PRECIPITATION KINETICS AND YIELD STRENGTH MODEL FOR NZ30K-Mg ALLOY

Abstract

Age-hardening effect is considerably strong in magnesium alloys containing Nd, making it possible to develop magnesium alloys with low cost and high strength. Although there have been massive researches about the precipitation product sequence and strengthening models in magnesium, aluminum and other light alloys during their ageing processes, those of NZ30K-Mg alloy, a newly-developed magnesium alloy, has not been carefully investigated. The present work mainly focuses on the model of precipitation kinetics and strengthening of NZ30K-Mg alloy. The precipitation kinetics has been investigated using electrical resistivity testing during continuous heating with different heating rates and formulated based on the isoconversional method. Two related model parameters, modified pre-exponential factor A'αrand activation energy Eαrwere respectively determined. The precipitation behavior of NZ30K-Mg alloy during ageing processes can also be intrinsically explained from the variations of Eαrand ln A'αr with the precipitation fraction. This kinetics model with two above-mentioned parameters can accurately describe the precipitation of strengthening phase during different ageing processes. The yield strength of under-aged and peak-aged NZ30K-Mg alloy have been tested and the results show that the testing samples isothermally aged at different temperature from 180 to 250 ℃ have almost the same peak yield strength of about 150 MPa, indicating that the strengthening effect of under-aged and peak-aged NZ30K-Mg alloy is only determined by the precipitation fraction within a certain range of temperatures. The precipitation strengthening model of NZ30KMg alloy has been carefully derived, and the parameter C in the model has then been determined by least squares method based on the tested yield strength data. The value of C is about 93 MPa. The prediction of yield strength of under-aged and peak-aged NZ30K-Mg alloy has been performed and fit well with the tested ones, demonstrating the effectiveness of precipitation strengthening model and its engineering application prospects.