Effect of Rare Earth Upon As-Cast Microstructure and High Temperature Performance of ZA40 Alloy
In testing comparison, this study is made on the modification effect of 0.05 wt.% La-rich rare earth (RE) upon as-cast microstructure, mechanical properties of materials at high temperatures (100, 150, 200°C) and wear resistance under different temperatures (100and 200°C) of ZA40 alloy containing Si. The addition of RE shows that it can not only effectively refine Al-rich and Cu-rich phase, but also reduce the amount of needle-like eutectic silicon, and then make the size of massive primary silicon phase smaller. Thus, there are the improvement of strength, ductility and wear resistance of materials at high temperature. The analyses are also conducted on the mechanism of underlying RE elements.
Hot deformation and processing map of ZA27 alloy
The hot deformation behavior of ZA27 alloy were investigated by compressive tests on Gleeble-1500 thermal-mechanical simulating tester in the temperatures range of 200-350 ℃,strain rate range of 0.01-5 s-1 and engineering strain of 60%.Based on Murty criterion,the processing map for the ZA27 alloy was obtained.The results show that the flow stress decreases with the increase of temperature,while increases with the increase of strain rate.The flow instability regimes appear at two different regimes: at the temperature of 200-210 ℃and the strain of 0.01-0.1 s-1,as well as at the temperature of 250-350 ℃ and the strain rate of 1-5 s-1.The dynamic recrystallization is the main reason for flow softening and stable flow.The hot deformation of ZA27 alloy can be carried out safely in the region with a strain rate of 0.1-1 s-1 and temperature range of 250-350 ℃.
Oxidational wear modelling: Part II. The general theory of oxidational wear
Abstract This paper describes the application of an oxidational wear model to a selection of the results of previously published experiments in which high-chromium ferritic steel pins were slid, with no conventional lubrication, against rotating austenitic stainless steel disks. 400 °C was the chosen disk temperature (TDS) and 2 m s −1 the chosen linear speed of sliding ( U ) at the pin-disk interface. The applied load ( W ) was varied between 12.5 N and 100 N. The wear model involves parameters which are very difficult to measure experimentally such as, for example, the number of contacts ( N ) that comprise the real area of contact and the temperature ( TF ) at the real area of contact. It also involves the thicknesses ( THP and THD ) of the oxide film formed at the real area of contact on the pin and disk respectively. In order to obtain independent estimates of N and TF , the experimental division of heat ( DE ), i.e. the proportion of heat that flows into the pin from the pin-disk interface during sliding, is compared with a previously published theoretical expression for the division of heat ( DTH ) and a numerical method used to deduce values of N and TF which will make DTH equal to DE for the values of THP and THD as measured by scanning electron microscopy. The resulting graphs of “ N vs . W ” show smooth rises of N with increasing load, whereas the radius of each individual contact ( A ) remains fairly constant at about 6 × 10 −6 metres, a trend expected from most models of sliding contact. The graphs of “ DTH vs . W ” and “ TF vs. W ” also reveal a general increase of DTH and TF with increasing load. Using the computed values of N and TF , together with the measured values of THP , it is shown that a consistent and realistic set of values can be obtained for the tribological activation energies ( QPM ) for the mild oxidational wear of this tribosystem, provided that one allows for the oxidation that occurs when an area of contact is momentarily “out-of-contact”, i.e. provided one assumes the general theory of oxidational wear. The graph of “ QPM vs. Load” reveals that an approximately smooth curve can be drawn between the QPM at 12.5 N (namely QPM = 51.46 kJ mol −1 ) and the QPM at 100 N (namely QPM = 44.49 kJ mol −1 ), which indicates an internal consistency. It is also about half the static oxidational energy for ferrous materials, which means that it is much easier to oxidise these surfaces tribologically than statically.