Research ArticleCuZr-based bulk metallic glass and glass matrix composites fabricated by selective laser melting
Introduction
When a liquid is strongly supercooled, it solidifies either via crystallization or vitrification. The latter becomes possible when the melt is cooled comparably fast and crystallization is kinetically suppressed [1]. Depending on the exact cooling conditions, the supercooled liquid can also only partially crystallize whereas the remaining supercooled liquid congeals into a glass. The outcome are glass matrix composites [2].
Similar to polymeric or oxidic systems, also metallic alloys can be completely or partially quenched into a glass. Especially the synthesis of bulk metallic glass (BMG) matrix composites has been intensely investigated in the past decades, since they do not suffer from the intrinsic brittleness (in tension) of monolithic BMGs and largely retain their desirable high strength [3]. The lack of ductility in BMGs originates from a severe strain localization in so-called shear bands [4]; their fast propagation leads to catastrophic failure under unconstrained loading conditions [4]. It has been demonstrated that uniformly dispersed crystals interfere with the nucleation and propagation of shear bands and thereby often enhance plastic deformation [[4], [5], [6]]. Thus, certain BMG matrix composites, among which are mostly β-type Zr/Ti alloys, show an attractive combination of high strength and plasticity [7,8].
Other promising glass formers in this respect are derived from the alloy Cu50Zr50. Such CuZr-based alloys tend to precipitate the B2 CuZr phase on cooling [9]. The B2 CuZr crystals undergo a deformation-induced martensitic transformation, which reflects in a pronounced work-hardening capability [9]. Depending on the microstructure of such CuZr-based BMG matrix composites, the work hardening of the crystals overcompensates the work softening of the glass, leading to an improved overall deformation behaviour [10,11].
Unfortunately, the critical casting thickness of CuZr-based alloys is rather limited because the polymorphic crystallization process, which competes with vitrification, renders them relatively poor glass formers [9]. This is a major shortcoming, which obstructs most potential applications.
Selective laser melting (SLM), a powder-bed fusion technique, represents a processing route, which can overcome this issue [12]. Because only small powder volumes are consecutively melted by a laser, selective laser melting has relatively high inherent cooling rates [13], which typically suffices to process a wide variety of metallic glass formers. The obtainable sample size is now only limited by the dimensions of the building chamber [14], which significantly extends the potential of metallic glasses and related alloys to be used in applications. As a result, this approach has gained increased scientific attention and, to date, Fe-, Zr-, Ti- and Al-based glass-forming alloys have been successfully processed by SLM [12,[15], [16], [17], [18], [19], [20], [21], [22]].
Selective laser melting not only bears great potential for producing large BMG samples with complex geometries, but also BMG matrix composites can be synthesized in this way. This can be done by mixing glass-forming and (high-melting) conventional powder [23]. The great advantage is that through the respective volume fractions and particle sizes, the resulting composite microstructures can be tailored and the mechanical properties can be adjusted [24,25]. Another pathway to BMG matrix composites is to select the SLM parameters such that the glass-forming alloys partially crystallize during processing. The crystals usually precipitate in the heat-affected zone [18]. Yet, owing to the relatively complex thermal history, it is not straightforward to implement a high relative density with the optimal size and volume fraction of crystals in such in-situ BMG matrix composites [18,26].
In the present work, we choose a slightly different approach: a glass-forming Cu46Zr46Al8 (at.%) powder is processed by SLM and the influence of the processing parameters on porosity, phase formation, structural heterogeneity and mechanical properties is investigated. To extend the range of different thermal histories and to deliberately produce BMG matrix composites, a remelting step with a slightly higher energy input than for the original scan is implemented. All results are compared with those of as-cast Cu46Zr46Al8 rods. Remelting is known to improve chemical homogeneity (and with it glass formation) in Fe-based glass formers [27] and, additionally, to have a beneficial effect on the relative density of conventional alloys processed by SLM [28,29]. Therefore, it is an interesting extension of one-step selective laser melting and allows an even larger variation of microstructures and materials’ properties.
Section snippets
Pre-alloy and suction casting
Cu46Zr46Al8 (at.%) ingots were prepared by arc melting (Edmund Bühler GmbH) in a Ti-gettered argon atmosphere using high-purity elements (purity at least 99.9 %). Each ingot was remelted at least three times to ensure chemical homogeneity. After alloying, the ingots were cracked into pieces with a weight of about three grams to produce cylindrical samples. Cu46Zr46Al8 rods with a length of 35 mm and a diameter of 3 mm were prepared in an arc-melting device equipped with a suction casting
Powder properties and porosity in the SLM samples
As shown in Table 1, the actual composition of the gas-atomized powder only slightly deviates from the nominal composition of Cu46Zr46Al8. Selective laser melting does not have a significant effect on the chemical composition. The oxygen content of the initial powder is 332 ± 11 ppm and increases to 491 ± 15 ppm after selective laser melting while it remains virtually constant after remelting (493 ± 17 ppm).
The characteristics of the powder such as particle size distribution and morphology
Discussion
During selective laser melting, there is an apparent uptake of oxygen, which indicates the alloy’s affinity to the residual oxygen in the building chamber [43,44]. This is also found in other additively manufactured glass formers [16] and affects the phase formation as well as the mechanical properties, as we discuss below.
The first aspects we want to tackle here are the relative density and the characteristics of porosity. Flowability of the powder is critical for the quality of samples
Conclusions
Fully amorphous Cu46Zr46Al8 bulk samples and glass matrix composites with relative densities exceeding 98 % were successfully fabricated via selective laser melting (SLM). By tuning the SLM parameters, B2 CuZr nanocrystals with diameters between 30 nm and 100 nm precipitate in the glass. Their formation is not confined to the heat-affected zone but instead occurs in a rather uniform manner in the glassy matrix. The crystalline volume fraction can be altered by adjusting the processing
Acknowledgements
The authors are grateful to P. Wang, P. Xue and H.Y. Chen for insightful discussion. Moreover, valuable technical support by A. Voß, H. Bußkamp, B. Bartusch, S. Donath and H. Merker is appreciated. L. Deng acknowledges financial support by the Chinese Scholarship Council (CSC). S. Pauly and K. Kosiba acknowledge support by German Research Foundation (DFG) (Nos. PA 2275/4-1, PA 2275/6-1 and KO5771/1-1). L. Zhang acknowledges support by the National Natural Science Foundation of China (Nos.
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