Laves Phase Ticr <sub> 2 </ Sub> Particles Enhance The Preparation Of Ti-based Alloys And Their Organizations, Performance And Mechanical Behavior

Alloys with Laves phase y-TiC^ and chromium content from 18% to 30% were fabricated by means of non-consumable electrode arc melting and an anneal 700癈x20h. It was found that the as-cast hypereutectoid Ti-Cr alloys were solely composed of p-Ti and the Laves phase y-TiCr2 would precipitate along the grain boundary to form a net when they were annealed at 700 癈 and then an eutectoid transformation p-Ti梐-Ti+y-TiCr2 would happened in the matrix to form eutectoid bulks in a radiative shape during air cooling from 700癈 to room temperature. The higher the chromium content was, the more the quantity and the larger the size of y-TiCr2 and theIhigher the hardness of the alloy were.The high temperature oxidation behaviors of the Ti-Cr alloys with Laves phase y-TiCr2 in the range of 6500C~780癈 and the influence of yttrium addition from 0.0125 to 0.0500% on the high temperature oxidation resistance of the alloys were investigated, respectively. The results on oxidation behaviors revealed that chromium content had significance to the oxidation resistance of the alloys. The scaling rates of the alloys with less than 26%Cr were higher than that measured for pure titanium, but for the alloys with more than 26%Cr their scaling rate was lower by 1~2 times, under the same oxidizing conditions, which was contributed to the formation of both chromium oxides and multiple titanium-chromium oxides that all could effectively block the diffusion of oxygen atoms in the internal oxidation layer. On the basis of above results an oxidation model of hypereutectoid Ti-Cr alloys with Laves phase y-TiCi'2 was proposed.Yttrium addition had a significant impact on the high temperature oxidation resistance of hypereutectoid Ti-Cr alloys with Laves phase y-TiCr2 and chromium content lower than 26%Cr. In the range of this study, the lower the Cr content and the oxidation temperature and the more Y addition in the alloys with chromium content below 21%, the more effectively the high temperature oxidation resistance could be improved, which was contributed to the fining of the grains of the oxides in the scale and the improvement of the scale's adherence to the matrix of the alloy as well as the formation of multiple titanium-chromium oxides, such as C^TiO.s and Cro284Tio.7i4Oi 857, in the internal layer. However, for the alloys with chromium content above 21%, there was an optimal chromium addition. Yttrium addition in alloys with chromium content higher than 26% showed no effect in improving the high temperature oxidation resistance.The results on both oxidation behaviors of the alloys and yttrium's influence revealed that oxidation temperature significantly affected the scaling rates of the hypereutectoid Ti-Cr alloys, and the mass gain was increased by times with a temperature change from 650癈 to 700 癈 or from 700 癈 to 780癈 for the same exposure duration.In order to fabricate super-saturation Ti-Cr alloyed powders with nano-grains the mechanical alloying of both Ti-20%Cr and Ti-30%Cr alloys with elemental titanium and chromium powders as raw materials was investigated. It was found that the intensity of the first strongest (Oil) peak of titanium in the XRD pattern rapidly decreased during the first 2 hour ball milling, but that of the second strongest (010) peak increased, which then became the first strongest peak, as those of the peaks of chromium did at the same time. The amorphization of titanium powders gradually took place as ball milling time reached 30 to 40 hours and with a milling rate 200r/min, a milling-ball : powder weight ratio 15 : 1 and 10 mm in radius steel balls as milling balls, nano-powders could be obtained as the milling time approached to 80 hours. However no Laves phase had been ever founded during even such a long time ball milling as 100 hours. Theoretic calculation figured out that It was in the first 10 hour ball milling period that the efficiency of grain fining, crystal lattice deformation and alloying appeared highest and chromium powders with a grain size less than 20 nm could be fabricated, but succes...