Research On The Spark Plasma Sintering Process Of TiAl Based Amorphous Powders Prepared By Mechanical Alloying

Ti based bulk metallic glasses is a new kind of promising functional materials, owing to its excellent mechanical, physical and chemical properties. Limited by the critical cooling rate, Ti based bulk amorphous alloys with large dimension and free of toxic element Be was difficult to prepare by traditional preparation technologies, which seriously prevent further extension of industrial applications. Using the technology of powder metallurgy, a new kind of non-toxic of Ti based bulk metallic glasses with large three-dimensional size, high strength and good glass forming ability can be produced. In this paper, mechanical alloying(MA) combined with spark plasma sintering(SPS) was employed to prepare a new kind of Ti based bulk amorphous alloys wealthy of aluminum and its amorphous/nanocrystalline composite materials.Ti-Al-Zr-B-Y amorphous alloy powder was prepared by MA, and the influence of Zr, B and Y on the process of ball milling and the glass forming ability was studied. Ti, Al, Zr, B and Y elemental powder of high purity were accurately measured according to nominal components and the pre-mixed powders together with stainless steel balls were put into stainless steel vessels of the high-energy planet ball mill to produce Ti-Al-Zr-B-Y amorphous alloy powder. X-ray diffraction technology(XRD) was used to study the phase composition of the milled powder. Scanning electron microscopy(SEM) and transmission electron microscopy(TEM) were employed to study the microstructures of the milled powders. Differential scanning calorimetry(DSC) was conducted to analyze the related thermodynamic parameters of the milled amorphous alloy, determine the characteristic temperatures such as the glass transition temperature, Tg, and the initial crystallization temperature, Tx1, etc. The glass forming ability and the thermal stability of the milled amorphous alloy was analyzed. Minor addition of alloying elements of Zr, B and Y can accelerate the formation process of amorphous and improve the glass forming ability of Ti Al based amorphous alloy. Minor addition of B and Y refined the particles of Ti Al based amorphous alloy powder. Ti50Al46.4Zr3B0.6 amorphous alloy has the maximum supercooled liquid region △T of 100 K.Ti-Al-Zr-B-Y amorphous alloy bulks were produced by SPS, and the effect of alloying elements of Zr, B and Y on the mechanical properties of bulk amorphous alloys was investigated. Based on the measured Tg and Tx1, the as-milled Ti-Al-Zr-B-Y amorphous alloy powder was put into a cemented carbide mold and pre-compacted under a pressure of 20 MPa, the preloaded mold was put into the sintering furnace and consolidated rapidly by SPS into specimens of the cylindrical shape with a diameter and height of about 20 and 7 mm, respectively. Measuring techniques of XRD, SEM and TEM were used to observe the phase structure and microstructure of the sintered specimens. EDAX energy spectrometer was employed to determine the actual chemical component of the sintered amorphous alloys. Using the Archimedes drainage method, density of the obtained amorphous bulks was measured, and the relative density was calculated. The experiment of measuring the room temperature compression strength of Ti-Al-Zr-B-Y sintered specimens was carried out by a Gleeble machine. The compressive samples were the cylindrical shape with a diameter and height of about 5 and 7 mm, respectively, the strain rate was 5 x 10-3 s-1. The elastic modulus and micro-hardness of the sintered samples were measured by a nano-indenter. The sintered bulks were completely amorphous alloy when sintered near Tg, the relative density and the compressive fracture strength increased with the increasing temperature. Nanocrystal phases of ?-Ti Al and ?2-Ti3 Al were separated out when the sintering temperature approached to Tx1. Minor addition of Zr, B and Y increased the compressive fracture strength of the amorphous alloy bulks, and minor addition of B and Y simultaneously resulted in the largest improvement of the compressive fracture strength. Ti50Al46.2Zr3B0.6Y0.2 amorphous alloy bulks has the maximum compressive fracture strength of 1362 MPa.Ti50Al47Zr3 alloy bulks were selected to study the effect of the sintering parameters on the room temperature mechanical properties of the sintered amorphous bulks and the amorphous / nanocrystalline composite bulks. The sintered process parameters were optimized, and the process parameters selection rule for obtaining amorphous and amorphous / nanocrystalline composite bulks with excellent properties was concluded. SPS sintered amorphous alloy bulks with high mechanical properties, need a high sintering pressure not less than 500 MPa, heated to an appropriate temperature near Tx1 in the undercooled liquid region, and then hold an appropriate time. By properly increasing the sintering temperature and holding an appropriate temperature, the Ti Al based amorphous / nano-crystalline composite bulks with high strength could be obtained. The measured maximum compressive fracture strength for Ti50Al47Zr3 amorphous alloy and amorphous / nanocrystalline composite bulks were 1265 and 1379 MPa, respectively.