On deformation behavior of Fe-Mn based structural alloys

Abstract

This article is a literature review of recent advances in novel Fe-Mn based alloys, drawing specific examples on Hadfield and TWIP steels, and high entropy and shape memory alloys. A critical discussion of these alloys merits a uniform treatment owing to their remarkable mechanical attributes, i.e. a substantial combination of strength and ductility. It is interesting to note that their microscopic deformation mechanism also shares commonality in terms of twinning-dominated straining in addition to slip-based plasticity. In the past two decades alone, a synergy of materials science and mechanics based studies has significantly helped uncover the microstructural origin, effects of alloying etc. therein. In this paper, we discourse elaborately on the reported microstructure-property correlations. By surveying these developments, we point out how the current knowledgebase can pave the way for strategizing future materials advancements as well as expanding applications.

Introduction

Fe-Mn based alloys constitute several important classes of engineering materials with remarkable mechanical characteristics, which make them attractive in numerous industrial applications today (Fig. 1). Hadfield steels, which exhibit an unusually high work hardening rate under loading, are still being used in heavy industrial applications, such as caterpillar tracks, railroads, mining equipment, and crusher jaws, where high strength and wear resistance are necessary [1], [2]. Twinning-induced plasticity (TWIP) steels find their applications mostly in the automotive industry, for example as the chassis, due to an excellent blend of high strength and lightweight [3], [4], [5]. The utilization of Fe-Mn based shape memory alloys (SMAs) are being actively investigated as seismic dampers [6]. Lastly, although not yet actively employed in industrial settings, high entropy alloys (HEA) hold considerable promise in cryogenic applications (e.g. in space-bound vehicles with liquefied fuel) due to their superior toughening attributes [7], [8], [9] at low temperatures. Given the diversified applications and potential of this class of materials, considerable research has been conducted to establish their structure-property correlations. The strength and ductility of these four classes of materials are compared with other structural alloys in Fig. 2. In this paper, we present an overview of the current knowledge base.

Discovered in 1925 by Sir Robert Hadfield [10], Hadfield steels possess austenitic (fcc) lattice structure with high Mn content. The high strain hardening, for which these steels find widespread industrial usage, has been attributed to interaction of dislocation slip with carbon interstitial and twinning systems [11], [12], [13], [14], [15], [16]. These alloys present themselves as interesting case studies due to their propensity for increased strengthening attributes. This is primarily a direct consequence of alloying, which leads to considerably low stacking fault energy, an attribute very conducive to twinning [15]. From research perspective, considerable amount of works are recorded in the literature on the single and polycrystalline deformation mechanism, effects of alloying with nitrogen, aluminum etc. [17], [18], [19], [20], [21].

Specifically, extensive microstructure characterizations have been performed to establish a consistent picture of the deformation mechanism underlying the superior hardening effects in these materials [2], [18], [19], [20], [22], [23]. For instance, the details of twin nucleation, formation of dislocation walls, interactions between these defects, as well as solute hardening and texture effects are examined. These revelations have benefitted significantly from modern characterization techniques, such as digital image correlation (DIC), electron microscopy, and electron backscatter diffraction (EBSD). In this paper, we discuss the key findings on the empirical mechanical behaviors of Hadfield single- and poly-crystal materials along with the microstructure characterizations. A brief case study of constitutive modeling will also be presented to exemplify the ongoing research.

TWIP steels are also austenitic steels having fcc crystal structure with a Mn content upwards of 20 wt% [24]. Similar to Hadfield steels, they possess low stacking fault energy (20–40 mJm⁻²), which gives rise to prevalent twining-based mechanism during plastic deformation [25], [26], [27]. Consequently, a greater degree of slip obstruction is encountered due to a reduction of their mean free path for free gliding, which results in the superior flow strength [28], [29], [30]. Extensive studies have been directed at understanding, for example, the role of grain size/orientation, alloying, dislocation substructure evolution, promotion/suppression of twinning [3], [4], [31], [32], [33], [34], [35], [36], [37], [38], [39]. It was established that the gradual evolution of dislocation structure eventually leads to the deformation twinning at more advanced stages of deformation. Numerous studies confirmed these microscopic trends. In this paper, we cover the experimental hardening response of TWIP steels from representative works, and the associated microstructural changes revealed through techniques such as EBSD, electron microscopy.

Shape memory alloys are known for their remarkable strain recovering ability by mere removal of load and/or by heating [40]. Fe-Mn based SMAs are particularly well-known due to the low costs of the constituent chemical species such as Fe, Mn, Si, Al, Cr. The main utility of Fe-Mn based SMAs alloys is related with engineering applications suitable near room temperature [41], [42]. The microscopic deformation mechanism of strain recovery in these alloys is characterized by reversible fcc-to-hcp or bcc-to-fcc transformations [43], [44], [45], [46], [47] (fcc, bcc, hcp being face-centered cubic, body-centered cubic and hexagonal close-packed). Most SMAs (i.e. Ni-Ti and Cu based ones) have ordered lattice structure, and the shape recovery is dictated by two-way martensitic transformation. By contrast, the Fe-Mn SMAs are characterized by disordered lattice and a dislocation-assisted transformation mechanism. Since the microscopic mechanism is at the heart of the manifestation of global properties, a study of these materials in a single volume can provide important insight into how to further engineer the mechanistic characteristics to develop superior alloys. For instance, the mesoscopic factors that control the dislocation motion would also play a significant role in the shape recovering attributes such as the obstruction by grain boundaries, non-shearable precipitates and Cottrell atmosphere [48]. Drawing on specific case studies, we discuss the experimental constitutive characteristics of these alloys as well as their deformation mechanism(s).

Discovery of high entropy alloys is a very recent development [8], [9], [49], [50], [51], [52]. What is unique of this class of alloys is that, unlike conventional alloys where elements are added to a base metal, HEAs are fabricated by mixing constituents in (nearly) equiatomic proportion. In this article, we cover the characteristics of equiatomic CoCrFeMnNi high entropy alloys (HEA), which have found recent interest due to their exceptional mechanical strength and ductility [8], [53]. HEAs are single phase solid solutions with (near) equiatomic composition typically comprising five transition metals (or upwards) [7], [9]. These alloys are characterized by high entropy of mixing, which in turn enhances solubility of individual chemical species. The properties of a HEA are typically considerably different from its constituent elements. Despite multiple types of solutes, each element has the same propensity to occupy a certain lattice site. Due to volumetric mismatch among different solute atoms from different constituent species, the lattice structure can be highly distorted [54]. The immediate effects of such distortion has been associated with high strengthening attributes [55], [56]. In addition, due to a slow diffusion compared to conventional metals and alloys, the propensity of microscopic precipitate formation is rather high. One unique characteristic of HEAs is that a dramatic change in mechanical properties can be achieved by tweaking the alloying contents [57], [58], [59].

From research standpoint, several outstanding issues are pinpointed. For instance, unlike binary or ternary alloys, no precise phase diagram has been uncovered to guide the design of multi-element HEA towards optimum properties. Another issue remains in the form of alloying-induced variation in the stacking fault energy, which in turn would largely control the mechanism of plastic deformation (e.g. slip-mediated versus twinning-dominated flow). A clear correlation between the composition and the fault energetics is necessary to accomplish desired plastic attributes. From current literature, the HEAs with bcc structure possess high strength with low ductility, while fcc HEAs have low strength with considerable elongation. This trend poses a possible strategy for the overall toughness improvement by mixing fcc and bcc types [60] by manipulating fabrication variables.