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Preparation of a micro-pillar from a carbon-enriched retained austenite grain (a grain diameter of approximately 2 mm). (a) EBSD IPF map of the Q&P steel microstructure. (b)
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We report evidence of a displacive phase transformation from retained austenite to martensite during preparation of quenched and partitioned steel micro-pillars by using a focused ion beam (FIB) technique. The BCC phase produced by the FIB damage was identified as martensite. The invariant-plane strain surface relief associated with the martensitic...
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... FIB-induced phase transformation during micro-pillar fabrication was observed for all three micro-pillars in the present study. Fig. 3 shows another example of the retained austenite-tomartensite transformation during the micro-pillar fabrication. The selected retained austenite grain had a grain size of approximately 2 mm. Supported by the absence of surface relief, the retained austenite did not immediately transform to martensite after a Ga þ ion beam scan of the ...
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... 3 shows another example of the retained austenite-tomartensite transformation during the micro-pillar fabrication. The selected retained austenite grain had a grain size of approximately 2 mm. Supported by the absence of surface relief, the retained austenite did not immediately transform to martensite after a Ga þ ion beam scan of the surface (Fig. 3(c) and (d)). Fig. 3(d) shows that the retained austenite grain was smaller than the inner diameter (3 mm) of the ring pattern of the FIB scanned area, indicating that this retained austenite grain was less influenced by the Ga þ ion beam compared to the coarse retained austenite grain shown in Fig. 1. However, despite the smaller grain ...
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... of the retained austenite-tomartensite transformation during the micro-pillar fabrication. The selected retained austenite grain had a grain size of approximately 2 mm. Supported by the absence of surface relief, the retained austenite did not immediately transform to martensite after a Ga þ ion beam scan of the surface (Fig. 3(c) and (d)). Fig. 3(d) shows that the retained austenite grain was smaller than the inner diameter (3 mm) of the ring pattern of the FIB scanned area, indicating that this retained austenite grain was less influenced by the Ga þ ion beam compared to the coarse retained austenite grain shown in Fig. 1. However, despite the smaller grain size and thus likely ...
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... was less influenced by the Ga þ ion beam compared to the coarse retained austenite grain shown in Fig. 1. However, despite the smaller grain size and thus likely increased austenite stability, the retained austenite grain also transformed to martensite in the later stages of the micro-pillar fabrication process, as shown in the EBSD results in Fig. 3(f) and (g). The KeS orientation relationship existed between the parent retained austenite and product martensite variants ( Supplementary Fig. S1), similar to the first example of the micropillar fabrication shown in Figs. 1 and 2. Furthermore, the observed martensite variants were twin-related ( Supplementary Fig. S1). Fig. 4 shows SEM ...
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... In previous related studies, Monte Carlo (MC) simulation based on binary collision theory (such as implemented in the SRIM software) has been commonly used as an auxiliary analysis method to understand high-energy ion irradiation effects [26]. Experiments show that the implantation depth of ions is directly related to the degree and direction of bending of nanopillars or thin films. ...
... An FCC/BCC phase boundary was identified using EBSD before PFIB was employed to isolate this boundary. During TKD of the resultant tip, it was observed that the FCC region had transformed to BCC, likely due to a mechanical transformation induced by the ion milling process, an effect previously observed and characterised by Seo et al. [43]. This is why both grains on either side of the boundary in Fig. 4b are indexed as BCC. ...
... Ion implantation has been known for a very long time [22][23][24] and is still one of the ways to modify materials [25]. Ion implantation improves the physical and mechanical properties of the material surface (hardness, heat resistance, wear resistance, fatigue strength, corrosion resistance, etc.) [20,[26][27][28][29][30]. The main advantages of ion implantation include the absence of adhesion problems [31] (since ions are implanted into the material ...
In this work, the corrosion resistance of AISI 321 stainless steel is increased through. the two-stage implantation of oxygen ions and of both aluminum and boron ions together. During ion implantation, a modified layer with a thickness of about 200 nm is formed, which affects the properties of material. The increase in corrosion resistance is confirmed by prolonged acid corrosion tests at pH 3.5 and by accelerated electrochemical tests using a potentiostat. The corrosion rate of the implanted sample is 0.708 μA/cm2, in contrast to the non-implanted sample (1.26 μA/cm2). The modified surface layer is examined using X-ray photoelectron spectroscopy (XPS), secondary-ion mass spectrometry (SIMS), and transmission electron microscopy (TEM). Aluminum and boron are implanted to a depth of more than 250 nm. It is found that the modified surface of the stainless steel substrate contains oxides of implanted ions (Al2O3) and oxides of substrate ions (Cr2O3 and NiCr2O4).
