique
deformation mechanisms. Furthermore, deformation and thermal stability of
binary nanotwinned Al alloys can be enhanced by adding a third or fourth solute
element. </p>
<p>Interfaces can
facilitate twin formation in Al as well. High-density twins and stacking faults
were introduced into Al by using Al/Ti layer interfaces. Nanotwinned Al/Ti
multilayers have ultra-high strength, superb deformability and thermal
stability. This thesis provides promising pathways to fabricate Al alloys and
composites with high strength and good thermal stability.</p>
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INVESTIGATION OF BIODEGRADABLE IRON-MANGANESE ALLOYS WITH VARIOUS POROSITYSabrina M Huang (6843719) 05 August 2019 (has links)
<p>Bioresorbable iron-manganese
(Fe-Mn) alloys are considered as a new class of biomaterials for the
development of orthopedic fixation devices due to their promising mechanical
properties, comparable to the human cortical bone, and the ability to degrade in
the physiological environment and release small quantities of metallic
ions/particles that are absorbable by the host. The greatest challenge for
developing an ideal resorbable Fe-Mn alloy is to increase the degradation rate
of the alloy without compromising the alloy biocompatibility, that is, causing
zero or minimal local and systemic toxicity to the tissue. Another challenge is
to improve osteo-integration through inducing a cascade of events leading to
tissue ingrowth.</p>
<p> </p>
<p>The incorporation of porosity into the Fe-Mn alloys aimed to
increase the corrosion rate and to provide the three-dimensional structure for cellular
activity and nutrient transport. The Fe-30wt.%Mn alloys with 0-, 5-, 10-, and
60-volume percent porosity were produced through the space holder
technique in powder metallurgy.
The space-holder material, ammonium bicarbonate (NH<sub>4</sub>HCO<sub>3</sub>),
was sieved to a particle size ranging 355~500 µm. The microstructures and
mechanical properties of the alloys, as well as the influence of the degree of porosity
on the alloy corrosion rates comparing to the concentrations of the degraded
metal ions were
investigated. Although the Fe-30Mn alloys containing 60-vol% porosity exhibited
the lowest average ultimate compressive strength of 381 MPa among the tested
groups, they were still mechanically stronger than a typical human wet compact
bone. Furthermore, the alloys had the highest average corrosion rate of 0.98 ± 0.20
mm/year, compared to 0.13 ± 0.07 mm/year for the non-porous Fe-30Mn alloys. Nevertheless,
the extract from the 60%-pore group had a cytotoxicity effect to the bone
marrow stem cells (BMSCs) at an average normalized cell viability of 58%, which
was below the standard viability of 70%, considered as cytotoxic in the
indirect cytotoxicity study. The cytotoxicity study also corresponded to the
highest level of transition metal ions Mn<sup>2+</sup> released into the media for
the 60%-pore group at an average ion released rate of 7 mg/day, compared to the
other groups presenting similar Mn<sup>2+</sup> released rates about 4 mg/day
after 1 day of incubation. The extreme case of the 60%-pore group demonstrated
the tradeoff between the corrosion rates and biocompatibility. On the other
hand, the 10%-pore group showed an average ultimate compressive strength of 737
MPa comparable to the stainless steel 316L, an average corrosion rate of 0.260
± 0.09 mm/year, which was 2-fold higher than the non-porous group, and
an average cell viability of 86% close to the non-porous group. It is promising
based on the above results,
however, the osteo-integration of the 10%-pore group in terms of cell-to-cell
and alloy-to-cell interactions was not ideal. </p>
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Effect of Ultrasonic Shot Peening on Mechanical Properties and Corrosion Resistance of MG Alloy SheetJianyue Zhang (6632399) 10 June 2019 (has links)
<div>Magnesium alloys are regarded as the most promising structure materials in transportation and aerospace fields because of their low densities and high specific strengths. However, the unsatisfactory mechanical performance and corrosion resistance restrict their applications. Grain refinement is an effective way to improve the mechanical properties and widen the applications. Among which, ultrasonic shot peening shows a great potential in producing refined grains or even nanocrystalline. A nanocrystalline forms at the surface after ultrasonic shot peening treatment. The formed nanocrystalline has been proved to dramatically affect the mechanical properties, such as hardness, mechanical stress, wear resistance and fatigue life. </div><div><br></div><div> </div><div>In this dissertation, the microstructure evolution of AZ31 Mg alloy after the ultrasonic shot peening as well as its effect on the mechanical properties are investigated. The grain size, the twin structure, the surface roughness and the residual stress distribution after ultrasonic shot peening are characterized. A gradient nanostructure is achieved through ultrasonic shot peening and the thickness of this gradient nanostructure increases with prolonging the