Microstructure and properties of the in situ formed amorphous-crystalline composites in the Fe–Cu-based immiscible alloys
Research highlights
► Amorphous-crystalline microstructure of systems with a liquid miscibility gap. ► Fractal-like microstructure of the ribbons melt-spun from a homogeneous melt region. ► Decrease of volume fraction of precipitates with increasing melt ejection temperature.
Introduction
Amorphous metallic alloys offer attractive mechanical and physical properties due to lack of lattice defects, mainly dislocations. Metallic glasses, formed upon quenching of the melt at cooling rates high enough to hinder crystallization process, are easily obtained in a form of melt-spun ribbons or rods. Despite high strength of metallic glasses its application is still very limited because of low plasticity caused by highly localized shear banding [1], [2]. In order to improve ductility of these materials, a composite microstructure, consisting of the crystalline phase dispersed in the amorphous matrix, is desired. Such microstructure can be obtained by in situ formation of a crystalline phase during cooling of the melt [3], [4], [5], [6], [7]. Final properties of composites are determined by morphology, volume fraction and distribution of reinforcing particles [8]. Based on the crack mechanics of polycrystalline materials, it is expected that spherical shape of precipitates would be desired. However during heterogeneous nucleation from a liquid state, precipitates tend to adopt dendrite shape.
Formation of spherical precipitates is possible in alloys with a liquid miscibility gap [9], [10]. Such alloys, which are characterized by a positive heat of mixing between two major elements, decompose into two melts of different compositions. Because of lack of strain energy in the liquid state and relatively low surface energy between two melt, precipitates tend to adopt an ideal spherical shape. For sufficient glass forming ability (GFA) of an alloy, crystallization of both melts should be restrained and two-phase metallic glass could be formed. However positive heat of mixing between two basic elements, required for liquid/liquid phase separation, decreases GFA of alloys. Therefore two-phase metallic glasses reported so far were mainly formed by a melt spinning process [11], [12], [13], [14], [15]. In order to receive amorphous-crystalline composite structure, further vitrification of the amorphous phase with lower thermal stability is necessary.
Two coexisting melts have different chemical compositions and glass forming abilities. If GFA of one of the melt will be not sufficient, the amorphous-crystalline composite microstructure can be obtained during cooling [16], [17], [18], [19]. This paper shows the microstructures and mechanical properties of the melt-spun Fe30Cu32Si13B9Al8Ni6Y2 and Fe44Cu18Si13B9Al8Ni6Y2 alloys. Two basic elements, i.e. iron and copper, exhibit positive heat of mixing as high as +13 kJ/mol [20], but the Fe–Cu phase diagram does not contain a liquid miscibility gap [21]. However Turchanin et al. [22] proved that undercooling of the homogeneous melt leads to decomposition into the Fe-rich and the Cu-rich melts. Selection of the alloying elements is explained in detail in Ref. [19].
In order to evaluate mechanical properties of the melt-spun ribbons, nanoindentation studies were carried out. It should be noted however that hardness of thin ribbons should not be compared with results obtained for bulk materials.
Section snippets
Experimental
Two alloys, Fe30Cu32 and Fe44Cu18, with nominal composition of Fe32Cu32Si13B9Al8Ni6Y2 and Fe44Cu18Si13B9Al8Ni6Y2 (at.%), respectively, were prepared by arc melting of a mixture of high purity elements (99.9% or higher) under titanium gettered argon protective atmosphere. Rapidly solidified ribbons were prepared by single roller melt spinning facility (Edmund Bühler melt spinner HV) equipped with infrared radiation pyrometer under an argon atmosphere at a linear wheel speed of 40 m/s and a
Results
The microstructures of the melt-spun Fe30Cu32Si13B9Al8Ni6Y2 (Fe30Cu32) alloy, observed on the cross sections, are presented in Fig. 1, Fig. 2. Coarse-segregated, non-uniform areas observed in the microstructure of ribbons melt-spun from 1230 and 1330 °C (Fig. 1d and e), indicate that the two melts, the Fe- and the Cu-rich, coexisted in the crucible prior to rapid cooling. On the other hand microstructures of the ribbons melt-spun from 1400, 1450 and 1520 °C (Fig. 1a–c) are composed of bright
Discussion
LM microstructures (Fig. 1, Fig. 4) indicate at the importance of the melt-ejection temperature for alloys with the liquid miscibility gap. Based on microscopic observations of the melt-spun ribbons it is concluded that the critical temperatures for attaining homogeneous melt in both alloys are located between 1330 and 1400 °C. In order to obtain relatively uniform microstructure with spherical precipitates distributed within the matrix, alloys should be heated above the miscibility gap. When a
Conclusions
Rapid cooling made possible in situ formation of the amorphous-crystalline composites with a fractal-like microstructure. Microstructural observations of the Fe30Cu32 and Fe44Cu18 alloys revealed liquid/liquid phase separation into the Fe-rich and the Cu-rich melts. Ribbons should be cooled from a homogeneous melt temperature region in order to obtain microstructure composed of the spherical precipitates distributed within a matrix. Both melts may constitute either a matrix or precipitates,
Acknowledgments
The study was supported by Polish Ministry of Science and Higher Education under project No. 11.11.110.790.
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