Preparation Of Zr-Al-Ni-Cu-Y Amorphous Alloy Powders By Mechanical Alloying And Investigation Of Low-pressure Sintering Process

Metallic glasses have extraordinary disordered atomic configuration and no crystalline defects such as grain boundaries in conventional crystal metals. The structurally and chemically homogeneous glassy phase characteristics of metallic glasses provide many excellent properties compared to crystalline counterparts. Metallic glasses have been one of research fronts in materials science, of which, Zr-Al-Ni-Cu amorphous alloy has attracted more attentions because there exist no precious metal elements or toxic elements. Furthermore, the alloy has strong glass forming ability. The preparation of metallic glasses by conventional molding is affected by many rigorous preparation requirements, and it is hard to industrialize the production. Mechanical alloying (MA) is a powder metallurgy processing technique that includes repeated cold welding, fracturing and rewelding of powder particles in a high-energy ball mill. Currently, MA has now been widely used to synthesize amorphous alloy powders. In this paper, amorphous Zr-Al-Ni-Cu alloy powders are prepared by MA at low vacuum with commercial pure element powders. The progress and mechanism of amorphization in mechanical alloying has been investigated by X-ray diffractometry, differential scanning calorimetry, scanning/transmission electron microscopy and the corresponding thermodynamic principle.It can easily obtain bulk samples with complex shapes by powder consolidation techniques. A number of studies have been carried out on the consolidation of amorphous alloy powders using various consolidation techniques. Powder consolidation is a relatively new method to obtain large-sized amorphous alloy with excellent properties. Based on cost considerations, the high-cost methods such as ultra-high-pressure consolidation, extrusion consolidation and spark-plasma sintering are not adopted in the research. In order to reduce the preparation cost, this paper explores a more economical consolidation method for Zr-based amorphous alloy powders by conventional low-pressure sintering.The study successfully prepared Zr-Al-Ni-Cu-Y amorphous alloy powders by mechanical alloying, and explored the beneficial effects of Al partially substituted by Y in Zr5oAl15Ni10Cu25on glass forming ability. The optimum substitution amount of Y is1.25at%, which provides a convincing experimental evidence for improving glass-forming ability of Zr-based amorphous alloy by minor alloying elements additions. The influence of external additive particles on the thermal stability of amorphous powders was investigated, and the optimum type and content of additive particles for Zr-Al-Ni-Cu have been determined. The amorphous alloy powders were consolidated by low-pressure sintering process. And then the paper explored the influence of additive particles and sintering temperature on morphology and mechanical properties of bulk Zr-Al-Ni-Cu-Y composite. The main conclusions of this paper are listed as below.1. Numerous experiments have indicated that an amorphous alloy can exhibit high glass forming ability when its composition lies along the binary eutectic line with low melting temperature in hypoeutectic composition. Based on these studies, a new composition Zr5oAl15Ni10Cu25is presented. And Zr5oAl15-xNi10Cu25Yx amorphous alloy powders can be synthesized from low purity commercial powders by mechanical alloying after60hours milled under the condition of low rotational speed (170r/min) and ball-to-powder weight ratio of20:1.2. The influence of Al partial substituted by Y in Zr5oAl15Ni10Cu25on glass forming ability is investigated. It can be found out that an appropriate amount of Y addition can make the atoms in Zr5oAl15Ni10Cu25arrange more disordered and scavenge the oxygen impurity in the system via the formation of innocuous yttrium oxides, thus improve the glass forming ability of Zr50Al15-xNi10Cu25Yx. The optimum addition amount of Y is1.25at%. Below or beyond that amount, the glass-forming ability of the system will decline.3. In order to optimize the addition amount of yttrium, a simple analysis based on the thermodynamic principle is carried out. It can find that the the mixing enthalpy of the system is monotonously decreased with the Y content increased, but the mixing entropy first increases and then decreases. The maximal mixing entropy appears at x=7.5. As can be seen from the fitting curve of equivalent free energy, free energy approaches to its lowest value at x=1.25, that is to say, the yttrium amount is appropriate for amorphous alloys formation from the point of view of thermodynamic principle.4. The impact of external additive particles on the thermal stability of Zr50Al15-xNi10Cu25Yx amorphous alloy has been explored. Results show that an appropriate amount (5wt.%) of the external additive particles (Si3N4) can effectively increase the crystallization activation energy of the amorphous matrix and thus improve the thermal stability. And good thermal ability is helpful for the consolidation of the amorphous alloy powders under heating process.5. The consolidation of Zr50Al15-xNi10Cu25Yx amorphous alloy powders by low-pressure sintering has been explored. Because of the sintering temperature is much higher than the crystallization temperature, all amorphous powders have been crystallized in the sintering process and no bulk amorphous alloys are obtained.6. The role of external additive particles on consolidating Zr5oAl15-x Ni10Cu25Yx amorphous alloy powders by low-pressure sintering process is investigated. It is found out find that the external additive particles can act as a framework in the amorphous alloy powders, thus strengthen the amorphous matrix. Among the three different additives, Si3N4has the best performance for consolidating Zr50Al15-xNi10Cu25Yx amorphous alloy powders. The highest flexural strength value is up to115.22MPa. Because of the particle size of the additives are large than the amorphous powders, it is hard to disperse the external additive particles homogeneously in the amorphous powders by means of hand milling. The inhomogeneous external particles can greatly restrict the structure and performance of the products.