Total Deformation and its Role in Heavy Precipitation Events Associated with Deformation-Dominant Flow Patterns

In this paper, it is elucidated that the total deformation （TD）, defined as the square root of the sum of squared stretching deformation and squared shearing deformation, is an invariant independent of the coordinate system used. An idealized flow field is then constructed to demonstrate the confluence effect of a non-divergent and irrotational deformation field on moisture transport. To explore the characteristics and role of TD, one heavy rainfall case that occurred in the middle and lower reaches of the Yangtze River （MRYR） over China, associated with a front with shear line, is analyzed using the Weather Research and Forecasting （WRF） model output data. It is found that right before the occurrence of precipitation, the effect of the confluence induced by deformation on moisture transport provides a favorable condition for precipitation. During the precipitation, both location and orientation of the zone of large TD coincide with the confluent shear line. The rainhands are nearly parallel with, and located lightly to the south of the zones of large TD and the confluent shear line. The TD in the lower troposphere increases in value as precipitation persists. When TD approaches its maximal value, the next 6-hour precipitation reaches its peak correspondingly. A tendency equation for TD is derived. The analysis of linear correlation and RMS difference between individual terms in the total deformation equation and the sum of the terms shows that the pressure gradient plays a major role in determining the local change of total deformation.

冷轧预变形对7xxx系铝合金板材析出行为的影响

采用硬度测试、扫描电镜、X射线衍射和DSC研究了冷轧预变形对7xxx系合金析出行为的影响。结果表明,40%冷轧预变形处理使合金120℃时效达到峰值的时间由18h缩短至6h。同时SEM统计表明,经冷轧预变形处理的合金在更短的时间内MgZn2相析出的体积分数达到更高的值;利用差示量热法结合JMA方程定量计算得出,经40%预变形后合金的η′相析出激活能由123kJ/mol降至111kJ/mol,MgZn2相的析出激活能由82kJ/mol降至67kJ/mol。

AA7021铝合金热变形行为及微观组织演变机理的研究

采用Gleeble-3500热模拟试验机对AA7021铝合金在变形温度为350~490℃、应变速率为0.01~10 s^-1的热变形条件下进行热压缩试验。建立基于应变的本构方程以及材料热变形特征的热加工图,并对热加工图中安全区和失稳区的显微组织进行分析。结果表明,在安全区有形变诱导析出;在变形失稳区内,当变形温度较低、应变速率较高时,由于应变热效应的作用,形成了绝热剪切带。另外,在应变速率大于1 s^-1的区域中发现导致铝合金热加工性能变差的原因有局部流变、大粒子破碎脱粘、微观裂纹等。在热变形过程中,随着温度的升高,铝合金的动态软化机制由动态回复转向动态再结晶。AA7021铝合金在中温、高温热压缩过程中共存着多种软化机制,但动态回复占主导地位。

铈对Cu-Zr合金热变形激活能和性能的影响

在Gleeble-1500D热模拟机上对Cu-1%Zr和Cu-1%Zr-0.15%Ce两种合金在变形温度分别为550,650,750,850和900℃,应变速率分别为0.001,0.010,0.100,1.000和10.000 s^(-1)条件下进行等温热压缩实验,分析了Ce对Cu-1%Zr合金热变形激活能的影响。通过透射电子显微镜,研究了合金在时效过程中的析出相和位错组态。结果表明:Cu-1%Zr和Cu-1%Zr-0.15%Ce两种合金具有相似的热压缩变形特征。高Zr和微量Ce的添加使Cu-Zr合金的热变形激活能显著提高。与Cu-1%Zr合金相比,添加0.15%Ce使合金的热变形激活能提高了约34%。添加Ce,使导电率下降了约5%IACS,显微硬度略有提高。通过导电率的变化,计算出时效过程中析出相体积分数,求得550℃时合金的析出动力学方程和导电率方程。