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。相变增韧需要应力诱导颗粒的马氏体相变，这在四方氧化锆陶瓷中是众所周知的。本研究的主要目的是寻找具有应力诱导相变的新型氧化物陶瓷。本文研究了(R_&lt；1-x&gt；La_x)_4Al_2O_9和Gd_4Al_2O_9/MgO复合材料中正交相与单斜相的关系、室温正交相失稳的可能因素以及热性能和力学性能。在(Ho_&lt；1-x&gt；La_x)_4Al_2O_9和(Gd_&lt；1-x&gt；La_x)_4Al_2O_9的组成范围分别为0*x&lt；0.5和0*x&lt；0.3的条件下，在空气中1700ﾟC反应烧结30h，得到了具有正交结构的单相固溶体。在这些组成范围以上，钙钛矿型LaAlO_3以第二相的形式共存。亚稳正交p-…在(Ho_&lt；1-x&gt；La_x)_4Al_2O_9和(Gd_&lt；1-x&gt；La_x)_4Al_2O_9中，当x=0和x=0.2时，当晶粒度大于6.2um(x=0)、3.3um(x=0.2)和2.7um(x=0.2um)时，均能淬火到室温。在抛光过程中发生了应力诱导相变。这一应力诱导相变伴随着约0.5%的体积增加。DSC测试结果表明，Gd_4Al_2O_9的热相变是可逆的，且具有滞后性。转变温度定义为放热峰和吸热峰的平均温度为1110ﾟC，对应于从低温单斜晶系向高温单斜晶系的相变。另一方面，在1430ﾟC加热和1420ﾟC冷却时观察到第二相变温度，这对应于从单斜晶系向斜晶系结构的相变。较少

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