treated time. The grains at the top surface after 5 min treatment is refined to 45 nm and further refined to 42 nm for 10 min and 37nm for 15 min treatment from the XRD result. A lamellar nanocrystaline is below the top surface and a lot of tensile twins are found at the heavily deformed grains below the nanocrystalline layer. Below the twinned layer, a residual stress is distributed as deep as 400 $\mu$m in the matrix. A rough surface is obtained and the surface roughness of the 5 min treated sample was 5.934 $\mu$m, increased to 6.161 $\mu$m for10 min and 6.236 $\mu$m for 15 min. The nanocrystalline leads to the improvement of the microhardness, from 65 HV of the as-received to 123 HV, 127 HV and 145 HV for 5 min, 10 min and 15 min treatment, respectively. The tensile stress and compression stress are also improved remarkably. The yield stress is increased from 127.7 MPa of as-received to 198 MPa of 10 min treated sample and the compression stress is improved from 73 MPa to 100 MPa. The improved yield stress is attributed to the grain refinement and the work hardening of the nanocrystalline. </div><div><br></div><div> </div><div> </div><div>The wear resistance of AZ31 Mg alloy is improved greatly after ultrasonic shot peening process. The coefficient of friction and the wear rate of the ultrasonic shot peening treated sample are both lower than that of as-received. The width of the wear track of ultrasonic shot peening treated is also narrower than that of as-received, and the worn surface has a lower surface roughness. In as-received samples, abrasion and oxidation dominate the wear mechanism at low sliding speed and low applied load. The increase of sliding speed or applied load resulted in the delamination. Severe wear such as thermal softening happens with the further increase of load value or sliding speed. In ultrasonic shot peening treated samples, oxidation, the abrasion and delamination are also existing while no severe wear is found. The improved wear resistance of the ultrasonic shot peened sample is due to the improved hardness and a higher activity of oxidation during wear process. The nanocrystalline on the top surface leads to the the transition boundary between the mild wear and severe wear to a higher sliding speed and higher applied load. </div><div><br></div><div><br></div><div>The corrosion resistance of AZ31 Mg alloy before and after ultrasonic shot peening is tested in 3.5 $\%$ NaCl solution. The corrosion resistance after ultrasonic shot peening is reduced greatly because of the Fe particles at the top surface, which was exfoliated from the shot during the treating process. After a 40 $\mu$m thick polishing, Fe particles are removed totally and the corrosion resistance is improved, compared with that of as-received. The anodic current density of the nanocrystallized surface after polishing is reduced because of the compression residual stress and a rapid formation of protective layer. Meanwhile, the grain boundary acts as a physical barrier for corrosion and reduces the corrosion rate. </div><div><br></div><div><br></div><div>The bending behavior of AZ31 Mg alloy before and after ultrasonic shot peening is studied by a V-bending test. The ultrasonic shot peening treated sample has a similar bending performance even though the ductility has been reduced after ultrasonic shot peening. A single side ultrasonic shot peening (either at the inner side or the outer side) changed the bending behavior because of its asymmetric structure. The ultrasonic shot peening at the inner side for 5 min improves the bendability and longer treated, such as 10 min and 15 min degenerates the bendability to as-received. The improved bendability of 5 min treated sample is due to the drawing back of the neutral layer. The ultrasonic shot peening at the outer side for 5 min also improves the bending performance and a longer treatment of 15 min further enhanced the bendability. The improved bendability after outer side treatment is due to the high yield stress of nanocrystalline at the convex, resulting in the smaller strain here. </div>
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Investigation of Energetic Materials and Plasmonic Nanostructures Using Advanced Electron Microscopic TechniquesXiaohui Xu (5930936) 17 January 2019 (has links)
<p>Investigation of laser-matter interaction has been an
important research topic which is closely related to applications in various
fields including industry, military, electronics, photonics, etc. With the
advent of ultrafast transmission electron microscope (UTEM), in situ
investigation of the interaction between pulsed laser and nanostructured
materials becomes accessible, with unprecedented spatial and temporal
resolution. Here, we studied two categories of materials with the help of UTEM,
namely, energetic materials and plasmonic nanostructures. The results
demonstrate that UTEM provides a novel and convenient way for the investigation
the structural and morphological change of energetic materials under external
stimuli at nanoscale. Also, UTEM makes it possible to visualize the
light-induced welding between plasmonic nanostructures at real time, which
