| Effects of Zn and Mn content on the tensile properties and microstructure of Mg-Zn-Mn wrought magnesium alloys, precipitate evolution during one- and two-step aging and crystallographic characteristics of?-Mn phases have been systematically ivestigated by means of optical microscopy (OM), scanning electron microscopy (SEM) equipped with energy dispersive spectronmetry (EDS), X-ray diffraction (XRD) and Transmission electron microscopy (TEM). In order to getting a far better understanding of precipitation feature, in-situ resistivity measurement and hardness test were used to quantitatively determine the phase transformation kinetics in Mg-6wt.% Zn-1wt.%Mn(ZM61) alloy during one- and two-step aging , in which Johnson-Mehl-Avrami (JMA) equations and‘Time-Temperature-Transformation'(TTT) curves are deduced. Moreover, yield strength evolution of ZM61 alloy during the early stage of one-step aging was simulated by modelling.Research on the effect of Zn content shows that:?the tensile properties of as-extruded alloys are not sensitive to Zn content, both yield strength?0.2 and tensile strength?b show slight variation and elongation is maintained at 13% with the increase in Zn content. After T6 treatment,?0.2 and?b increase largely and?falls slightly as Zn content rises from 4wt.% to 6wt.%,however, as Zn content rises from 6wt.% to 9wt.%, only a small increase in?0.2 and?b occurs but?falls largely. In contrast to one-step aging, two-step aging has better strengthening effect but lower?. Consequently, 6 wt. % is the best Zn content for the Mg-xZn-1Mn alloys to possess the optimum combination property.?While ZM61 is treated by one-step aging at 180?, hardness reaches to peak value at 3.5h, the main phase transformation before peak aging is the increase in number density and length of rod-like particles,the number density rises to maximum at peak aging, the main phase transformation during over-aging is the coarsening of rod-like particles and increase in the number of disc-like particles, it is worth noting that severely coarsened rod-like particles always heterogeneously nucleated on?-Mn particles.?While ZM61 is treated by two-step aging at 180, only 1 hour is needed for hardness to approach peak value, a vast number of G.P. zones with 5nm in diameter are present in as-pre-aged microstructure, no disc-like particle can be seen in microstructure as aging time is less than 20min, indicating that G.P. zones can only act as the heterogeneous nuclei towards rod-like particles, disc-like particles gradually increase during over aging.?As ZM61 is treated by one- and two-step aging at 230?, time needed to rearch peak aging is shortened obviously (one-step aging: 2h, two-step aging 0.5h). As two-step aging runs for 7h, although the alloy has been under over aging, almost all precipitates are short rods, meanwhile, disc-like particles are rare, indicating that both high temperature and G.P.-zone-nucleation-mechanism retard the precipitation of disc-like particles.?In the as-one- and two-step-aged microstructure, dislocation-nucleation-mechanism results in regular alignment of precipitates along dislocations.Research on the effect of Mn content shows that:??-Mn particles can be precipitated out during homogenization, hot-extrusion and solutionization; the cross-section morphologies of?-Mn precipitates include spheric shape, polygon and rod.?It has been found by investigating the crystallographic characteristics of an?-Mn particle with regular hexagon morphology that: the six facets of this?-Mn particle all belong to?110??-Mn crystal plane family, if all exterior surfaces belong to?110??-Mn, the three-dimensional morphology of this?-Mn particle will be a icosahedron. The orientation relationship between this?-Mn particle and matrix is (1 00)Mg// ( 321)Mn; [ 041]Mg //[111]Mn.?It has been found by investigating the crystallographic characteristics of a rod-like?-Mn particle that: the rod lies on the base plane of matrix, the long axis is along the close packed direction of matrix.?In as-aged microstructure, it is common that?-Mn can act as the heterogeneous nuclei of?1?rods, it has been found by investigating the crystallographic characteristics of heterogeneous nucleation that: there exist low-index crystal planes between?1?and?-Mn that can form coherent interface, consequently, if heterogeneous nucleation occurs, initial incoherent interface between?-Mn and matrix will be replaced by coherent interface, which can largely reduce surface energy.The investigation in isothermal phase transformation kinetics of ZM61 alloy during one- and two-step aging shows that:?by means of in-situ resistivity measurement, the Avrami indexes of nucleation, growth and coarsening stages during one-step aging are about 0.3, 1 and 0.5, respectively, indicating that the phase transformation mechanism during growth stage is the elongation of?1' rods and that during coarsening stage is the coarsening of?2' discs.“Temperature-Time-Transformation”(TTT)curve of one-step aging indicates that:‘Nose'does not appear in the temperature range 90250?, the precipitation kinetics under 250?is controlled by diffusibility of Zn in Mg and nucleation stage is largely shortened while aging temperature is higher than 150?. The Avrami indexes of growth and coarsen stage of two-step aging are 1 and 0.5, indicating that two-step aging does not change the phase transformation mechanism. TTT curve of two-step aging shows that‘nose'still does not appear in the temperature range 130250?and precipitation kinetics is still controlled by the diffusibility of Zn.?By means of hardness test, most Avrami indexes of one- and two-step aging at different temperature are near to 1, indicating that the phase transformation mechanism before peak aging is the elongation of?1' rods. The phase transformation activation energy of early stage of one- and two-step aging are all near 80KJ/mol. TTT curve shows that‘nose'is not present; two-step aging largely accelerates the phase transformation rate.Mechanical properties evolution of ZM61 during the early stage of one-step aging is investigated by modeling, the results show that:?the predicted results of model using‘strengthening equation 2'(Orowan strengthening equation presuming a random distribution of precipitates on gliding planes) indicate that:the increasing velocity of the rod length is much faster than that of rod diameter; the‘fv-t'curve deduced by‘homogeneous nucleation theory'(HNT) rises by exponential form, while‘fv-t'curve deduced by JMA equation rises by‘S'form. As fv is deduced by JMA equation, yield strength curve has a maximal point, as fv is deduced by HNT, yield strength rises much slowly in the early stage of aging but much faster while peak aging is approached and no maximal point appear. Analysis of the formation of maximal point shows that: both volume fraction and diameter of?1' rods determine the yield strength, increase in volume fraction of?1' rods is not certain to raise yield strength, since increase in rod diameter will reduce yield strength.?By contrasting the predicted results of model using‘strengthening equation 2'and model using‘strengthening equation 1'(Orowan strengthening equation presuming that precipitates are arranged to form regular-triangle arrays), it can be found that: the yield strength values predicted by‘strengthening equation 1'are entirely larger than those predicted by‘strengthening equation 2', indicating that precipitates arranged as regular-triangle arrays bring about extremely large strengthening effect. |
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