Study on Stress-Induced Phase Transformation of Rare Earth Aluminates

相变增韧是提高脆性陶瓷断裂韧性和断裂强度的有效途径之一。相变增韧需要颗粒的应力诱导马氏体相变，这在四方氧化锆陶瓷中是众所周知的。我们研究的主要目的是寻找具有应力诱导相变的新型氧化陶瓷。本文研究了（R_<1-x>La_x）_4Al_2O_9和Gd_4Al_2O_9/MgO复合材料的正交相和单斜相之间的关系、室温下正交相亚稳化的可能因素以及热力学性能。在1700℃的空气中反应烧结30h，合成了具有正交结构的单相固溶体，（Ho_<1-x>La_x）_4Al_2O_9体系的组成范围为0*x<0.5, (Gd_<1-x>La_x)_4Al_2O_9体系的组成范围为0*x<0.3。在这些组成范围以上，钙钛矿型LaAlO_3作为第二相共存。在晶粒生长高于临界粒径(x=0为6.2 μ m, x=0.2 in （Ho_<1-x>La_x)_4Al_2O_9为3.3 μ m, x=0为2.7 μ m, x=0.2 in (Gd_<1-x>La_x)_4Al_2O_9为2.4 μ m）的条件下，亚稳正交晶相能够淬火至室温。在室温下对所有正交相烧结体表面进行抛光后，样品的晶体结构转变为单斜晶结构；在抛光过程中发生应力诱导相变。这种由应力引起的从正斜向单斜的相变伴随着体积增大约0.5%。DSC测量结果表明，Gd_4Al_2O_9的热相变是可逆的，且存在滞后。以放热峰和吸热峰的平均温度为1110 C，表明材料由低温单斜晶向高温单斜晶转变。另一方面，在1430°C加热和1420°C冷却时观察到第二次转变温度，这对应于从单斜晶到正交晶的相变。少

Transformation toughening is one effective approach to improving the fracture toughness and fracture strength of brittle ceramics. Transformation toughening requires the stress-induced martensitic transformation of particles, as is well known in tetragonal zirconia ceramics. The main purpose of our study is to find new oxide ceramics with stress-induced phase transformation. In the present study, the relationship between orthorhombic and monoclinic phase, the possible factors for metastabilization of orthorthombic phase at room temperature and thermal and mechanical properties in (R_<1-x>La_x)_4Al_2O_9 and Gd_4Al_2O_9/MgO composites are examined. Single phase solid solutions with orthorhombic structure were synthesized by reaction sintering at 1700ﾟC for 30h in air within the composition range of 0*x<0.5 for (Ho_<1-x>La_x)_4Al_2O_9 and 0*x<0.3 for (Gd_<1-x>La_x)_4Al_2O_9 system. Above these composition ranges, perovskite-type LaAlO_3 coexisted as second phase. Metastable orthorhombic p … More hase was capable of quenching to room temperature under the conditions of grain growth above the critical particle size of 6.2mum for x=0, and 3.3mum for x=0.2 in (Ho_<1-x>La_x)_4Al_2O_9 and 2.7mum for x=0 and 2.4mum for x=0.2 in (Gd_<1-x>La_x)_4Al_2O_9. After polishing the surface of all sintered bodies with orthorhombic phase at room temperature, the crystal structure of samples changed to a monoclinic structure ; i.e.a stress-induced phase transformation occurred during the polishing process. This stress-induced phase transformation from orthorthombic to monoclinic structure was accompained by an increase of volume of about 0.5%. From the results of DSC measurements, the thermal phase transformation of Gd_4Al_2O_9 is reversible and has hysteresis. The transition temperature, defined as the average temprarure of exothermic and endothermic peaks, was 1110ﾟC,which correspond to the phase transformation from low temperature monoclinic to high temperature monoclinic structure. On the other hand, a second transition temperature was observed at 1430ﾟC on heating and at 1420ﾟC on cooling, which would correspond to the phase transformation from monoclinic to orthorhombic structure. Less