Date of Award

2019-01-01

Degree Name

Doctor of Philosophy

Department

Engineering

Advisor(s)

Raja D. Misra

Abstract

The concept of low lattice misfit and high-density of nanoscale precipitates obtained through solution treatment was adopted to obtain ultrahigh strength maraging steel without compromising elongation. An "ultrahigh strength-high toughness" combination was successfully obtained in 19Ni3Mo1.5Ti maraging steel with ultimate strength of ~1858 MPa and static toughness of ~110 MJ·m-3. Maraging steel had extremely high density (2.3Ã?1024 m-3) of nanoscale precipitates with minimum lattice misfit of less than 1% at the solutionization temperature of 820 oC. Two kinds of nanoscale precipitates, namely, η-Ni3(Ti,Mo) and B2-Ni(Mo,Fe) contributed to ultrahigh strength. The size of nanoscale precipitates governed the movement of dislocations, cutting versus by-passing. Theoretical estimate of ordering and modulus contribution to strengthening suggested that ordering had a dominant influence on strength. The toughness was closely related to the characteristic evolution of nanoscale precipitates such that the high density of nanoscale precipitates contributed to increase of elastic deformation and low lattice misfit contributed to increase of uniform deformation. The nanoscale size and low lattice misfit of precipitates were the underlying reasons for the high-performance of maraging steel. Moreover, the combination of high-density of nanoscale precipitates and low lattice misfit is envisaged to facilitate the futuristic design and development of next generation of structural alloys. The low lattice misfit (0.6% ~ 0.9%) precipitates interacted with dislocations leaving stacking fault ribbons within precipitates and built a large long range of back stress producing a high strain-hardening response. Additionally, nanoscale twinning occurred. The above contributions to ductility are envisaged to be in addition to the significantly reduced elastic interaction between the low lattice misfit nanoscale precipitates and dislocations that reduces the ability for crack initiation at the particle-matrix interface. EBSD studies suggested that preferred orientations of {101}, fraction of high-angle grain boundary (HAGB) and total length of grain boundary per unit area (μm/μm2) were increased with increase of aging temperature, which was beneficial to both strengthening and toughening of maraging steel. Three types of reverted austenite, granular reverted austenite at grain boundaries (γG1), lamellar reverted austenite in the matrix (γL) and globular reverted austenite (γG2) were observed depending on the aging temperature and time. At low temperatures (560 °C and 640 °C), only γG1 and γL were observed. While at high temperature (700 °C), γG1 and γL decreased with holding time increased and were completely transformed to γG2 at equilibrium condition. The observation of three different morphologies of reverted austenite were a consequence of competition between the nucleation rate and growth rate of reverted austenite at different aging temperatures. The weak texture of maraging steel with high Schmid factor at high aging temperature implied that γL and γG2 are the likely reasons for superior toughness and ductility. We underscored that it is important to consider and control the morphology and content of reverted austenite, besides other microstructural features when interpreting the mechanical behavior.

Language

en

Provenance

Received from ProQuest

File Size

184 pages

File Format

application/pdf

Rights Holder

Bing Yu

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