【ZHOU HUIJIU FORUM】Invited Speaker: Prof.Christopher Hutchinson
【讲座一】The fatigue behaviour of underaged Aluminium alloys
时间: 9:30-11:30 am, Nov. 8th, 2016
Aluminium alloys are widely used in moving structures due to their high specific strength, stiffness and environmental resistance. In these applications, the fatigue behavior is also extremely important and is often the cause of component failure. In Steels, the fatigue ratio (e/UTS) is ~0.5. For precipitate strengthened Al alloys it is closer to 0.3. It is now known this is because of the instability in the precipitate structure of Al even when cycling at room temperature.
However, the ability to change the precipitate microstructure by cyclic straining at room temperature is also something that can be exploited to improve the fatigue response. It has been shown that cyclic loading can catalyze profuse dynamic precipitation in underaged Al alloys which can drastically strengthen the alloys. It will be shown in this presentation that the resulting high cycle fatigue behavior is also improved, in some cases leading to lives an order of magnitude greater than the peak aged state.
The results of both LCF and HCF tests on 2xxx and 7xxx series alloys are reported in this presentation. Small angle x-ray scattering (SAXS) and transmission electron microscopy (TEM) has been used to characterize the evolution of the precipitate state during cyclic loading and to help rationalize the fatigue behavior. The distribution of plasticity has been monitored using optical profilometry. The effect of dynamic precipitation during cycling the localization of plasticity, and the evolution of surface roughness leading to crack initiation is discussed. The use of dynamic precipitation as an homogenizer of plasticity is emphasized.
时间: 9:30-11:30 am, Nov. 9th, 2016
One of the current challenges in the automotive steel industry consists of developing new steel solutions with ultra high resistance, in particular, yield strength (YS>1400MPa), while maintaining sufficient ductility and in-use properties such as weldability. Such developments are potentially good candidates for lightweight solutions leading to CO2 savings in the automotive industry. Maraging steels meet many of these design requirements but their high alloying contents means they are prohibitively expensive.
In this context, the current work investigates the development of lean (ie. low solute) ‘Maraging type’ steels. Using a Genetic Algorithm coupled to a computational thermodynamic database, a range of alloy chemistries have been designed that exploit nanoscale precipitation of the G Phase, an intermetallic silicide, in a lath martensite matrix, leading to yield strengths above 2GPa, in comparatively low solute compositions. The precipitation process has been investigated in detail using a combination of atom probe tomography (APT) and small angle x-ray scattering (SAXS) and the mechanical response measured using tensile and compression testing.
Aging lath martensites in the range 500-600°C for several hours, after quenching from 1200°C leads to the formation of G phase precipitates with radii in the range 15-40Å and number densities as high as 1025 m-3 which are amongst the highest number densities so far observed in engineering alloys. The result is a yield strength of 2GPa.
In-situ SAXS experiments allowed measurements of the kinetics of precipitation and when coupled with atom probe measurements of the precipitate compositions, using the theory for strengthening from shearable obstacles, the individual G-phase precipitate strength, as a function of precipitate size, was extracted. The particle strength was observed to depend approximately linearly on size (as is usually assumed) but a deviation at particles radii below 15Å was observed.
The challenges and prospects for lean Maraging metallurgy are discussed.