Effects of load partitioning and texture on the plastic anisotropy of duplex stainless steel alloys under quasi-static loading conditions

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Abstract

The plastic anisotropies of two hot-rolled Lean Duplex Stainless Steels (commercially known as LDX 2101 and LDX 2404) were investigated by applying compressive strains, at 10−3 s−1 rate, along the rolling- and transverse-directions (RD and TD). The microstructural changes were elucidated by Electron Backscatter Diffraction (EBSD) as a function of strain level and loading direction. In both alloy grades, the austenite phase shows a weaker texture development than the ferrite phase; the later develops {001}<110> and {110}<110> textures in LDX 2101 and LDX 2404 alloys, respectively. Also, in both alloys, the yield stress along the TD is larger by 10% than along the RD. Anisotropies are also detected in the rate of property changes with deformations; after 30% true strain, the flow stress along the RD in LDX 2101 alloy starts exceeding the stress along the TD. Microstructural studies indicate that the load partitioning, grains shape, phase boundaries and austenite to martensite phase transformations are the origins of the anisotropic phenomena in LDX 2101 alloy, whereas the crystallographic texture of ferrite phase, phase boundaries and load partitioning are the plausible origins of plastic anisotropies in LDX 2404 alloy.

Introduction

Duplex Stainless Steels (DSSs) are dual phase alloys with almost equal volume fractions of austenite (γ) and ferrite (α) phases. This alloy class exhibits attractive mechanical (yield stress, ultimate tensile stress and ductility) and corrosion properties compared against single phase stainless steel alloys [1,2]. The overall deformation mechanism of DSSs is governed by the deformation mechanisms of individual phases and the interactions between them [[3], [4], [5]]. The Face Centred Cubic (FCC) structure of the austenite phase deforms via several modes (dislocation slips, shear banding and phase transformations) [3,6,7], whereas the Body Centred Cubic (BCC) structure of the ferrite phase undergoes deformations by dislocation slips on many operating slip systems [6,8]. Furthermore, the residual stresses that develop during the manufacturing process are distributed differently in these two phases. It has been observed that tensile and compressive residual stresses develop in the austenite and ferrite phases, respectively. This difference arises due to their differences in thermal expansion coefficients [5,9]. Thus, a coexistence of the two phases makes the overall deformation behaviour in DSSs rather complex.

It has been shown in numerous investigations that steels exhibit mechanical anisotropies as a result of the crystallographic anisotropy and manufacturing related anisotropy [10]. Crystallographic anisotropies depend on the crystal structures, initial microscale stress distributions, morphology and grain size [5,11]. Manufacturing related anisotropy, which causes a texture, develops during the manufacturing process, where grains within metal blocks get aligned along the loading reference axes of deformation. For example, grains in the hot- and cold-rolled plates rotate and align with the rolling coordinate systems [12,13]. The anisotropies of DSSs have been studied extensively [5,[13], [14], [15], [16], [17], [18], [19]], these include the behaviour under quasi-static tensile [5,13,14,18], fatigue [16] and shock [17,19] loadings. Hutchinson et al. [14] reported that the tensile stress along the TD of a cold-rolled DSS 2205 is 10% higher than along the RD. Similar findings on DSS 2205 were reported by Mateo et al. [16], the fatigue life along the TD was determined to be longer than along the RD. These differences result from the crystallographic textures within the ferrite phase that develop during rolling processes and the coexistence of the austenite and ferrite phases [13,14]. Similar finding was also reported via in-situ tensile testing of DSS 2304 by X-ray diffraction [5], i.e. higher yield strength was measured along the TD as a result of texture development in the ferrite phase. Another interesting phenomenon was discovered from the in-situ investigation that both γ and α phases accommodate identical deformation levels when the tensile loading is applied along the RD, whereas an uneven strain distribution is detected between the phases when the loading is applied along the TD and 45° to the RD. They reported that more plastic strain is accommodated in the austenite phase when the loading is parallel to the TD, while the plastic deformation is mainly accommodated by the ferrite phase when the loading is applied along 45° to the RD. In contrary with previous studies, it was observed that the incipient spall strength (dynamic tensile strength under uniaxial strain condition estimated via plate impact experiment [20]) of DSS 2205 is higher along the RD than along the TD; this was due to favourable void growth conditions at a particular phase boundary orientation [16]. Although anisotropic properties are widely reported for DSSs, there are some exceptions too. For example, Papula et al. [13] found that the tensile properties (yield stress and ultimate tensile strength) in a cold-rolled Lean Duplex Stainless Steel 2101 (LDX 2101) are almost similar along the RD and TD.

