Development of Precipitation Hardening Parameters for High Strength Alloy AA 7068.

The mechanical properties after age hardening heat treatment and the kinetics of related phase transformations of high strength AlZnMgCu alloy AA 7068 were investigated. The experimental work includes differential scanning calorimetry (DSC), differential fast scanning calorimetry (DFSC), sophisticated differential dilatometry (DIL), scanning electron microscopy (SEM), as well as hardness and tensile tests. For the kinetic analysis of quench induced precipitation by dilatometry new metrological methods and evaluation procedures were established. Using DSC, dissolution behaviour during heating to solution annealing temperature was investigated. These experiments allowed for identification of the appropriate temperature and duration for the solution heat treatment. Continuous cooling experiments in DSC, DFSC, and DIL determined the kinetics of quench induced precipitation. DSC and DIL revealed several overlapping precipitation reactions. The critical cooling rate for a complete supersaturation of the solid solution has been identified to be 600 to 800 K/s. At slightly subcritical cooling rates quench induced precipitation results in a direct hardening effect resulting in a technological critical cooling rate of about 100 K/s, i.e., the hardness after ageing reaches a saturation level for cooling rates faster than 100 K/s. Maximum yield strength of above 600 MPa and tensile strength of up to 650 MPa were attained.

Simultaneous measurement of anisotropic magnetoresistance and observation of magnetic domains by Kerr microscopy.

We report on a new instrument, which consists of a Kerr microscope combined with resistance measurements. This setup allows for the recording of magnetic domains while measuring the anisotropic magnetoresistance (AMR). For this purpose the development of a special sample holder and the extension of the measurement software was required. The sample holder is equipped with electrical contacts in such a way to apply a current, measure the voltage, and use it in the Kerr microscope. The extension of the measurement software enables the recording of resistance and Kerr images simultaneous. The new setup allows for a better microscopic understanding of the AMR behavior.