New Aspects of the martensitic Transformation of Zirconia

掺少量氧化钇的氧化锆在300 ℃左右加热或低温冷却时，既有等温相变，也有非热猝发相变。在这项工作中，氧化钇含量，晶粒尺寸和试样尺寸的影响进行了定量测量的两种模式的转变看洞察马氏体的形核。制备了直径为0.3 ~ 0.8mm、Y_2O_3含量为1.4 ~ 1.6mol%、晶粒尺寸为0.5 ~ 1.0 μm的小球形试样。采用显微硬度计研究了等温保温过程中的等温转变动力学，并通过视频记录试样在连续冷却过程中的猝发转变过程和温度来测量Mb温度，等温转变开始于一定的保温时间，并在较短的时间内遵循S形曲线直至完成。转变开始时间在TTT图中呈现典型的C曲线。随着氧化钇含量的降低和晶粒尺寸的增加，C曲线向短时间侧移动。在孵化期的老化效果被发现是累积的，这表明在孵化期的成核的持续增长。Mb温度也显着增加，减少氧化钇含量和晶粒尺寸的增加，这表明一个显着的不稳定的影响，减少氧化钇含量和晶粒生长。另一方面，Mb温度仅随试样尺寸的减小而略有下降。Mb的威布尔模量为80，显著高于类似材料的断裂强度（Mb = 6）。这表明马氏体的胚具有与导致断裂的那些缺陷本质上不同的分布。

Zirconia doped with a small amount of yttria exhibits both isothermal and athermal burst type transformation depending on being heated at around 300 ℃ or subzero cooled. In this work, the effect of yttria content, grain size and the specimen size were quantitatively measured for both modes of transformation to look insight into the nucleation of martensite. Small spherical specimens of 0.3〜0.8 mm diameter containing 1.4〜1.6 mol%Y_2O_3 and the range of grain size 0.5〜1.0 μm were prepared. The isothermal transformation kinetics was studied by micro-dilatometer during isothermal holding and Mb temperature was measured by video recording the burst transformation together with temperature while the specimens were continuously cooled.Isothermal transformation started after some incubation time and followed a sigmoidal curve until its completion in a relatively short time. A transformation start time exhibited a typical C-curve in a TTT diagram. The C-curve shifted toward short time side with decreasing yttria content and with increasing grain size. The effect of aging during the incubation period was found cumulative, suggesting a continuous growth of nucleation during the incubation period. The Mb temperature was also significantly increased with decreasing yttria content and with increasing grain size, suggesting a significant destabilizing effect of the reduction of yttria content and grain growth. On the other hand, Mb temperature decreased only slightly with decreasing specimen size. The Weibull modulus for Mb was 80, which was considerably higher than that for the fracture strength of a similar material (m〜6). This suggests that the embryos of the martensite have an essentially different distribution from those defects responsible for the fracture.