Research PaperHeat treatment of laser powder-bed-fused Co–28Cr–6Mo alloy to remove its microstructural instability by massive FCC→HCP transformation
Introduction
After a decade of intense development, additive manufacturing (AM) is nowadays capable of producing sophisticated parts not only from plastic materials but also from metallic materials. By technologies such as laser powder bed fusion (L-PBF) or electron beam powder bed fusion (E-PBF), also known as selective laser melting (SLM) and electron beam melting (EBM), respectively, and many others, a wide range of metals and alloys can be processed with high quality [1]. Among them, the most attention is paid to materials intended for demanding applications in the medicine or aerospace industry, such as Ni-based superalloys (Inconel-718), Co–Cr alloys (Co–28Cr–6Mo), steels (maraging 300, 316 L) or Ti-based alloys (Ti–6Al–4V) [2]. Despite numerous benefits that the AM of metallic materials provides, there are specifics which require special attention of material researchers.
From a vast number of studies on different additively manufactured (AMed) metals and alloys, it is obvious that characteristic microstructures are yielded by AM [2]. Due to high cooling rates reaching up to 108 K/s, highly oversaturated solid solutions are generally formed, with a minimum of secondary phases being precipitated (if so, very fine particles are formed). At such rates, most materials show cellular solidification mechanism (Al–12Si, Al–10Si–Mg, 316 L austenitic stainless steel, Co–28Cr–6Mo) resulting in very fine cellular microstructures formed by cells of a solid solution and cellular boundaries enriched in alloying elements (either in the form of specific phases or as atomic segregation) [3]. However, there are some materials which do not behave as binary systems (the solute/s is/are not ejected from the solvent; solubility is not negligible at solidification temperature), for which the cellular solidification does not occur. For example, in Ti–6Al–4V, the microstructure is formed by a diffusionless (martensitic) transformation of the high-temperature solid solution β mediated by atomic shear of a cubic crystal lattice into a hexagonal one. Metastable phases are often present in AMed microstructures at room temperature. Furthermore, a directional heat dissipation results in directional solidification; most materials thus show columnar grains, spread either across the thickness of a layer or multiple layers [4].
Owing to the beneficial effects (highly oversaturated solid solutions, nano- to micro-sized secondary phases forming cell boundaries, or metastable phases) of AMed microstructures, AMed materials are often highlighted for enhanced mechanical performance [5]. However, not many studies draw attention to the instability that these effects might cause when exposing the materials to elevated temperatures. There might be different causes for such instability. Generally, cellular microstructures are unstable and tend to decompose during annealing [3]. However, there were studies [6], [7], [8] showing that some AMed materials are unstable at even slightly elevated temperatures (even below 373 K for Al alloys) at which the cellular microstructure is preserved. Such instability stems from a high supersaturation of solid solutions forming cell cores. With even little energy given to the material, solid solutions tend to reach their stable composition by expelling solutes in excess. For example, hypo-eutectic Al–Si alloys were shown to precipitate Si in the form of Si needles, or Cu/Mg in the form of well-known precipitation-strengthening phases (Al2Cu, Mg2Si) at temperatures around 373 K, but even lower [6], [7]. For other materials, the instability is associated with the presence of metastable high-temperature phases at room temperature. For example, the martensitic microstructure of Ti–6Al–4V alloy decomposes and transforms at temperatures at which the conventional two-phase microstructure is otherwise stable [8], [9]. For stainless steel 304 L, Amine and Newkirk [10] showed that microstructure and mechanical properties were dramatically affected at a temperature (573 K) much lower than expected when compared to samples of wrought steel. Gradual hardness decrease with aging time might be associated with the decomposition of metastable δ-ferrite retained in the laser powder bed fused (L-PBF) microstructure.
This study focuses on an AMed Co–28Cr–6Mo alloy (prepared by L-PBF technology) in which both these instability factors take their role. In our previous study [11], we have shown that the instability is manifested by gradual material hardening with increasing temperature. The onset of the instability was observed already at 673 K. The cellular microstructure did not decompose before exceeding 1073 K. Between 673 and 1073 K, the hardening was caused only by transforming the metastable FCC phase into the stable HCP phase. Above 1073 K, the second factor joined – precipitation strengthening induced by σ-phase precipitates formed after the disintegration of the Mo-enriched intercellular network. In the present study, we demonstrate the effect of such microstructural changes on changes in mechanical properties.
With respect to the application of this Co–Cr alloy for components operating at elevated temperatures [12], inducing material stability is desirable prior to its commissioning. Considering the above-stated changes in the material microstructure observed during the exposition to elevated temperatures in its as-built state, two conditions should be fulfilled to provide its stability: (i) fully transformed HCP matrix, (ii) microstructure homogenization (elimination of intercellular segregation). Hence, the solution is a suitable heat treatment.
