The macroscopic mechanical properties of steel are highly dependant upon the microstructure, morphology, and distribution of each phase present.
Nanomechanical testing provides a straightforward solution for quantitatively characterizing each of these phases. Additionally, nanomechanical testing can serve a critical role in the identification of defects resulting from surface engineering processes such as the carburizing or nitriding of critical components.
Hardness mapping provides additional quantitative information concerning microstructure that is less evident using Modulus Mapping on materials with similar modulus values or a thick native oxide layer. Hardness mapping allows for visual observation of phase segregation and distribution, which assists in tailoring microstructure and the optimization of processing variables.
The mapped sample consisted of a cross sectioned ferrite/martensite dual-phase steel. The sample was polished using a 1μm diamond paste and a polishing cloth prior to testing. Nanoindentation tests were performed using a Hysitron TriboIndenter®, a cube corner diamond indenter probe, and a trapezoidal load function. A peak load of 65μN was used for all indentation tests. A 10x10 automated grid of indents was performed on a defect-free region of the sample identified by SPM imaging and indents were spaced 1μm apart.
Indentation results were analyzed and mapped using the Hysitron TriboAnalysis™ software package.
Results and Discussion
Figures 1 and 2 show hardness and reduced modulus maps of a ferrite/martensite multiphase steel sample, respectively. Both maps were obtained utilizing the same array of 100 automated indentations. The multi-phase structure of the surface can clearly be seen in Figure 1 due to large differences in hardness between the ferritic and martensitic phases. The presence of multiple phases is not evident in the modulus map shown in Figure 2.
Figure 1. Hardness map showing distinct ferrite and martensite phases.
Figure 2. Reduced modulus map of a martensitic/ferritic dual- phase steel.
Figure 3 depicts a histogram of the hardness and reduced modulus values used to construct the maps. The bimodal distribution of the hardness histogram in relation to the Gaussian-like distribution of the reduced modulus histogram further demonstrates the benefits of hardness over modulus characterization on this material type. The frequency of values within the two hardness groups of Figure 3B gives some indication of the localized volume fraction of each phase present.
The identification and distribution of phases within a steel sample were readily identified by hardness mapping, which was not obtainable using modulus alone.
Additionally, this technique could be used for defect identification on surface engineered materials and can lead to a better understanding of the effects of the processing parameters on the surface attributes of components.
Figure 3. Histogram of (A) reduced modulus and (B) hardness values from 100 indentations performed on a ferritic/martensitic steel sample.