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We investigated the local preliminary hardening of ferrite near the ferrite–martensite interfaces in a dual-phase (DP) steel. Geometrically necessary dislocations (GNDs), generated due to interfacial misfit between different phases, may cause preliminary hardening of ferrite around such interfaces. However, for nano-hardness distribution, the hardened zone was not evidently detected by scattering measurement. Thus, we factorized nano-hardness scattering to estimate the actual ferrite hardness near ferrite–martensite interfaces.
First, nano-hardness was measured around a martensite island using a conical nano-indenter in the DP steel containing 10% martensite by volume. Taking into account the scattering, the nano-hardness measurement converged to the hardness of ferrite, exceeding the distance corresponding to the nano-indenter radius. Thus, a preliminary hardening zone was not detected. Subsequently, the surface of the nano-indented microstructure was polished and observed using scanning electron microscopy (SEM) by analyzing electron back scattering diffraction (EBSD). This analysis confirmed the presence of the nano-indented microstructure under ferrite. Moreover, it established that the majority of the irregularly higher nano-hardness was caused by the buried martensite under ferrite. The value of the kernel average misorientation (KAM), which is proportional to the GND density for other irregularly higher nano-hardness points, was higher for the nano-indented microstructure as compared to that of the buried martensite. On the other hand, the ferrite was expanded under the nano-indented points for the majority of the irregularly lower nano-hardness, with some exceptions. Further, soft martensite was observed to induce irregularly lower nano-hardness locally around the interface.

Rights:

• Creative Commons BY Attribution 4.0 International
  Creative Commons BY Attribution 4.0 International

Keyword: dual phase steel, grains and interfaces, scattering, nanoindentation, serial sectioning

Date published: 2021-01-15

Publisher: Iron and Steel Institute of Japan

Journal:

• ISIJ INTERNATIONAL (ISSN: 09151559) vol. 61 issue. 1 p. 473-480

Funding:

Manuscript type: Publisher's version (Version of record)

MDR DOI:

First published URL: https://doi.org/10.2355/isijinternational.ISIJINT-2020-546

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Updated at: 2024-01-05 22:13:56 +0900

Published on MDR: 2023-02-28 11:55:09 +0900