Columnar to equiaxed transition and grain refinement of cast CrCoNi medium-entropy alloy by microalloying with titanium and carbon
Introduction
The CrMnFeCoNi high-entropy alloy (HEA) and its medium-entropy subsystems with the face-centered cubic (FCC) structure (also referred to as multi-principal-element alloys in the literature) have attracted extensive attention during the past decade and a half (e.g. Refs. [[1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25], [26], [27], [28]]). Among these alloys, the CrCoNi medium-entropy alloy (MEA) exhibits the best strength-ductility/toughness combination at cryogenic temperatures [7,14,19,29]. It can therefore serve as a base for the development of promising engineering alloys in the future.
For nearly all engineering alloys, casting is an important step that enables the production of complex-shape components. Typically, cast alloys exhibit coarse and anisotropic solidification microstructures with grains aligned along the heat-flow direction. This produces bulky columnar structures outside and a relatively small region of coarse equiaxed grains in the center. The coarseness of the microstructure increases as the solidification/cooling rate decreases (i.e., as the size of the casting increases). Coarse columnar microstructures form also in cast HEAs [30] and are generally undesirable from a mechanical properties standpoint. Cast microstructures can be broken down by deformation processing and recrystallization after which the tensile properties of at least some HEAs are rather good, as was shown for the first time with CrFeCoNi and CrMnFeCoNi [2,3]. Grain refinement in HEAs results in classical Hall-Petch strengthening [2,5,31] as well as simultaneous increases in strength and ductility [32,33].
While wrought alloys can benefit from grain refinement during processing, cast alloys offer limited opportunities to manipulate the grain morphology and refine the grain size after the castings are made (especially if they have complex shapes). Consequently, the as-cast grain size, to a large extent, determines the final properties. It is important, therefore, to refine the as-cast grain size, not only to improve mechanical properties but also to decrease the tendency for hot tearing and to have a more dispersed and refined porosity distribution after solidification. The columnar to equiaxed transition (CET) and grain refinement of cast high- and medium-entropy alloys have received little attention so far [34]. Consequently, the methods and mechanisms that can be deployed in such materials to alter their as-cast grain size/shape remain unclear.
There are several methods for refining the as-cast microstructures such as inoculation/heterogeneous nucleation, rapid solidification, application of external fields (e.g. mechanical, electromagnetic, ultrasonic and stirring (e.g. Refs. [[35], [36], [37]]). Some of these are difficult to implement in the generally used laboratory-scale processes, which typically involve induction/arc melting and drop casting in a closed vacuum furnace. In these cases, alternative approaches using intrinsic grain refinement methods are needed. Heterogeneous nucleation has long been known to be a powerful method for grain refinement and has been used in foundries for years. Another method involves the use of certain solutes that segregate during solidification and has been shown to be an effective way to reduce grain sizes in various alloys [[38], [39], [40], [41], [42]]. In the latter case, the notion is that growth-restricting solutes induce constitutional undercooling, the extent of which depends on the specific species and amounts, and the undercooling promotes grain refinement. Here, we selected the CrCoNi alloy as a model MEA to investigate the CET and grain refinement via trace additions of Ti and C. Our results show that it is indeed possible to achieve significant grain refinement by the addition of optimum levels of Ti and C and the resulting fine-grained MEA has superior tensile properties.
Section snippets
Experimental
Alloys with compositions (CrCoNi)100-2xTixCx in at.%, where x = 0–1 (denoted as TixCx MEAs hereafter) were prepared by arc melting a mixture of the pure metals Cr, Co, Ni and Ti (purity >99.9 wt%) and Cr3C2 powder (purity >99 wt%) in a high-purity argon atmosphere. After the raw materials were loaded in the arc melter, the chamber was first evacuated to ∼5 × 10−2 Pa and then backfilled with pure Ar. This process was repeated twice. After the final backfill, the Ar pressure in the chamber was
Results and discussion
Fig. 1a is a schematic diagram showing a cross-section located 30 mm from the bottom of the casting where microstructural examination was performed. On this section, microstructures were examined both close to the mold-ingot interface [red square labeled (b)] and near the center of the cast ingot [blue square labeled (c)]. EBSD images of as-cast TixCx MEAs taken at these locations are shown in Fig. 1 b and c, respectively. Close to the mold (Fig. 1b), the base CrCoNi MEA exhibits anisotropic
Conclusions
- (1)
Microalloying additions of Ti and C induce a columnar to equiaxed transition in the shape of the as-cast grains of the CrCoNi medium-entropy alloy. The alloy with composition (CrCoNi)99.2Ti0.4C0.4 shows uniform equiaxed grains with a relatively fine grain size of ∼75 μm. In contrast, the base CrCoNi alloy without Ti and C has long columnar grains with average lengths of ∼1000 μm and widths of ∼120 μm.
- (2)
The yield strength, ultimate tensile strength and elongation to fracture of the (CrCoNi)99.2Ti
Acknowledgements
Research sponsored by the U.S. Department of Energy, Office of Science, Basic Energy Sciences, Materials Sciences and Engineering Division, E.P.G., and the China Scholarship Council (201506165006), X.W.L. G.L. and J.P.M. acknowledge funding from the German Research Foundation (DFG) through project B5 and B7 of the SFB/TR 103.
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