helps to reveal more details about the mechanisms involved. Furthermore, we
studied the formation of some novel structures by combing different gold and
silver nanostructure.</p>
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DESIGN AND PROCESSING OF NICO-BASED SUPERALLOYS FOR THE STUDY OF SOLUTE SEGREGATION AT PLANAR DEFECTS DURING HIGH TEMPERATURE DEFORMATIONSae Matsunaga (11820032) 18 December 2021 (has links)
<p>Ni-based superalloys have been
widely used for high temperature applications such as turbine blades for jet
propulsion and power plants due to their excellent creep, fatigue, and
corrosion resistance. But as the demand for higher temperature capability and
strength increases, there remains a need to better understand high temperature
deformation mechanisms and improve and strengthen superalloys at these elevated
temperatures. Recently, a correlation has been observed between solute
segregation at planar defects (stacking faults, antiphase boundaries, etc) and
enhanced high temperature creep properties – known colloquially as phase
transformation strengthening. Experimentally, regardless of alloy composition,
strong Co segregation at planar defects along with Cr has been observed. In
addition, it has been suggested by density functional theory work that Co would
promote Cr concentration at stacking faults by forming strong Cr-Co bonds.
Based on these findings, it was hypothesized the presence of Co provides a significant
thermodynamic driving force for segregation to planar defects. </p><p>In order to further investigate
the correlation between solute segregation and deformation mechanisms the
fabrication of a planar front single crystal Ni-based superalloy and its microstructure,
alloy composition, and microhardness properties of the as-zone melted and
solution heat treated states were investigated and compared to the
directionally-solidified state to study the effect of microsegregation on these
alloy characteristics. Next, new Co-containing, Cr-free alloys are designed to
optimize g-g’ volume fraction,
size, and morphology to mimic microstructures observed in single crystal
superalloys. The general alloy design strategy and approach are outlined, and the
composition, microstructure, phase transformation temperatures, and mechanical
properties of new Cr-free and Co-containing alloys are reported. A new set of
Cr-free alloys have thus been designed, with modifications of Nb, Ta, and Ti
additions ranging from 3 to 7 at.% to investigate the role of these elements on
the phase transformation strengthening mechanism at elevated temperatures.</p><p></p>
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Dynamic Deformation and Temperature Field Measurement of Metallic MaterialsYizhou Nie (7909019) 22 November 2019 (has links)
<p>In this dissertation, we first used
high-speed X-ray phase contrast imaging and infrared thermal imaging techniques
to study the formation processes of adiabatic shear bands in aluminum 7075-T6
and 6061-T6 alloys. A modified compression Kolsky bar setup was developed to
apply the dynamic loading. A flat hat-shaped specimen design was adopted for
generating the shear bands at the designated locations. Experimental results
show that 7075-T6 exhibits less ductility and a narrower shear band than
6061-T6. Maximum temperatures of 720 K and 770 K were locally determined within
the shear band zones for 7075-T6 and 6061-T6 respectively. This local high
temperature zone and the resulting thermal instability were found to relate to
the shear band formation in these aluminum alloys. Secondly, a high-speed laser
phosphorescence thermal imaging technique is developed and integrated with the
compression Kolsky bar setup. The temperature field measurement during dynamic
loading are performed at 100 – 200 kHz frame rate with a spatial resolution of
13 µm/pixel. The
dynamic compression of copper shows 312 K temperature rise among the material
surface. Experiments with thermocouple are also conducted and the results
verifies the laser measurement. In the dynamic shear of aluminums, the
temperature evolution during adiabatic shear band formation was observed and
the results are compared with infrared measurements. The shear band was found
forming at approximately 400 K and 440 K for 7075-T6 and 6061-T6, respectively,
while the maximum temperature is measured as 650 K for 7075-T6 and 800 K for
6061-T6. Although the maximum temperature agrees with the infrared results, thermal
softening is not considered as the main cause of the ASB formation due to the
low temperature when the shear band forms.</p>
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MICROSTRUCTURE DEVELOPMENT IN MULTI-PASS LASER MELTING OF AISI 8620 STEELMatthew L Binkley (9182462) 29 July 2020 (has links)
<p>An existing thermal model for laser melting and additive
manufacturing (AM) was expanded to include phase transformation and hardness
predictions for an alloy steel and coupled to experimental results. The study was performed on AISI 8620, a
popular case-hardening, steel to understand microstructural and property
effects for potential repair applications.
