Alloy design, characterization, and mechanical properties of advanced ultra-high strength steels strengthened by nano-precipitates

Abstract

Ultra-high strength steels are of paramount importance in aerospace, automotive, nuclear power, and other high-tech industries. These steels are also future key materials for lightweight engineering design strategies and corresponding CO2 reduction. In this thesis, three types of advanced ultra-high strength steels were studied: Cu precipitation strengthened steels, NiAl precipitation strengthened steels, and co-precipitation strengthened steels. Cu precipitation strengthening in steels has attracted considerable attention and has become the cornerstone for the development of high-strength low-carbon steels. Improvement of Cu precipitation strengthening requires an understanding of the nanoscale precipitation mechanisms. Cu and Ni are found to have synergistic effects on nanoscale precipitation and mechanical properties of Cu-rich nano-cluster-strengthened ferritic steels, and new steels with ultra-high strength and good ductility have been developed. Our results indicate that Ni effectively increases the number density of Cu-rich nano-clusters by more than an order of magnitude, leading to a substantial increase in yield strength. Cu and Ni are also found to be beneficial to grain-size refinement, resulting from lowering the austenite-to-ferrite transformation temperature, as determined from thermodynamic calculations. In addition, the strengthening mechanisms of Cu and Ni were quantitatively evaluated in terms of precipitation strengthening, grain boundary strengthening, and solid-solution strengthening. NiAl-strengthened ultra-high strength steels usually have high Ni and Al contents to form hardening particles. Several new low-Ni steels are reported, which achieve an outstanding combination of low cost, ultra-high strength, and good ductility through the precipitation of high number densities of nanoscale NiAl precipitates as characterized by atom probe tomography. The precipitation parameters are tuned by optimizing both compositions and heat-treatment parameters. The strengthening effects are analyzed quantitatively in terms of order strengthening, modulus strengthening, and coherency strengthening. Advanced steels with 1.5 GPa strength and good ductility are not only highly desirable for a wide range of critical applications but in favor of the lightweight design for reducing CO2 emissions. However, available gigapascal strength steels either are prohibitively expensive or have yet to exhibit the desired weldability needed to make them viable for a widespread use. We report on design strategies and structure-property studies of a new class of gigapascal strength ferritic steels essentially strengthened by nanoscale co-precipitation, which achieves a unique combination of 1.5-2 GPa strength, high ductility, good weldability, and low material cost. A computational-aided alloy design approach was applied for tailoring co-precipitation of nano-carbides, nano-intermetallics, and nano-clusters. The superior mechanical and welding properties together with their low cost make the currently developed nanoscale co-precipitation strengthened steels attractive for industrial applications.

Date of Award	14 Feb 2014
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	Chain Tsuan LIU (Supervisor)