Impact of beryllium additions on thermal and mechanical properties of conventionally solidified and melt-spun Al–4.5 wt.%Mn–x wt.%Be (x = 0, 1, 3, 5) alloys

Highlights

• Thermal and mechanical properties of Al–Mn–Be alloys were investigated.

• IQC Al–Mn–Be alloys were synthesized by the CS and MS techniques.

• The volume fraction of IQC increases continuously with Be content.

• The melting points of the QC i-phase were determined between 652 °C and 675 °C.

• The maximum H_V and σ values were found to be 124 kg/mm² and 458 MPa with the addition of 5% Be.

Abstract

The influence of beryllium (Be) addition on the quasicrystal-forming ability, thermal and mechanical properties of Al–4.5 wt.%Mn–x wt.%Be (x = 0, 1, 3, 5) alloys was investigated in this study. Quasicrystalline Al–Mn–Be alloys were synthesized by the conventionally casting and melt spinning techniques. The microstructures of the samples were characterized by scanning electron microscopy (SEM) and the phase composition was identified by X-ray diffractometry (XRD). The phase transition during the solidification process was studied by differential scanning calorimetry (DSC) and differential thermal analysis (DTA) under an Ar atmosphere. The mechanical properties of the conventionally solidified (CS) and melt-spun (MS) samples were measured by a Vickers micro-hardness indenter and tensile-strength tests. The Al–4.5 wt.%Mn alloy has a hexagonal structure and minor dendritic icosahedral quasicrystalline phase (IQC) precipitates surrounded by an α-Al matrix. Addition of Be into the Al–4.5 wt.%Mn alloy generates intermetallic Be₄AlMn and IQC phases with the extinction of the hexagonal phase, and the fraction of IQC increases continuously with the increase in Be content. A considerable improvement in microhardness and tensile strength values was observed due to the addition of Be in different percentages into the composition.

Introduction

For the development of structural materials it is of great importance that they be produced with minimal processing cost and that properties such as elevated temperature strength, density and stiffness are improved. Achievement of these properties can lead to weight reductions resulting in extended lifetime, fuel reduction, etc. The need for these improvements is a major drive for process and alloy development in structural aluminum alloys [1]. One way of achieving this objective is the use of rapid solidification (RS) which is an attractive method for the fabrication of metallic alloys with unique properties for numerous industrial applications. Microstructure refinement, solid solubility extension, and chemical homogeneity of the products are some of the advantages of the RS process [2]. Furthermore, it is possible to produce metastable materials such as quasi-crystals, nano-crystals and amorphous alloys by cooling metallic melts at cooling rates exceeding 10⁴ K s⁻¹ [3], [4]. Since the first discovery of an icosahedral quasicrystalline phase (IQC) in a rapidly solidified Al–Mn system by Shechtman et al. [5], there have been numerous works on the development of high-strength Al alloys containing the quasicrystalline (QC) phase as an effective reinforcement [6], [7]. Due to their unique aperiodic long-range atomic order, quasicrystals (QCs) exhibit a broad variety of physical, chemical, mechanical and tribological properties that are unusual for metallic systems [8], [9]. Quasicrystal-forming alloys consequently have high potential for applications like high temperature thermal barriers, low-friction and wear resistant coatings [8], [9], [10], composite biomaterials [11] or catalysts [12]. The possibility to employ QC phases in alloys for the automotive industry and studies of QC particles’ interactions with an aluminum-based matrix have also been investigated. It is obvious that the formation of QC phases and their potential for applications are currently very interesting topics [13]. Until now, hundreds of alloys have since been observed to form QC phases; however, only a few systems formed stable quasicrystalline phases, and the majority of them consisted of Al-based alloys. Among them, Al–Mn–Be alloys are of great importance because the QC phase in the ternary Al–Mn–Be alloys has a primitive IQC structure and has a significantly lower critical cooling rate and Be was found to greatly increase the QC forming ability in well-known Al–Mn QC alloys [14], [15], [16], [17]. In addition to Be, as the important alloying element, some other elements such as Pd [18], Mn, Ce [19], [20], and Sr [21] have been added to aluminum as the ternary alloying elements due to their influence on the formation of IQC phase improving mechanical properties at room and elevated temperatures. A number of studies have been carried out to examine the effect of Be on Al–Mn alloys, and most of them employed conventionally solidified or rapidly solidified Al–Mn–Be alloys. The microstructure and impression creep behavior of Be-containing Al–Mn alloy were studied by Song et al. [14] and Kim et al. [22]. They reported that an icosahedral QC phase formed during the melt spinning of Al–Mn–Be alloys. The observed improvement in QC forming ability was attributed to the fact that Be addition significantly facilitates the formation of the quasicrystal and it can be formed in a significant amount by injection casting in a Cu mould. However, in these alloys a hexagonal approximant phase was also present in the Al-rich solid solution, in addition to the quasicrystalline particles. The effect of Be additions on the microstructure of the Al–Mn system has been investigated by Zupanic et al. [16], [23]. They found that a quasicrystalline phase formed only in Al₉₂Mn₃Be₅ and Al₈₉Mn₆Be₅ melt-spun ribbons with thicknesses between 30 μm and 200 μm and that Be additions shift the minimum amount of Mn for the formation of quasicrystals by melt-spinning to as little as 2.5 at.% Mn, in contrast to 6 at.% Mn in the Al–Mn system alone. In addition, they found that the highest particle density and the smallest size of quasicrystalline particles were attained on the wheel side of the thinnest ribbons in the alloy Al₈₉Mn₆Be₅, where the highest microhardness was measured (≈290H_V) [23].

Until now, a wide range of reinforcements have been used to produce Al–Mn alloys with the CS and MS methods, such as QC forming ability and Vickers microhardness ternary and quaternary QC alloys [14], [15], [16], [17], [18], [19], [20], [21], [22], [23]. These studies were devoted to the production of QC and hexagonal approximant phases containing alloys by non-equilibrium techniques and the effect of Be in different amounts on the microstructure and mechanical properties of Al–4.5Mn alloy has been rarely reported. Therefore, the aim of this paper is to study the influence of beryllium (Be) addition on the quasicrystal-forming ability, thermal and mechanical properties of Al–4.5 wt.%Mn–x wt.%Be (x = 0, 1, 3, 5) alloys. The reasons we use these wt.% are following: (i) It is known [24] that i-phase may form at a Mn content as low as 5 wt.%. We decided to decrease the Mn content to 5 wt.% in order to further lower the liquidus temperature and to increase the potential for forming the stable IQC-phase. (ii) The beryllium in the range of 1–5 wt.% is the one of the most widely used casting aluminum alloys because of it proved to considerably increase the QC forming ability [14], [15], [16], [17]. Moreover, the best of our knowledge, there is no study related to the effect of Be on the microstructure and mechanical properties, especially Vickers microhardness with tensile-strength tests, of CS and high cooling rate MS Al–4.5 wt.%Mn–x wt.%Be (x = 0, 1, 3, 5) alloys. Thus, in this paper we study how Be addition, together with high cooling rate treatments, affects the microstructural and mechanical properties of Al–4.5Mn alloys.