The experimental samples were
polished, etched with nital and picral for comparison, imaged, and Vicker’s microhardness
was taken at 0.5 and 0.2 kg loads. The
etched images revealed a transformation zone slightly larger than the melt zone
in all cases including a gradient in transformation along the outer edges of
the transformation zones. The microhardness
measurements revealed that the lower energy cases provided a higher hardness in
the melted region even after tempering due to multiple passes. But the overall hardness was higher than what
is to be expected of a fully martensitic structure in AISI 8620. The phase transformation model qualitatively
shows a similar microstructure where molten regions turn completely to
martensite. The model also predicts a
transformation zone larger than the melt pool size, as well as the
transformation of pearlite but not ferrite near but not in melt pool. This observation is experimentally verified
showing a heat affected zone where pearlite is clearly transformed but not
ferrite outside the transformation zone comprised of complete martensite. The hardness model predicts a lower hardness
than the experiments but is similar to what is expected based on published
Jominy End Quench tests. The cases in
the regime dominated by conductive heat transfer show good agreement with the predictions
of melt pool shape and hardness by the thermal model. However, at higher powers and lower speeds,
the fluid flow influenced the shape of the melt pool and the hat transfer in
its vicinity, and the model was less accurate.</p>
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SOLIDIFICATION BEHAVIORS OF PROEUTECTIC AL3SC AND AL-AL3SC EUTECTIC IN HYPEREUTECTIC AL-SC UNDERCOOLED MELTAoke Jiang (10716237) 28 April 2021 (has links)
<p>The lack of a thorough understanding of the solidification behaviors of the proeutectic Al<sub>3</sub>Sc and the Al-Al<sub>3</sub>Sc eutectic in a hypereutectic Al-Sc alloy stimulates the present dissertation. The major findings for the single-phase growth of the proeutectic Al<sub>3</sub>Sc is summarized as follows: At a low cooling rate (~1 ºC·s<sup>-1</sup>), the proeutectic Al<sub>3</sub>Sc phase’s formation was governed by the lateral growth, exposing six flat {100} facets. At an intermediate cooling rate (~400 ºC·s<sup>-1</sup>), the proeutectic Al<sub>3</sub>Sc grew in a dendritic manner, with well-defined backbones extending in eight <111> directions and paraboloidal dendrite tips, although the dendrite tips and side-branches turned into faceted steps at a late growth stage,when the lateral growth prevailed. At a high cooling rate (~1000 ºC·s<sup>-1</sup>), the proeutectic Al<sub>3</sub>Sc primarily crystallized into an entirely seaweed-structured particle, which was composed of interior compact seaweeds and exterior fractal seaweeds. In order to verify the proposed dendritic and seaweed growth mechanisms for the proeutectic Al<sub>3</sub>Sc, various morphological stability criteria were used, and fair agreement between the observed and the estimated characteristic length scales was reached.</p><p>On the Al-Al<sub>3</sub>Sc eutectic side, it was found that a rod-typed Al<sub>3</sub>Sc eutectic phase prevalently existed in an as-cast hypereutectic Al-Sc alloy that solidified via both slow cooling in air (~1 ºC·s<sup>−1</sup>) and rapid cooling in a wedge-shaped copper mold (up to ~3000 ºC·s<sup>−1</sup>). Al-Al<sub>3</sub>Sc eutectic dendrites were identified within a narrow region near the edge of the wedge. The eutectic dendrites had an equiaxed dendritic contour and a rod eutectic structure inside. Quantitative assessments revealed that an interface undercooling of 48.2 ºC was required to form the eutectic dendrites, or in other words, to enter the coupled zone of the Al-Al<sub>3</sub>Sc phase diagram. Furthermore, a phenomenon of scientific interest was discussed: When crystallizing under a near-equilibrium condition, the eutectic Al<sub>3</sub>Sc phase formed a non-faceted morphology, in contradiction to its faceted nature. Based on the competitive growth criterion, it was deduced that the non-faceting of the eutectic Al<sub>3</sub>Sc phase essentially reduced the interface undercooling for the resultant regular eutectic, in comparison to an otherwise irregular eutectic that would contain a faceted eutectic Al<sub>3</sub>Sc phase.</p>
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