Studying The Thermal Stability, Crystallization/Relaxation Behaviors And Glass Forming Ability Of Cu-Zr-Al-Ag Amorphous Alloys By Electrical Resistance And Internal Friction Measurements

Due to the structure of long-range disorder and short-range order, amorphous alloys have special properties. However, whether exploring the phenomenology and grounded cognition of amorphous materials, developing new bulk amorphous alloys (BMGs) or improving the glass forming ability (GFA) needs further research. Because of the good GFA, superhigh mechanical properties and relatively low cost, the recently developed Cu-Zr-Al-Ag BMGs have the engineering application prospects. Nevertheless, the thermal stability, relaxation and crystallization behaviours, and the essence of GFA of these BMGs are still lack of thorough research.As the sensitive physical quantities to structure, electrical resistivity and internal friction can reflect the material structure changes. In this paper, taking the CuxZr84-xAl8Agg (x=42,40,38,36) BMGs as the main research object, the electrical resistivity and internal friction methods were first applied to study the their thermal stability, relaxation and crystallization behaviours, GFA, and the impact of melt state on the above aspects to ascertain the aforementioned physical processes and their differences and natures, and to reveal new phenomena and laws, which can provide the scientific and technical basis for the further research and engineering application of these BMGs and even the amorphous field. The main innovative results and conclusions are summarized as follows:1. In the process of continuous heating up to 1100 K, there is a distinct change of the temperature coefficient of resistivity at the glass transition temperature of the CuxZr84-xAl8Ag8 BMGs. Hereafter, there are two sudden drops of resistivity indicating that besides the first-stage crystallization the second-stage crystallization will take place at the higher temperature for the four BMGs, which can’t be detected by the DSC method due to the limit of the measurement temperature range. The phase analysis shows that the first-stage crystallization process is mainly the precipitation of the metastable phases Cu10Zr7 and AgZr from amorphous matrix; while the second-stage crystallization is mainly the process that the metastable phases Cu10Zr7 and AgZr transform into the stable phases CuZr2, AlCu2Zr, CuAl2, and unknown phases.2. The continuous heating crystallization kinetics of the CuxZr84-xAl8Ag8 four BMGs was studied by the resistance method combined with the thermal analysis method. The characteristic temperatures and the calculation results of the Kissinger equation show that as the composition becomes richer in Zr, the supercooled liquid region widens, the glass transition apparent activation energy decreases, and the onset crystallization apparent activation energy increases, which indicates that the thermal stability and GFA are enhanced. Density measurement result reveals that the reason can be ascribed to the more close-packed structure and larger negative mixing enthalpy. The Avrami exponent n and incubation time τ of the four BMGs are obtained by fitting the isothermal resistance crystallization kinetics data. The incubation time of the BMG with the higher thermal stability significantly increases. The values of n reflect the similarity and difference of the nucleation and growth mechanism during the crystallization process. During the first-stage isothermal crystallization process the nucleation of the four BMGs exhibits a decreasing nucleation rate with time evolution; the crystallization of the first two BMGs is diffusion-controlled, contrasting with an interface-controlled growth for the latter two BMGs. During the second-stage isothermal crystallization process the crystal growth of the four BMGs is diffusion-controlled; the crystallization of the first two BMGs is the growth and transformation process of existing particles, contrasting with a decreasing nucleation rate with time evolution for the latter two BMGs.3. The relaxation behaviour of the Cu36Zr48Al8Ag8�'�Cu42Zr42Al8Ag8 BMGs from room temperature to the supercooled liquid region was studied synchronously by the internal friction and resistance methods. The results show that:(1) With the temperature increasing, the internal friction temperature spectra successively exhibit slowly linear increasing, nonlinear increasing at a higher rate, radically increasing, and an IF peak occurring whose feature is affected by the loading frequency. The relaxation behavior corresponds to the atomic jumping or flowing which is highly limited in local areas, slow β occasionally bridging in local areas, a relaxation starting with Tg, and the combined effect of a relaxation and crystallization, respectively. When the loading frequency is so low that the laboratory time approaches the time of a relaxation, the internal friction peak is a relaxation peak. While the loading frequency is so high that a relaxation appears difficultly, the internal friction peak is a first-order phase transformation internal friction peak.