Metallurgy Commons^™

Microstructure-Based High Temperature Processing And Failure Analyses Of Engineering Alloys Under Complex Conditions, Dong Han

Doctoral Dissertations

From traditional petrochemical applications to contemporary manufacturing techniques, advanced structural materials are subjected to intricate thermomechanical and environmental conditions. In these engineering domains, high-temperature deformation plays a pivotal role, profoundly influencing the ultimate outcomes of industrial applications. Nevertheless, the underlying mechanisms of this deformation remain elusive, largely due to the absence of a microstructure-based mechanistic understanding of high-temperature processing and failure of materials.

This dissertation endeavors to comprehensively examine these mechanisms through computational methodologies, focusing on three quintessential topics: grain boundary cavitation failures (2 examples: stress relaxation cracking (SRC) and grain size dependence), high-temperature hydrogen attack (HTHA), and additive friction …

Experimental And Computational Investigation Of High Entropy Alloys For Elevated-Temperature Applications, Haoyan Diao

Doctoral Dissertations

To create and design novel structural materials with enhanced creep-resistance, fundamental studies have been conducted on high-entropy alloys (HEAs), using (1) thermodynamic calculations, (2) mechanical tests, (3) neutron diffraction, (4) characterization techniques, and (5) crystal-plasticity finite-element modeling (CPFEM), to explore future candidates for next-generation power plants.

All the constituent binary and ternary systems of the Al-Cr-Cu-Fe-Mn-Ni and Al-Co-Cr- Fe-Ni systems were thermodynamically modeled within the whole composition range. Comparisons between the calculated phase diagrams and literature data are in good agreement. The Al_xCrCuFeMnNi HEAs have disordered [face-centered-cubic (FCC) + body-centered-cubic (BCC)] crystal structures. Excessive alloying of the Al …

Microstructure Control And Correlation To Creep Properties In Grade 91 Steel Weldment After Thermo-Mechanical Treatments And An Fe-30cr-3al Alloy Strengthened By Fe2nb Laves Phase, Benjamin Allen Shassere

Doctoral Dissertations

Type IV cracking in weldments of steel pipes after creep deformation is a concern in modern fossil-fueled power plants. Two possible methods for minimizing or eliminating Type IV cracking will be discussed. The first method alters the initial microstructure of typical Grade 91 steel base metal before welding, while the second provides baseline microstructure characteristics and creep performance of a new alloy that is strengthened by the intermetallic Fe2Nb Laves phase. The initial microstructure of the Grade 91 steel can be controlled by Thermo-Mechanical Treatments, which aids in precipitation of fine (5-10 nm) MX particles in austenite before transformation to …

Microstructures And Mechanical Behavior Of Nial-Strengthened Ferritic Alloys At Room And Elevated Temperatures, Zhiqian Sun

Doctoral Dissertations

In order to improve the thermal efficiency and decrease the greenhouse gases emission, it is required to increase the steam temperature and pressure in fossil-energy power plants. In the United States, research has been performing in order to push steam temperature to 760 Celsius degree and steam pressure to 35 MPa. However, the highest operational temperature for current commercial heat-resistant ferritic steels is ~ 620 Celsius degree. In this sense, new advanced ferritic alloys with better creep resistance are needed, considering such service conditions in next-generation ultra-supercritical fossil-energy power plants.

Coherent B2-ordered NiAl-type precipitates have been employed to reinforce the …

Characterization Of The Fine-Scale Weld Deposit Microstructure And Its Influence On The Elevated Temperature Properties Of 2.25cr - 1mo - 0.25v Weldments In Heavy Wall Pressure Vessels, Dewey Joshua Burgess

Doctoral Dissertations

The research herein was conducted to characterize the fine-scale microstructure of 2.25Cr‑1Mo‑0.25V (22V) submerged arc weld deposits and to study the influence of the microstructure on creep behavior and reheat cracking susceptibility. Scanning electron microscopy and transmission electron microscopy examinations concentrated on carbide morphology and evolution as a function of time and temperature, since the majority of properties that are associated with 22V weld deposits are attributable to the carbide type and location throughout the microstructure.

Five distinct carbides were observed in the range of heat treatments studied: MC, M2C, M23C6, M7C3, and M6C. It was shown that each carbide …

Computational And Experimental Study Of Structure-Property Relationships In Nial Precipitate-Strengthened Ferritic Superalloys, Shenyan Huang

Doctoral Dissertations

Ferritic superalloys strengthened by coherent ordered NiAl B2-type precipitates are promising candidates for ultra-supercritical steam-turbine applications, due to their superior resistance to creep, coarsening, oxidation, and steam corrosion as compared to Cr ferritic steels at high temperatures. Combined computational and experimental tools have been employed to investigate the interrelationships among the composition, microstructure, and mechanical behavior, and provide insight into deformation micromechanisms at elevated temperatures.

Self and impurity diffusivities in a body-centered-cubic (bcc) iron are calculated using first-principles methods. Calculated self and impurity diffusivities compare favorably with experimental measurements in both ferromagnetic and paramagnetic states of bcc Fe. The calculated …