Overall, the aforementioned anisotropies in DSSs vary with loading conditions and alloy grades. However, the effects of various factors, such as, austenite phase transformation, grains’ shape and phase boundary interactions, on the mechanical anisotropies of DSSs remain unexplained. This gap is also indicated by other researchers, for instance, Tian et al. [4] reported that load partitioning in commercial DSSs can be more complex when it involves austenite/martensite transformations. Further, newer grades of DSSs, such as Lean Duplex grades (LDXs) and hyper duplex grades, are evolving and their behaviours are largely unknown.

LDX alloys have recently been developed via partially substituting nickel and molybdenum with nitrogen and manganese [[21], [22], [23]]. Both manganese and nitrogen are austenite stabilizers. Nitrogen also enhances the mechanical and corrosion properties [24]. Manganese also increases the solubility of N. These substitutions make LDXs grades available at lower costs and suitable for wide engineering applications [21,[25], [26], [27]]. However, the mechanical response and the mechanical anisotropies of these new alloy grades need to be systematically assessed. This study investigates the anisotropic responses and the microstructural evaluations in the newest grades of commercial LDXs (LDX 2101 and LDX 2404 alloys) under quasi-static loading conditions. The microscopic investigations were conducted using Electron Backscatter Diffraction (EBSD) to understand the localised deformation induced strain fields within the phases. The micro-hardness testing was conducted to correlate the flow stress with the EBSD revealed deformation phenomena.

Section snippets

Materials and methods

In this study, a 20 mm thick LDX 2101 (UNS S32101, EN 1.4162) hot-rolled plate and 6 mm thick LDX 2404 (UNS S82441, EN 1.4662) hot-rolled coil-plate produced by Outokumpo® were investigated. The bulk chemical composition of the materials is shown in Table I. Both plates were heat treated at 777 K (1050 °C) and then water quenched to achieve equal volume fractions of the γ and α phases. The principal cross-sections of the plates, RD, TD and Normal Direction (ND), are scanned by EBSD and

Initial characterization

EBSD characterizations of the as-received samples show equal area fractions of the γ and α phases in both LDX 2101 and LDX 2404 samples. The 3D phase maps in Fig.(1) show that the size of the phase fields is larger in LDX 2101 than LDX 2404. This indicates a higher density of phase boundaries in LDX 2404. The phase boundary density in a given reference plane is quantified by measuring the length of phase boundary in a unit area in the EBSD map. The data from each sample is summarized in Table II

Conclusions

This paper reports the anisotropic behaviours and the microstructural evolutions in two grades of Lean Duplex Stainless Steels (LDX), LDX 2101 and LDX 2404, under quasi-static compressive loading conditions along the RD and TD. The main findings are:

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    In the as-received alloys, the hot rolling and annealing at 777 K develop strong {001}<110> and {110}<110> textures in the ferrite phase in LDX 2101 and LDX 2404, respectively.

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    Both LDX 2101 and LDX 2404 alloys show anisotropic response over the

Acknowledgments

Authors would like to acknowledge the support by the Air Force Office of Scientific Research under grant number FA2386-17-1-4095. The financial support of the Australian Government Research Training Program Scholarship, Australia for the PhD study of the first author is also acknowledged.

References (37)

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