... This feature of the cellular structure of L-PBF 316 L has to date not yet been reported, to the authors knowledge. Nonetheless, it is known that austenitic stainless steels are prone to a Ga þ -induced ferritic transformation, [48][49][50][51][52] mainly due to the stabilization of ferrite by Ga, but the extent of the transformed regions may also be affected by the strain induced during implantation. [50] Knipling et al. concluded that the austenite composition is the most important factor controlling the Ga-induced transformation, while the second and third factor respectively being the beam dose and crystal orientation. ...
In the current study, the 3D nature of the melt pool boundaries (MPBs) in a 316 L austenitic steel additively manufactured by laser‐based powder bed fusion (L‐PBF) is investigated. The change of the cell growth direction and its relationship to the MPBs is investigated by transmission electron microscopy. A hitherto unreported modulated substructure with a periodicity of 21 nm is further discovered within the cell cores of the cellular substructure, which results from a partial transformation of the austenite, which is induced by a Ga⁺ focused ion beam. While the cell cores show the modulated substructure, cell boundaries do not. The diffraction pattern of the modulated substructure is exploited to show a thickness ≥200 nm for the MPB. At MPBs, the cell walls are suppressed, leading to continuously connecting cell cores across the MPB. This continuous MPB is described either as overlapping regions of cells of different growing directions when a new melt pool solidifies or as a narrow planar growth preceding the new melt pool.
... Therefore, it is only possible to fabricate a single small size pillar (at most ¼ of the grain size) from each selected grain in the DSS microstructure. It is well known that the use of Ga þ ion milling could impact on the mechanical properties of the micropillars through residual ion implantation, crystal defects or unwanted phase transformations [27][28][29]. However, it has been reported that the damage is less important in pillars with diameter <500 nm [28] and at low ion currents and accelerating voltages [29], hence we believe our fabrication conditions and small pillar dimensions have minimised these potential artefacts of the fabrication method we have used. ...
... It is well known that the use of Ga þ ion milling could impact on the mechanical properties of the micropillars through residual ion implantation, crystal defects or unwanted phase transformations [27][28][29]. However, it has been reported that the damage is less important in pillars with diameter <500 nm [28] and at low ion currents and accelerating voltages [29], hence we believe our fabrication conditions and small pillar dimensions have minimised these potential artefacts of the fabrication method we have used. ...
The mechanical properties of the austenite and ferrite phases in a high Cr content 2205 duplex stainless steel (DSS) have been investigated using single crystal micropillars of size 200–2000 nm. Austenite pillars with axial orientation [123] show a strong size effect with strength increasing as pillar size decreases, following a power-law relation, with exponent -0.62. Pillars of diameter d = 300 nm show a strength >2.4 GPa. The strength of ferrite pillars with axial orientation [001] also shows a power-law with exponent -0.56 and strength > 1.5 GPa at d = 260 nm. However, [123]-orientated ferrite showed no size effect and a mean strength of 760 MPa. Both phases show localised multiple slip on non-parallel slip planes with high Schmid factors, with the ferrite showing a wider spacing of the slipped regions. On comparison with previous work, the strength of both phases in this alloy are shown to have greater strength than lower Cr content steels. Combining our data with results in the literature for larger diameter micropillar specimens supports the hypothesis that the high Cr content of the DSS leads to a larger bulk stress and a reduction in the magnitude of the apparent size effect exponent with larger pillar diameters.
This paper presents studies concerning the influence of the Ga⁺ ion beam on the stability of retained austenite in K110 cold work tool steel (X155CrVMo12). The material was first austenitized at 1200 °C to dissolve a significant amount of the alloy's carbides to obtain the largest possible volume fraction of retained austenite, which was 93% after quenching in oil. Changes in the steel's crystal structure upon interaction with Ga⁺ ions were observed using a combination of SEM-FIB-EBSD techniques. Selected grains in the material were sputtered with Ga⁺ ions of various accelerating voltages (2 kV – 30 kV) using two different doses, namely 50 and 100 pC/μm². It was found that the application of 5 kV Ga⁺ ion beam is enough to induce partial transformation of austenite to BCC phase. The use of higher voltages (8–30 kV) resulted in the complete transformation of austenite. Low acceleration voltages of 2 kV, even with a high dose of 500 pC/μm², did not induce any phase transformation and only deformed austenite's crystal structure. Moreover, it was shown that the crystallographic orientation between austenite and the Ga⁺ ion beam induced BCC phase is not random.
