Effect of Nanocrystal Distribution on Mechanical Properties of Ti-Based Metallic Glasses

Abstract

Ti45Zr5Cu45Ni5 metallic glasses in which Ta and Al were substituted for Cu were evaluated in terms of mechanical properties, thermal properties and microstructures in order to determine the factors contributing to an improvement in plasticity. Samples are examined by compression testing, differential scanning calorimetry, X-ray diffractometry and electron microscopy. Mold-cast Ti45Zr5Cu44Ni5Ta1 bulk specimens were confirmed to consist of a metallic glass matrix and nanocrystals homogeneously dispersed at high density within the matrix. The yield stresses of both Ti45Zr5Cu45� xNi5Tax and Ti45Zr5Cu45� xNi5Alx are approximately 1800 MPa, and the maximum plastic strain of 3.1% was obtained for the Ti45Zr5Cu44Ni5Ta1 specimen. The Ti45Zr5Cu45� xNi5Alx bulk specimens exhibited poorer plasticity due to the formation of larger crystalline grains. (doi:10.2320/matertrans.MF200616) Various kinds of Ti-based amorphous aloys and metallic glasses have been developed, 1-12) due mainly to the light weight and high corrosion resistance properties of Ti. Ti- based metallic glasses are therefore attractive candidates for light-weight, high-strength and biocompatible structural materials. However, fully metallic glasses exhibit only elastic deformation, and activation of many shear bands in metallic glasses is necessary for plastic deformation. The homogeneous dispersion of nanocrystals in metallic glasses is one method for preparing metallic glasses that undergo plastic deformation by the activation of shear bands. Recently, Ti-based bulk metallic glasses exhibiting plastic deformation have been reported, 6,11) although the micro- structures of the materials have yet to be clarified. If the plastic deformation in Ti-based bulk metallic glasses is caused by the dispersion of nanocrystals, it may be possible to increase the plastic strain by optimizing the density, distribution, and morphology of the nanocrystals. Many Ti alloys exhibit solid-state phase transformation from austenite � -Ti to martensite � 00 -Ti. Whenstabilizers are added to the Ti alloys, the martensite transformation temperature (� transus) decreases with increasingstabilizer content due to stabilization of the � -Ti phase. When such alloys are quenched, it is expected that � -Ti nanocrystals will precipitate. In the present study, Ta and Al asand � stabilizers were substituted for Cu in Ti45Zr5Cu45Ni5 and the resultant alloys were rapidly quenched to form bulk metallic glasses. The mechanical and thermal properties and micro- structures of the glasses were investigated with respect to the dispersion of nanocrystals. 2. Experimental Procedures Ingots with nominal compositions of Ti45Zr5Cu45Ni5, Ti45Zr5Cu45� xNi5Tax and Ti45Zr5Cu45� xNi5Alx (x ¼ 1; 2; 3; 4; 5) were prepared by Ar-arc melting, and cylindrical bulk specimens of 1 mm in diameter and 30 mm in length were prepared by Cu mold casting of the ingots. Differential scanning calorimetry (DSC) measurements were performed to determine the glass transition temperature (Tg), crystal- lization temperature (Tx), and supercooled liquid region (� Tx ¼ TxTg) at a heating rate of 0.67 K/s. The micro- structures of the specimens were characterized by X-ray diffractometry (XRD) using a rotating Cu anode, scanning electron microscopy (SEM), and transmission electron mi- croscopy (TEM). The mechanical properties of the speci- mens were examined at room temperature through compres- sion tests at a strain rate of 1 � 10 � 4 /s using an Instron-type machine.