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Dynamic Deformation Behavior of Ni-containing High Strength Steels

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Abstract:
This thesis documents a study of the high strain rate deformation behavior of Ni-containing steels of interest to the U.S. Navy for ship structures. A range of microstructures that resulted in different static mechanical properties in a family of Ni-containing steels was generated by modifying heat treatments for a given steel composition (10% Ni-0.1%C) and by modifying the steel composition in terms of Ni content (2.5%, 3.5%, 4.5%, 6.5% and 10%) as well as C (0.1% and 0.03%) content. The loss in strength from C decrease in the lower Ni versions was offset by adding Cu to produce precipitation hardening from the nanosize Cu precipitates. The influence of microstructure on the dynamic properties of these steels was studied through a combination of careful documentation of the initial microstructure, mechanical testing using the Kolsky compression bar set-up, and detailed post-deformation microstructure characterization. The dynamic deformation behavior of a steel with the composition 0.09%C-10.0%Ni-1.2%Mo-0.6%Mn-0.5%Cr-0.1%V (wt.%) was first examined in two different tempers. In the first case, the microstructure consisted of heavily dislocated martensite laths with a fine distribution of MC carbides. This microstructure was highly prone to shear localization in the strain rate regime 2 x 10 3 s -1 to 4 x 10 3 s -1 . The resulting adiabatic shear band, approximately 15 μm wide, consisted of submicron equiaxed grains composed of a mixture of fcc and bcc phases thought to be austenite and ferrite/martensite.In the second case, the tempered microstructure consisted of martensite laths although the dislocation density appeared lower; in addition to coarse MC carbides, M 2 C carbides and austenite islands were noted. At comparable strain rates, the steel was more resistant to localization and a well-developed shear band was not obtained; incorporating a notch in the specimen however facilitated the formation of a shear band. The microstructure within the band was again composed of equiaxed grains but diffraction patterns did not show the presence of the fcc phase, implying its transformation during deformation to bcc ferrite/martensite. This phase transformation of austenite to martensite may be responsible for the enhanced resistance to shear localization. This finding was followed by a methodical study to evaluate the role of precipitated austenite content, composition and size distribution on xvi discouraging shear localization. Mössbauer spectroscopy was used to determine austenite content in the heat-treated steels. Results confirmed that when the austenite content was in excess of 5 volume percent, shear localization could be postponed. For similar volume fractions that showed enhancements in shear localization resistance, tempering at a lower temperature resulted in less of a loss in static strength while displaying enhanced ballistic resistance. When lower levels of Ni were incorporated in the alloy, for example 2.5% Ni or 3.5% Ni instead of 10% Ni, the shear localization resistance improved but the improvement came at the expense of static strength. This study has shown that it is possible to develop a microstructure that provides a high static strength while maintaining acceptable levels of ballistic resistance; the improvement is thought to come from a martensite transformation of precipitated austenite, the transformation being triggered by deformation.
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Thesis (Ph.D. -- Brown University (2013)

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gupta, ratnesh k., "Dynamic Deformation Behavior of Ni-containing High Strength Steels" (2013). Materials Science Engineering Theses and Dissertations. Brown Digital Repository. Brown University Library. https://doi.org/10.7301/Z0PV6HQS

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