Laves相TiCr₂微粒增强Ti基合金的制备及其组织、性能和力学行为的研究

【作者】 肖平安；

【导师】 黄培云；

【作者基本信息】 中南大学 ， 材料学， 2002， 博士

【摘要】 本文通过非自耗磁控电弧熔铸和700℃×20h高温退火，制备出了Cr含量为18～30％的含Laves相γ-TiCr₂过共析Ti-Cr合金。研究发现，铸态过共析Ti-Cr合金的组织为单相β-Ti：这种铸态合金在700℃保温退火时，金属间化合物γ-TiCr2沿晶界析出，并形成连续分布，在随后的空冷过程中，合金内部会发生β-Ti→α-Ti+γ-TiCr₂共析分解，共析组织在合金基体中呈放射状分散析出。合金含Cr量越高，析出的γ-TiCr₂数量越多、尺寸越粗大，合金的硬度也越高。 评价了含Laves相γ-TiCr₂ Ti-Cr合金在650～780℃温度范围内的抗氧化性能，并研究了微量Y对其高温抗氧化性能的影响。研究发现，含Cr量对含Laves相γ-TiCr₂过共析Ti-Cr合金的高温抗氧化性能有显著影响。当Cr＜26％时，合金的高温抗氧化性能明显比纯Ti差，而含Cr＞26％时，在合金表面氧化膜的内氧化层中，能够生成较高体积分数的Cr₂TiO₅和Cr₂O₅Cr复合型氧化物，有效地提高了氧化膜阻碍O原子向合金内部扩散的能力，使得高温抗氧化能力显著提高，合金单位面积氧化增重仅为同样条件下纯Ti的1/2～1/3。在上述研究结果的基础上，作者提出了一种含Laves相γ-TiCr₂过共析Ti-Cr合金的氧化过程模型。 研究发现，在Cr＜26％的含Laves相γ-TiCr₂过共析Ti-Cr合金中加入微量Y，有效地细化了氧化物的晶粒度、改善了氧化膜的致密性及其与基体金属的附着力，并促使在内氧化层中生成Cr₂TiO₅和Cr_0.284Ti_0.714O_1.857复合型氧化物，提高了氧化膜对O原子扩散的阻碍能力，从而能够有效地提高合金的高温抗氧化性能。合金含Cr量越低、氧化温度越低时，微量Y的效果越明显，并且其作用与加入量成正比；但是，当Cr≥21％后，则只存在一最佳Y加入量。对于Cr≥26％的含Laves相γ-TiCr₂ Ti-Cr合金，加入微量Y不能起到改善高温抗氧化性能的作用。 抗氧化性能和微量Y的作用研究结果均表明，含Laves相γ-TiCr₂过共析Ti-Cr合金对氧化温度十分敏感，在650～780℃的温度范围内，氧化温度每提高50～60℃，合金的单位面积氧化增重成倍增加。 为了制备出过饱和固溶纳米（或纳米晶）Ti-Cr复合粉末，本文研究了过共析Ti-Cr合金的机械合金化（Mechanical Alloying）规律。研究发现，在球磨初期的2h内，会发生Ti的（011）主衍射峰强度迅速降低、（010）第二衍射峰强度提高并成为了最强峰和Cr的衍射峰强度提高的现象；当球磨时间达到30～40h时，Ti开始非晶化：在球磨速度为200r/min、球料比取15：1和使用20mm球径的淬火钢球的条件下，当球磨时间达到80h时，可制得粒度在10Onm以下的均匀纳米粉末，但是，即使MA的时间超过100h，也没有发现Laves相TICr:在固相直接合成。对粉末晶粒度、晶格微应变和合金化程度的分析计算发现，MA的前10h是粉末晶粒细化、晶格应变和合金化进行得最迅速的时期，经过该阶段的球磨后，Cr的晶粒度可以达到20nm:进一步延长球磨时间，对获得过饱和固溶的合金粉末是十分必要的。30一4Oh是通过MA制备纳米晶或者非晶过饱固溶Ti一Cr合金粉末的合理球磨时间。 在球磨速度和过程控制剂对过共析Ti一Cr合金MA的影响研究中发现，球磨速度、过程控制剂及其加入量对MA的效率、粉末的粒度和团聚程度、因为磨损带入杂质Fe的量和来源有显著影响。在其它条件相同的情况下，球磨速度越高，MA的效率越高，粉末的粒度也越小;不使用过程控制剂时，MA的效率最高，得到的粉末粒度最小，但是，团聚现象最为严重;使用酒精作为过程控制剂，能够有效地减少球磨带入的杂质量，但是硬脂酸不适合用做过共析Ti一C:合金MA的过程控制剂;试验确定的合理球磨速度范围为150一20Or/min。 本文以MA过饱和固溶纳米晶Ti一Cr复合粉末为原料，进行了原位TICr:颗粒增强Ti基合金热压合成工艺和高温退火工艺的优化研究。试验结果表明，在860一1300℃的温度范围内，通过热压均能够制备出原位TICrZ颗粒增强的近全致密Ti基合金。但是，热压温度越高，合金的晶粒尺寸越粗大、增C也越严重;按照优化得到的热压工艺:860’C又45min，能够制备出增强相粒度和基体晶粒度均小于200nm的TICrZ微粒增强微晶Ti基合金，增强相TICr:在基体中分布均匀。 高温退火研究发现，TICr:微粒增强微晶Ti基合金具有良好的组织热稳定性。高温退火有利于合金中物相的充分析出，并使合金的成分和组织均匀化，此时，原料粉末中的杂质Fe偏聚在6一TICr:中，形成Ti、C:和Fe平均原子比为l:1 .2:0.45的合金化金属间化合物。优化得到的高温退火工艺为:700℃x（10一12）h。 力学性能测量结果表明，TICr:微粒增强微晶Ti基合金的室温强度性能超过IMI834，抗拉强度可达1210.3MPa，抗压强度可达2287.2 MPa;抗压断裂延性应变可达21.7%，比熔铸+高温退火工艺制备的含Laves相Y一TICr:Ti一Cr合金提高一倍，比纯金属间化合物TICr:提高一个数量级;其600一700℃的高温压缩力学性能明显优于采用反应热压法RHP（Reaetive Hot Pressing）制备的原位（石B+TIC）增强钦基复合材料。更多还原

【Abstract】 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更多还原