Composition Design And Properties Of The Zr-Al-Ni-Cu Bulk Metallic Glasses

Amorphous alloys had attracted increasing attention since the late 1980's due to the advent of bulk metallic glasses, readily prepared by Copper mould casting. Due to their specific properties and application potentials, this field has become a focus and frontier in new materials research. However there has been little progress in recent year regarding new bulk amorphous alloys. This is principally due to the fact that their formation mechanism is largely unknown. The discovery of these amorphous alloys has relied on extensive trials based on empirical rules.In this paper we will try to understand the formation rule of bulk metallic glasses by analyzing Zr-Al-Ni-Cu alloy system. We introduce e/a-constant and atomic size-constant criteria that reflect both electronic structure and atomic structure factors for designing and optimizing amorphous forming compositions. Systematic studies have been carried out on thermal analysis, microstructure and properties.First, we surveyed from reported literatures the compositions with high glass formation ability and high glass thermostability in quaternary Zr-Al-Ni-Cu and ternary Zr-Al-Ni and Zr-Al-Cu. We have noted that they are located near a straight line in both ternary and quaternary alloy systems. This is strikingly similar to the e/a-constant phenomenon in quasicrystalline systems. Considering that atomic size also exerts strong influence, we established quantitative criteria for the formation of bulk metallic glasses in quarternary Zr-based alloy systems. The compositions with high glass forming ability and large thermostability satisfy a specific e/a ratio and an average atomic size for a given system, called respectively e/a-constant and atomic size constant criterion. The optimum amorphous compositions are located close to the intersection line of the e/a-constant plane and atomic size-constant plane in the quaternary phase diagram.According to these rules, we designed a series of new Zr-Al-Ni-Cu amorphous alloy with e/a ratio of 1.375 and average atomic size of 0.1496nm: l#-Zr65.5Al5.6Ni63Cu22.4, 2#-Zr65.3Al6.5Ni8.2Cu2o, 3#-ZresAl75NiioCu75 (Inoue alloy), 4#-Zr64.8Al8.3Ni0.4Cu15.5, 5#-Zr64.5Al9.2Ni13.2Cu13.1, and 6#-Zr63.8Alu.4Ni17.2Cu7.6. The alloys were prepared by suction casting method in the form of cylinder bars with top diameter of 3mm and bottom diameter of 4mm and length 30mm.inWe have used DSC and DTA to measure the glass forming ability and thermostability. The result revealed that all the alloys have high reduced glass transition temperatures and large supercooled liquid regions. This indicates that we obtain a series of bulk amorphous alloys with high glass formation ability and high thermostability. In particular, the best composition among the six is Zres.gAlii^Nin.aCu?^. It has the highest glass transition temperatures (Tg=671K), crystallization temperatures (TX=758K) and reduced glass transition temperatures (Trg=0.610).We have studied microstructure in as-cast and crystallized samples by XRD, SEM, and TEM. Except samples 1# and 2#, no crystalline phases are observed. Partially crystallized samples contain more than two kinds of intermetallic compounds. No new phase was found.We have studied friction and wear characteristics of the six amorphous samples and made comparisons with their crystallized samples of the same compositions. All the amorphous alloys have nearly the same friction coefficients under dry sliding condition. Crystalline alloys have higher microhardness and wear resistance than amorphous alloys. We have also studied corrosion behavior of the amorphous alloys and their crystallized samples. In 0.1 M Na2SO4 electrolytes, 2#, 3#, 4#, and 5# manifest better resistance than the others. In particular, sample 3* (Inoue alloy) is the best of the six amorphous samples. Its corrosion current density icorr value is only 0.0138)iA/cm2. The crystallized samples have smaller icorr value than their corresponding amorphous samples in samples 2# and 4#. Among the eight amorphous and crystallized samples studied, samples 3#...