Superior mechanical properties and deformation mechanisms of a 304 stainless steel plate with gradient nanostructure

Highlights

• A bulk GNS was formed in a 304 stainless steel plate of ∼1.90 mm in thickness.

• A superior strength-ductility synergy was obtained in the GNS 304 stainless steel.

• The GNS plate was gradually yielded from center to surface under tension.

• Extra strain-hardening and deferred necking occurred in the strained GNS plate.

• Dislocation activities dominate the strain-hardening behavior in the GNS sample.

Abstract

Spatially gradient microstructures have shown a promising application in enhancing strength-ductility synergy of engineering metals such as austenitic stainless steels. However, existing approaches are limiting in producing a thick gradient nanostructured (GNS) layer with a high strengthening capability, and the underlying deformation mechanisms are still not clear in GNS austenitic stainless steels. In this work, we developed a new approach, i.e., plate surface mechanical rolling treatment, to produce a bulk gradient nanostructure in a 304 stainless steel plate of ∼1.90 mm in thickness. Uniaxial tensile tests revealed that an ultra-high yield strength of ∼1073 MPa with a considerable uniform elongation of ∼21% was achieved in the GNS sample. Subsequently, the evolutions of microstructure, phase, microhardness, and local strain distribution were systematically studied in the GNS plate during tensile tests. The results demonstrated that the mechanical incompatibilities, relating with the gradient microstructure and martensite-enclosing-austenite domains, contribute to an extra strain-hardening capability, leading to the outstanding strength-ductility synergy in the GNS 304 stainless steel. Furthermore, analyses based on experimental observations and theoretical calculations revealed that dislocation activities, instead of deformation-induced martensite transformation, microstructure refinement, and twinning, play a dominant role in the strain-hardening mechanisms of the GNS plate during tension.

Introduction

Due to their outstanding properties including corrosion resistance, formability, work-hardening capability and biocompatibility, austenitic stainless steels, such as AISI 304 and 316 L, are widely used as key structural materials in industries (Gupta and Birbilis, 2015; Lo et al., 2009; Taleb et al., 2014; Yin et al., 2018). In addition, while their yield strength is relatively low, there are increasing needs on stainless steels with promoted mechanical properties, especially in aerospace, nuclear power, and some other highly specialized industries (Lo et al., 2009; Lu, 2010). Various approaches, such as work-hardening by cold rolling, martensite formation, nitrogen alloying, and grain size refinement, have been applied to promote the strength of austenitic stainless steels (Milad et al., 2008; Odnobokova et al., 2017; Ravi Kumar et al., 2009; Sunil et al., 2021; Yi et al., 2015). Unfortunately, such approaches typically result in significant decreases in elongation and toughness (Khan and Liu, 2016; Sakai et al., 2014). For example, while the yield strength was increased to > 1000 MPa, almost no uniform elongation was observed in nanostructured 304 or 316 L stainless steel samples prepared by severe plastic deformation (SPD) approaches (Qu et al., 2008; Ueno et al., 2011; Yan et al., 2012; Yi et al., 2015; Zheng et al., 2011). This is mostly related with the loss of work-hardening capability and the occurrence of early necking due to strain localization (SL) in nanostructures.

