Oxidation of amorphous HfNbTaTiZr high entropy alloy thin films prepared by DC magnetron sputtering

Highlights

• Amorphous HfNbTaTiZr high entropy alloy thin films were deposited by DC magnetron sputtering.

• A fine structure of round-shaped amorphous clusters ∼10 nm was observed.

• Oxidation at ambient atmosphere leads to a preferential formation of Ti-, Hf- and Zr- oxide nanoclusters.

• Oxidation takes place predominantly in large open volumes in between cluster aggregates.

Abstract

High entropy alloys represent a new type of materials with a unique combination of physical properties originating from the occurrence of single-phase solid solutions of numerous elements. The preparation of nanostructured or amorphous structure in a form of thin films promises increased effective surface and high intergranular diffusion of elements as well as a high affinity to oxidation. In this work, we studied HfNbTaTiZr thin films, deposited at room temperature by DC magnetron sputtering from a single bcc phase target. Films exhibit cellular structure (∼100 nm) with fine substructure (∼10 nm) made of round-shape amorphous clusters. The composition is close to equimolar with slight Ti enrichment and without any mutual segregation of elements. Oxidation at the ambient atmosphere leads to the formation of Ti, Zr, Nb, Hf, and Ta oxide clusters in the film up to the depth of 200–350 nm out of the total film thickness of 1650 nm. Oxygen absorption takes place preferentially in the large vacancy clusters located in between the amorphous cluster aggregates. The dominant type of defect are small open volumes with a size comparable with vacancy. The distribution of these defects is uniform with depth and is not influenced by the presence of oxygen in the film.

Introduction

High entropy alloys (HEA) called also complex concentrated alloys or multi-principal element alloys [1] are a group of materials with a combination of interesting properties [[2], [3], [4]]. While most conventional alloys are based on one principal matrix element with alloying elements of lower concentration, these alloys are a mixture of at least four elements in a similar atomic ratio. The high configurational entropy of this system causes the formation of a single solid solution phase at high temperatures [5]. A high number of elements promises a combination of interesting phases [6,7] and intriguing deformation behavior [8] when annealed. Therefore, new combinations of physical properties are expected, namely mechanical [2,3], oxidation [9], irradiation [10], and wear resistance [11].

HfNbTaTiZr alloy belongs to a group of refractory materials with a high melting point and thermal stability [9] with enhanced ductility and strength [12]. The casting of this alloy results in dendritic structure and subsequent homogenization annealing produces large grains and the alloy is susceptible to embrittlement from absorbed atmosphere impurities [13]. Moreover, local variations of the lattice parameter lead to the distribution of sizes of interstitials [1] forming an open structure favorable for hydrogen absorption. Interaction of refractory high entropy alloys with hydrogen has been widely studied [14,15].

For hydrogen absorption in metals, a large surface area together with small grains microstructure is advantageous for diffusion along grain boundaries into the material [[16], [17], [18]]. A nanocrystalline microstructure can be achieved by annealing of the amorphous phase. Amorphous HEA, often called metallic glasses (MG) have strong topological and chemical disorder and may have special properties different from bulk MG or HEA [4]. They can be prepared either by a high quenching rate of liquid phase [19] or by various deposition techniques [20]. It was found that the composition driven effects in sputtered thin films are decisive about the amorphization of the alloys, and the more complex is the composition of the alloy, the higher is the occurrence of amorphous content [21,22]. A mechanism of phase selection in high entropy alloys between solid solution and amorphous phase was proposed based on atomic size dispersion and mixing enthalpy [19]. Therefore, it is interesting to prepare an amorphous phase from an alloy typical for single solution formation. Moreover, the hardness of such an amorphous thin film is much higher than bulk material [23]. Oxidation of amorphous or nanocrystalline film may block effective hydrogen absorption in the film though.

Two HfNbTaTiZr thin films with different thicknesses were prepared by DC magnetron sputtering. The deposition at room temperature is favorable for the formation of a nanocrystalline to an amorphous structure [24]. The effect of various deposition conditions (deposition rate, substrate temperature) and film composition to the structure of thin films prepared by magnetron sputtering was studied e.g. in Refs. [21,25]. Oxidation of HEA films can be well studied by X-ray photoelectron spectroscopy combined with depth profiling [26]. Positron annihilation lifetime spectroscopy is a versatile tool providing information about open volumes, important for oxidation and hydrogen absorption properties, in both crystalline and amorphous structures.