氧化铝粉末系统微结构对θ到α相转换温度的影响

Microstructure-controlled Effects on Temperature Reduction for θ- to α-Al_2O_3 Phase Transformation

以一平均晶径15.2nm的θ-氧化铝粉末,通过(1)原取得(asreceived)粉末,(2)利用机械搅拌调制均匀分散粉料,(3)将均匀分散粉料制成高密度坯体(单轴压500MPa)三种方式,改变其粒体间的距离及其均质度,了解氧化铝粉末θ-到α-相转换温度的下降现象.研究系通过DTA曲线,配合等温试验,由晶粒成长速率式计算θ-晶粒成长活化能,来进行三系统的相变观察.结果显示,提高均质程度,及缩短晶粒间的距离,均有助于相变发生温度的下降及时间的缩短,而θ-晶粒所需成长活化能也减少.后者经计算,经均质化并拉近粒体间距离可致该活化能减半,由原需300kJ/mol降至150kJ/mol.

The inter-particle distance effects on the temperature reduction of θ-to α-phase transformation based on microstructure deviation were examined using DTA techniques.Three powder systems derived from the same θ-powder of average crystallite size 15.2 nm were prepared,with intention to create different microstructure for each powder systems as:(1) as-received,(2) pre-treated by homogenization with a mechanical stirring accompanied with pH adjustment for dispersion,and(3) homogenized and additionally uniaxial-pressed to compacts with higher bulk density.Activation energies of θ-crystallite growth occurring in the three powder systems were also obtained based on isothermal model of grain growth rate equation.It is found that the temperature reduction characteristics can be related to the homogeneity as well as inter θ-Al2O3 crystallite distances behaved by the θ-crystallites.With higher homogeneity and shorter inter-crystallite distance for the θ-powder systems may favor the occurrence of phase transformation at lower temperatures with a shorter duration.Furthermore,activation energies of θ-crystallite growth can be reduced.The reduction of the pressed samples prepared by homogenized powders can be up to 50% compared with the as-received samples.The values of the as-received and pressed samples were nearly 300 and 150 kJ/mol,respectively.