(2) In the initial phase of continuous heating the electrical resistivity of the Cu-Zr-Al-Ag BMGs depends linearly on the temperature. And then, there is a turnaround between them, which is highly consistent with the turning point of the internal friction temperature spectra. The primary analysis considers that the turing point corresponds to the starting temperature of slow β relaxation. After that, the electrical resistivity varies nonlinearly with temperature until at Tg the second turning point happens which corresponds to the activation of a relaxation. By computation and comparison, the BMG with a better GFA has a lower start temperature and activation energy of slow β relaxation and a higher internal friction peak. Compared with the internal friction method, the resistance method from the electronic level is very effective and more intuitive and accurate to determine the start temperature of slow β and a relaxation, which provides a new method and thought for studying the relaxation behavior of amorphous alloys, but still needs further exploration.(3) The frequency spectra at different temperatures are obtained by linearly interpolating the temperature spectra at different frequencies.With the temperature increasing, because an anelastic relaxation process happens and the relaxation parameter changes, the internal friction frequency spectra successively exhibit four trends of the linear increasing, first decreasing and then increasing, monotonously decreasing, and an anelastic IF peak occurring.4. Based on the Anderson’s localized theory of disordered materials and Mott-CFO model, develop and innovatively put forward a criterion for predicting the GFA of metallic glasses from the perspective of electron transport, which is termed as the relative electrical resistivity difference between the amorphous and fully crystallized states at room temperature (RT), i.e. Δρ=(ρamorRT-ρcrysRT)/ρcrysRT,ρamorRT and ρcrysRT denote the room-temperature electrical resistivities of an amorphous alloy and its corresponding crystal, respectively. The alloy with a larger Δρ always has a better GFA. It is demonstrated by the Cu-Zr-Al-Ag, La-Ce-Al-Co, and Fe-Mo-Y-B amorphous systems that the change trend of Δρ and the maximum diameter Dmax of a rod-like metallic glass is completely consistent. Compared with Trg, ΔTx, and γ, the coincidence degree between Δρ and Dmax is the best. The new criterion reflects the disordered degree and the structure difference of an amorphous alloy relative to the crystal. The new criterion basically reveals the density of state (DOS) difference at the Fermi energy EF between the amorphous and crystalline alloys, which may be exactly the essence of the GFA.5. The high-temperature DTA measurement result reveals that there is a liquid-liquid structure transition in the temperature range of a few hundred degrees above the liquidus temperature for the Cu36Zr48Al8Ag8 alloy melt. Hereby, the influences of melt overheating on the crystallization behavior, thermal stability and GFA of Cu36Zr48Ag8Al8 amorphous alloy were explored primarily by the resistance method, supplemented by DSC and XRD methods. The results indicate that with the melt overheat increasing, the GFA of the alloy increases firstly, then decreases, finally keeps invariability; the thermal stability of the amorphous alloy has no obvious changes firstly, then decreases significantly, finally keeps constant; and a "shoulder" firstly appears before the first-stage crystallization peak and then the first-stage crystallization peak is split into two peaks. Analysts say, on one hand, the chemical medium/short range orders existing in the low temperature melt are gradually being broken at high temperature, and the melt structure are more uniform and disordered, thereby the GFA of the alloy are enhanced. On the other hand, the content of oxygen measurement result reveals that with the increase of melt temperature the content of oxygen rises. The oxygen atoms form high-melting-point metal oxides with constituent elements. They act as crystal nucleation basement in the cooling process, worsen the GFA and thermal stability of amorphous alloy and induce the change of the crystallization behavior. This shows that if improve the GFA and thermal stability of amorphous alloys by using the melt overheating treatment these two factors should be considered.