Novel Mechanical Characterization of Austenite and Ferrite Phases within Duplex Stainless Steel

Abstract

The microstructure and micromechanical properties of the constitutive phases of a particular duplex stainless steel in various processing conditions have been characterized. Hardness (H), elastic modulus (E) and H/E cartography maps were obtained by using a high-speed nanoindentation mapping technique. Small-scale H and E evolution at different processing conditions has been investigated by statistical analysis of a large number of nanoindentations (10,000 imprints per sample). Two mechanically distinct phases, ferrite (α) and austenite (γ), were deconvoluted from this dataset using Ulm and Constantinides’ method, with the remaining values assigned to a third mechanical phase linked to composite-like (containing α/γ interphase boundaries) regions. These mechanical property phase assessments were supplemented by overlaying crystallographic phase maps obtained by electron backscattered diffraction. An excellent correlation between microstructure and small-scale mechanical properties was achieved, especially when considering the ratio H/E.

Influence of indentation size and spacing on statistical phase analysis via high-speed nanoindentation mapping of metal alloys

“…At first, we see a single elliptical distribution for the histogram at the finest spacing of 1 µm, which is elongated at a roughly constant hardness level of 4.5 GPa, that displays a significant tail towards the origin. This type of tail has been observed before in previous works [14,24] to correspond to the influence of roughness or porosity on the hardness results. However, in this work, the high-level vibropolishing that the DSS sample underwent suggests that roughness is likely not a factor here.…”

“…These figures offer a simple visual method to evaluate 'hot spots' in the indentation property space which statistically correspond to individual phases. As shown in previous work [14], these 'peaks' in the 2D histograms often take the form of elliptical, normal distributions which are elongated along the direction of the H/E ratio.…”

“…With correlative analytical methods, particularly EBSD in this case, it is a relatively simple matter to separate the two phases and obtain their individual properties. Indeed, good correlation between H/E ratio maps and EBSD phase maps has already been obtained on this material [14]. However, the focus of this work is to determine the relative effectiveness of various statistical methods alone.…”

“…Furthermore, in agreement with CSM indentations, the austenite (γ) phase displays higher hardness and lower elastic modulus compared with the ferrite (α) phase. More information about the correlation between the mechanical properties and the phases in DSS materials at different processing steps is provided elsewhere [14].…”

“…However, advances in instrumentation and electronics have now enabled the speed of indentation to be increased approximately 100 times, such that many thousands of indentations can now easily be performed within an hour [5][6][7]. This makes both techniques simultaneously more accessible and thus more popular with numerous applications emerging in the recent literature on a variety of materials: cements [8,9], hardmetals [6,10,11], Ni-Fe meteorites [12], titanium [13], duplex [14] and tool steels [15], and thermal barrier coatings [16].…”