| Author: | Zhou, Bingchen |
| Title: | Discontinuous precipitation and mechanical properties of NiAl-strengthened steels |
| Advisors: | Jiao, Zengbao (ME) Shi, Sanqiang (ME) |
| Degree: | Ph.D. |
| Year: | 2022 |
| Subject: | Precipitation hardening Steel, Structural Hong Kong Polytechnic University -- Dissertations |
| Department: | Department of Mechanical Engineering |
| Pages: | xvi, 136 pages : color illustrations |
| Language: | English |
| Abstract: | Nanoscale precipitation strengthened steels have attracted considerable attention in recent years, the development of which requires a fundamental understanding and precise manipulation of the precipitation behavior. NiAl has been recognised as one of the most effective phases used for precipitation strengthening because it has an ordered crystal structure and exhibits high coherency with the body-centred-cubed (BCC) Fe. NiAl precipitation occurs either continuously or discontinuously in NiAl-strengthened steels. Discontinuous precipitation (DP) refers to the formation of coarse rod-like precipitates formed along grain boundaries, whereas continuous precipitation (CP) leads to the uniform precipitation of spherical nanoparticles in the grain interiors. While most studies have focused on the CP behavior of NiAl-strengthened steels, research on their DP reaction is very limited. To date, the fundamental mechanisms for the formation and control of DP in NiAl-strengthened steels remain poorly understood, and the metallurgical factors affecting the interplay between the CP and DP behaviors have yet to be fully elucidated. The first part of this thesis aims to study the fundamental issues relating to the effects of Ni and Al contents and ratios on the nanoscale continuous and discontinuous precipitation, sublattice occupancy, and mechanical properties of NiAl-strengthened steels. The Fe-xNi-3Al and Fe-8Ni-yAl (x = 3 – 15; and y = 1 – 3, wt.%) steels were selected as model alloys, and their microstructures were characterized by a combination of scanning electron microscopy (SEM), transmission electron microscopy (TEM), electron back scattered diffraction (EBSD), and atom probe tomography (APT). It was found that increasing the Ni and Al contents results in a refinement of grain structure and an increase in the chemical potential gradient, both of which promote the DP reaction, leading to an increase in the volume fraction of DP regions. The APT results reveal that the composition of NiAl precipitates is far from the stoichiometric composition of binary NiAl; they contain a considerable amount of Fe and have different Ni/Al ratios, depending on the bulk alloy composition. First-principles calculations indicate that Fe is energetically favorable to occupy the Ni sublattice in the Al-rich environments and Fe prefers the Al sublattice in the Ni-rich environments. The mechanical results reveal that increasing the Ni content significantly improves the alloy ductility, which is accompanied by the transition of the fracture mode from cleavage to ductile dimpled failure. Our analyses indicate that the grain size and matrix composition are two important factors affecting the ductility of the NiAl-strengthened steels. The second part studies the interplay between the CP and DP reactions in NiAl-strengthened steels and proposes an effective method to suppress the DP reaction and accelerate the CP reaction of NiAl-strengthened steels. This research demonstrates that Cu is effective in not only promoting the nano-scale continuous NiAl precipitation but also in suppressing the coarse-scale discontinuous NiAl precipitation at grain boundaries, which results in the development of new NiAl-strengthened steels with high yield strength (1400 MPa) and good ductility (10%). The mechanisms for suppressing discontinuous NiAl precipitation are twofold. One is the acceleration of continuous NiAl precipitation through Cu partitioning, which swiftly reduces the matrix supersaturation, thereby decreasing the driving force for the DP reaction. The other is the reduction of grain boundary energy through Cu segregation, which inhibits the initiation of DP. Consequently, Cu increases the number density of NiAl nanoparticles by more than five-fold, which leads to a two-fold enhancement in the strengthening and an improvement in the over-aging resistance of NiAl-strengthened steels. The third objective is to investigate the effects of pre-strain on the DP reaction of NiAl-strengthened steels and to develop NiAl nanorods strengthened steels through the accelerated DP reaction. The temporal evolution of precipitate microstructure and grain structure of the pre-strained steels were characterized by SEM, EBSD, and APT. The microstructural results reveal that the DP regions develop in a fast pace, which is associated with a fast grain recrystallization. Mechanism analyses indicate that the pre-strain is effective in promoting the DP reaction by generating more nucleation sites for DP and accelerating the diffusion due to the high densities of dislocations and grain boundaries. As a result, annealing of the pre-strained steel results in a fine recrystallized grain structure and high volume fractions nano-sized NiAl rods in the matrix. The DP nanorods strengthened steels exhibit an outstanding combination of high tensile strength of 1650 MPa and good ductility of 12% at room temperature. The strengthening mechanisms of nano-sized NiAl rods were quantitatively analyzed. In summary, this research systematically investigated the metallurgical factors affecting CP and DP behavior and mechanical properties of NiAl-strengthened steels. This work not only sheds lights on the fundamental understanding of precipitation, strengthening, and ductilizing mechanisms of NiAl-strengthened steels but also provides useful guidelines for the design of advanced high-strength steels for technological applications. |
| Rights: | All rights reserved |
| Access: | open access |
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