In recent years, some kinds of heterogeneous nanostructures, such as gradient nanostructure (Cheng et al., 2018; Fang et al., 2011; Hasan et al., 2019; Li et al., 2020b), heterogeneous lamella structure (Wu and Zhu, 2021; Wu et al., 2015), and multi-phase nanostructure (Li et al., 2020a; Zhang et al., 2021) have been developed to produce metallic materials evading the strength-ductility trade-offs. Experimental observations and theoretical analyses revealed that such heterogeneous nanostructures might promote the accumulation of geometrically necessary dislocations (GNDs), resulting in extra back stresses and therefore extra strain-hardening capability during deformation (Li et al., 2020a, 2017; Wang et al., 2019; Wu and Zhu, 2021; Wu et al., 2014; Zhao et al., 2020). Among them, spatially gradient nanostructure has attracted extensive attentions (Fang et al., 2011; Cheng et al., 2018; Wang et al., 2020; Pan et al., 2021,). Numerous works showed that some flexibility and low-cost surface plastic deformation approaches might be applied to produce gradient microstructures on various materials, including pure metals, steels, and other alloys (An et al., 2021; Cheng et al., 2018; Fang et al., 2011; Fu et al., 2022; Hasan et al., 2019; Lei et al., 2019; Li et al., 2020b; Pan et al., 2021; Wang et al., 2020). This offers unique opportunities to enhance properties of materials and solve engineering problems in industries (An et al., 2021; Chen et al., 2011; He et al., 2021; Ji et al., 2019; Li et al., 2020b; Long et al., 2019; Meng et al., 2017; Wang et al., 2020, 2019; Yin et al., 2020; Yuan and Branicio, 2020; Zhang et al., 2021). Inspired by these works, gradient nanostructure has been formed on austenitic stainless steels to enhance their strength-ductility synergies (Chen et al., 2011; Lei et al., 2021a; Sun et al., 2022; Tian et al., 2021; Wang et al., 2022; Wu et al., 2016). For example, a yield strength of ∼700 MPa with a uniform elongation of ∼40% was obtained in a gradient nanostructured (GNS) 304 stainless steel plate of 0.5 mm in thickness processed by surface mechanical attrition treatment (SMAT) (Wu et al., 2016). And the yield strength was further increased to ∼950 MPa via a modified SMAT process (Zhu et al., 2019). In comparison, the yield strength and uniform elongation are ∼268 MPa and ∼63%, respectively, in the coarse-grained counterpart. Furthermore, it is noticed that the corrosion resistance of GNS austenitic stainless steels might be simultaneously enhanced with the strength-ductility synergy (Gupta and Birbilis, 2015; Lei et al., 2021a).

As we know, in addition to the influences of thickness and microstructure/hardness distribution of the GNS layer in the as-prepared state (Cheng et al., 2018; Hasan et al., 2019; Lyu et al., 2017; Wang et al., 2019; Wu et al., 2014), the deformation-induced evolutions of strain distribution and microstructure in a GNS material also significantly contribute to its mechanical properties. Typically, a combined mechanism of deformation-induced martensite (DIM) transformation from metastable austenite, dislocation activities, microstructure refinement, and twinning dominates the plastic deformation and work-hardening behavior of austenite stainless steels (Chen et al., 2016, 2011; Murr et al., 1982). And their roles become more complex in GNS and nanostructured samples. For example, our previous works showed that GNS promoted the formation of DIM and stacking faults in 316 L stainless steel during cyclic deformation, bringing a significant increase in strain-controlled fatigue properties (Lei et al., 2019, 2021b). However, (Ueno et al., 2011) found that dislocation recovery and microcrack formation along twin boundaries occurred in nanostructured 316 L samples, resulting in cyclic softening and decreased fatigue life in strain-controlled fatigue tests. In GNS 304 stainless steel with high strength, (Wu et al., 2016) noticed that the DIM transformation was prolonged to high plastic strains during tensile tests, so that a remarkable work-hardening effect was spread over a larger strain range, and the ductility was enhanced. In addition, Chen et al., (Chen et al., 2016) found the DIM transformation was suppressed, while twinning and detwinning were active, in GNS 304 stainless steel samples consisting of submicro- and nano-twins, also resulting in a significantly enhanced strength-ductility synergy. Up to now, the underlying mechanisms of gradient nanostructure influencing plastic deformation and work-hardening behavior are still not clear in austenitic stainless steels. And a deeper understanding on these issues is expected to further enhance their mechanical properties.

In the present work, a bulk gradient nanostructure with a large layer-thickness and a high hardness-gradient was prepared in a 304 stainless steel plate by a newly developed surface plastic deformation approach. Subsequently, its mechanical properties and deformation mechanisms were investigated. Specifically, an ultra-high strength with considerable ductility was revealed in the GNS sample, and the correlations between the microstructure incompatibilities, local strain distributions, and microstructure/phase evolutions were systematically studied to clarify its deformation and work-hardening mechanisms.