There have been several studies dealing with the heat treatment of L-PBF Co–28Cr–6Mo alloy [13], [14], [15], [16], [17], [18] but none of them focused closer on its effect on material stability. To achieve a complete material homogenization, Kajima et al. [13] and Takaichi et al. [14] reported annealing at 1423 K to be necessary; lower temperatures were insufficient. Such annealing fully relieves residual stresses, yields recrystallization, and provides the material with satisfactory ductility. However, the FCC phase dominates the microstructure. To increase the HCP fraction, an aging step should be included after the solution annealing followed by quenching. Béreš et al. [19] carried out a solution treatment at 1423 K/1 h followed by water quenching and artificial aging at 1073 K/4 h. Except for homogenization, the fraction of the FCC phase was significantly reduced by the formation of the finely distributed HCP phase in the {111} FCC planes. Zhang et al. [17] applied a higher aging temperature for a longer duration (1173 K/10 h) by which they achieved an almost fully HCP matrix with M23C6 precipitates. As isothermal transformation is known to lead to a localized stress concentration at HCP/FCC interfaces, a decrease in ductility has to be anticipated for this type of heat treatment [19]. A different approach was chosen by Li et al. [16] who applied direct multiple-step aging (723 K/45 min + 1023 K/60 min + 723 K/100 min) to improve the scratch resistance. Such treatment preserved the cellular microstructure while increasing the HCP phase fraction (however, only 10% of HCP are reported).
In the current paper built on our previous work [11], we present two types of heat treatment successful in inducing the stability of the studied L-PBF Co–28Cr–6Mo alloy at elevated temperatures; the first being a solution annealing followed by aging and the second, direct aging without the solution annealing step (isothermal annealing), for which a detail microstructural characterization was presented in the previous paper. For both of these heat treatment regimes, we demonstrate their impact on the mechanical properties of the material.
Section snippets
Material
The current work builds on the previous work presented in Ref. [11]. The studied material is a low-C Co–28Cr–6Mo alloy (ASTM F75), with the exact composition given in Table 1, prepared by laser powder bed fusion (L-PBF). Samples in the form of round tensile testing bodies conforming to the DIN 50125 standard (Fig. 1a) were built vertically in an SLM Solutions 280 HL machine. More details on specific process parameters can be found in Ref. [11]. For this part of the experimental work, samples
Microstructure after heat treatment
The microstructure after heat treatment is shown in Fig. 3, Fig. 4 in comparison with the as-built state in Fig. 2. In all these Figures, the microstructures are shown on a metallographic section parallel with the deposited layers, perpendicular to the building direction. Solution treatment (Fig. 3) led to a complete disappearance of macro- and microstructural characteristics of the L-PBF as-built state – line laser melting tracks (Fig. 2a,b) and cellular grain substructure (Fig. 2c,d) formed
Discussion
The exposition of both types of heat-treated specimens to elevated temperature manifested that both treatments were successful in inducing material stability. The results of the tensile tests and Vickers hardness measurement revealed that both heat treatments increased material strength but at the expense of ductility. Inducing material stability as well as variations in the mechanical response of the material are closely related to the microstructural changes induced by the relevant heat
Conclusion
The exposition of the L-PBF Co–28Cr–6Mo alloy in its as-built state to the elevated temperature of 1173 K (HVmax state) has been shown to yield significant precipitation strengthening, with a 53% increase in TYS. As shown in detail in our previous study [11], the original cellular microstructure formed by dual-phase (72%FCC-28%HCP) Co-matrix and Mo segregations at dislocation cell walls disappeared. Instead, uniformly distributed precipitates of σ-phase were formed in the matrix which was
CRediT authorship contribution statement
Michaela Roudnická: Conceptualization, Investigation, Data curation, Original draft preparation, Writing – Original Draft, Review & Editing. Jiří Kubásek: Investigation – EBSD. Libor Pantělějev: Investigation – tensile testing. Orsolya Molnárová: Investigation – TEM. Jiří Bigas: Investigation – heat treatment. Jan Drahokoupil: Investigation – XRD. David Paloušek: Resources, Project administration, Funding acquisition. Dalibor Vojtěch: Supervision, Project administration, Funding acquisition.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
Authors M. R., J. B. and D.V. wish to thank the Ministry of Education, Youth and Sports, Czech Republic (MEYS CR), for the financial support of this research (Specific university research - grants nos. A2_FCHT_2020_026 and A1_FCHT_2020_003). M. R. and O. M. acknowledge the CzechNanoLab Research Infrastructure supported by MEYS CR (project LM2018110). The work of J. D. was supported by Operational Programme Research, Development and Education financed by European Structural and Investment Funds,
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