Abstract

Alloys of composition Cu100-xHfx with 50 ≥ x ≥ 25 at.% and Cu55Hf45-xTix with 5 £ x £ 45 at.% have been investigated. All alloys investigated in both series were fully vitrified when melt spun to ribbon of thickness ~25 mm, except the binary alloy Cu75Hf25 that showed a crystalline + amorphous structure. The critical size for the binary alloys was limited to ribbons, while the critical diameter is 3 mm for the Cu55Hf25Ti20 and Cu55Hf20Ti25 alloys. On the other hand, 2 mm fully glassy diameter was found for the Cu55Hf30Ti15 and Cu55Hf15Ti30 alloys. The substitution of Hf by Ti causes an increase in the glass-forming ability (GFA). As the Ti content increases, the glass transition temperature (Tg) and crystallization temperature (Tx) decrease for both Cu100-xHfx and Cu55Hf45-xTix alloys. In contrast, the liquidus temperature (Tl) has a minimum value of 1163 K for the Cu55Hf20Ti25 alloy, resulting in a maximum Tg/Tl of 0.61. The alloys with the highest Tg/Tl value showed the best GFA in this Cu-based alloys series.
Keywords

Bulk Metallic Glasses, Suction Casting, XRD, DSC, Critical Glassy Diameter
Introduction

Metallic glasses are non-crystalline alloys formed by continuous cooling from the liquid state. Up until recently [1], such materials could only be formed in thin sections at cooling rates typically in the range 104-106 Ks-1. Such alloys, based on iron, nickel and/or cobalt have established numerous applications as ultra soft magnetic materials in power and high frequency cores for transformers, chokes, inductors and other similar devices [2]. In the past decade, researchers had paid more attention to the metallic glasses not only due to their superior chemical and physical properties, but because they can now be fabricated in bulk form. They are therefore considered as real practical engineering materials and open up new application opportunities. For instance, glassy alloy can be used for making magnetic tapes [3, 4] as it exhibits better wear resistance. Extensive experimental investigations have demonstrated that many compositions in these systems can be formed into the glassy state by using suction casting to form cylindrical or slab-shaped ingots. Maximum section diameters or thicknesses of fully glassy phase range up to 72mm [5], depending on the combination of constituent elements and their precise concentrations. Recently, Inoue et al. [6, 7] reported Cu-based alloys with high glass forming abilities, high tensile strength of over 2000 MPa and lower material cost bulk metallic glasses which can be prepared by copper mould casting. Their excellent mechanical properties and glass forming ability (GFA) make the production of precision mechanical parts possible such as high precision gears [6]. Driven partially by interest in engineering applications, there has been an ongoing effort to identify amorphous alloys with greater strength, elastic modulus, hardness, and ductility. Of particular interest are alloys based on such common metals such as Cu, Al, Co, Fe, Ni, etc. However, the information of a glassy phase in Cu-Hf-Ti alloys is based on 60 at.% Cu. The aims of this investigation are to produce bulk amorphous rods of Cu-Hf and Cu-Hf-Ti and determine the critical glassy diameter of these alloy families.
Experimental

Alloy ingots of nominal compositions Cu100-xHfx (x = 50, 45, 40, 35, 30 and 25 at.%) and Cu55Hf45-xTix (x = 5, 10, 15, 20, 25, 30 , 35 and 40 at.%) were prepared by arc melting mixtures of Hf (crystal bar), Cu (sheet) and Ti (Rod) having purities of 99.5 at.%, 99.99 at.% and 99.8 at.%, respectively. The arc melting was performed in a Ti-gettered high purity Argon atmosphere. Each ingot was re-melted at least four times in the arc melter in order to obtain good chemical homogeneity. Ribbon samples of mean thicknesses ~ 25 mm and width ~ 2 mm were prepared by melt spinning in a controlled Ar atmosphere. Copper die suction casting was employed to produce rods with a stepped profile having diameters decreasing from 4 to 3 to 2 mm, each with a total length of 50 mm. The phase constitutions of the rods were studied by X-ray diffraction (XRD). The thermal stability, defined by the glass transition temperature (Tg) and the crystallization temperature (Tx), was studied by differential scanning calorimetry (DSC) at a heating rate of 20 K/min. The solidus temperature (Tm) and liquidus temperatures (Tl) were determined by differential thermal analysis (DTA) at a heating rate of 20 K/min. Thermal characterization was performed using melt spun ribbons.
Results

The average thickness of the ribbon produced was 25 mm and almost all the ribbons produced were of uniform width. All the alloy ribbons formed, except for one sample, the binary alloy Cu75Hf25, were found to be nominally fully amorphous by XRD analysis. They showed high metallic lustre and could easily be bent through 1800 without fracture, thus showing very good ductility to a high strain and indicating a fully amorphous or almost fully amorphous thickness. However, the Cu75Hf25 alloy ribbon had very poor ductility and it was shown to be crystalline by the XRD analysis. Stepped rods were produced of diameters 2, 3 and 4 mm with a length of 50 mm. Figure 1 shows a photograph of a 2/3/4 mm diameter stepped rod produced by suction casting. The rods produced by suction casting showed good metallic lustre and a low incidence of fabrication defects.

The structure results by X- ray diffraction for all the ribbon and rod samples of the Cu100-xHfx alloy series with x = 25 – 50 are summarised in Table 1. All rods were crystalline or largely crystalline as several distinct peaks can be seen for all these traces with no clear evidence of a diffuse halo corresponding to a glassy phase. The ribbon samples are clearly fully amorphous in each case, with the exception of the composition Cu75Hf25 which shows a small fraction of crystalline structure, with one peak at ~ 43o 2 q corresponding to the phase Cu8Hf3 (111). Figure 2 shows DSC curves for the melt-spun ribbons of this alloy series and values of Tg and Tx are plotted as functions of Hf content in Figure 3. Although no clear glass transition is observed for 30 at.% Hf, the alloys containing 50 at.% to 35 at.% Hf exhibit distinct glass transitions, followed by the glass transitions region before crystallisation, however, the alloy containing 25 at.% Hf was found to be partially crystalline, giving a DSC curve with amorphous + crystalline structure. The Tg and Tx decrease with increasing Hf content from 35 at.% to 45 at.%, despite the fact that the Hf has a much higher cohesive energy than Cu. It is also seen that the supercooled liquid region defined by the temperature interval between Tg and Tx, DTx (=Tx − Tg), shows a relative constant value of ~ 30 K over the range 35 – 50 at.% Hf, but appears to narrow considerably for 50 at.% Hf. The substitution of Hf for Cu in the range 25 – 35 at.% Hf decreases the liquidus temperature, but increasing the Hf content beyond 35 at.% Hf then leads to an increasing Tl. The values of the reduced glass temperature (Trg=Tg/Tl) are plotted as a function of Hf content in Figure 4. Trg increases when decreasing Hf content from 0.55 up to 0.61 for x = 50 and 35 respectively, then decreasing to the value of 0.59 for x = 30. Table 2 shows the thermal analysis results for this alloy series.