The Research On Microstructure, Properties At High Temperature For Mg-Gd-Y-Zn Alloy

Mg-Gd based heat-resistant alloys, which show excellent performance at hightemperatures, can be employed as a new type of high-strength, lightweight structuralmaterials in high temperature occasions. It is a potential candidate for theheat-resistant magnesium alloy material, but the research is still far from adequate. Inthis paper, the heat treatment process, microstructure, mechanical properties, tensilefracture characteristics in in-situ transmission electron microscopy (TEM), as well ashigh temperature oxidation behavior were investigated for three typical Mg-Gd alloys.Under the strain-controlled low cycle fatigue experiment at high temperature, both theinitiation and propagation of fatigue microcrack, and the evolution of themicrostructure of the Mg-Gd-Y alloy were explored. In a high-temperature heattreatment, Mg-Gd-Y alloys exhibit preferred antioxidant ability. In order tounderstand the antioxidant, the microstructre and phase composition of scale werestudied using of scanning electron microscopy, electron microprobe, XRD etc.(1) Mg-1.0Gd-0.50Zn alloyMg-1.0Gd-0.50Zn alloy shows notable hardening effect during the artificialaging. At the peak ageing, precipitate γ″is parallel to the {0001}α-Mgbasal plane.Under the tensile test at175°C, the γ″does not impede the the twin shear. Thedeformation twinning pass through the γ″phase, and the γ″phase at the interface ofthe twins deflect about4.2°along the shearing direction.In order to examine the effect of γ″phase on the crack propagation, the in-situtransmission electron microscopy observations were carried out. The results show thatthe γ″phase, just a few nanometers thickness, can not significantly hinder crackgrowth.The γ″phase, which is parallel to the basal plane of the α-Mg, can be used as amarker to determine the relationship of local tensile stresses, the crack propagationdirection and crystal orientation. The in-situ TEM strain experiments were carried outto study the fracture behaviors of the alloy. When the crack propagatedperpendicularly to γ″phase (i.e. the crack extent along the [0001]α), the basaldislocations slips were difficult to operation for the Schmid factor was nearly equal tozero. As the local stress at the tip of crack exceeded the cohesion strength, theprismatic planes split layer-by-layer along the [0001]αto release the elastic energy. When the crack extended along the random orientation (non-[0001] direction),dislocations emissions led the foil near the tip of crack to thinning gradually. As thestress increased, the micro-voids initiated in the dislocation-free zone (DFZ). Finally,the main crack merged the void, and then propagated forward. In general, the resultsof in-situ TEM strain experiments show that the material of the crack front exhibt aquite different thinning manner for ductility and brittle fracture.(2) Mg-2.0Gd-0.60Y alloyThe Mg-2.0Gd-0.60Y alloy show a significant age-hardening during artificialaging at225℃. In peak aging, the precipiate β′phases play a major role instrenthening materials.The strain controlled low cycle fatigue tests were carried out for aMg-2.0Gd-0.60Y alloy at573K. The cyclic softening responses were observed underthe diverse total strain amplitudes at573K. For high strain amplitudes, the alloyshowed a stronger cyclic softening response than that at low strain amplitudes of0.3%and0.45%. At the lower strain amplitudes, the grain boundaries are considered as theprincipal microcracks resource because of the creep-fatigue cavity inititation at thegrain boundaries. At the higher strain amplitudes, the microcracks, which initiate atthe slip bands, propagate along the grain boundaries.Under the low cyclic loading, abundant twins appeared near the crack. Densedislocations slip leaves the precipitates in a thermodynamically unstable condition.Then those precipitates disintegrate to form a small equilibrium phase duringuninterruptedly cyclic loading at573K. At the precipitation-free zone, dislocationscells, dislocations nets as well as small angle boundaries appeared extensively. Due tothe dense dislocations pile up at the boundaries, strip-shaped grains of the dynamicrecrystallization distribute along the grain boundaries.The selective oxidation was detected under the high temperature for theMg-2.0Gd-0.60Y alloy. According to the XRD and the electron probe microanalysis(EMPA), the scales were believed mainly composed of (Gd0.18Y1.82)O3. The dense richrare-earth scale on the surface can reduce the oxygen inward diffusion, and thusimprove the oxidation resistance of the substrate at high temperatures. As the ingot ofalloy is placed in a box furnace at730℃, the alloy show a good flame retardant.(3) Mg-2.1Gd-1.1Y-0.82Zn alloyThe long-period stacking ordered phase (LPSO) are widely distributed along thegrain boundary after the solution treatment at500°C in the Mg-2.1Gd-1.1Y-0.82Znalloy. At peak aging, the priciapitate β′and β1is believed as the principal strengthening phase. Moreover, a small amount γ″phase, which exists along the α-Mg{0001}, can also be observed. The tensile and yield strength of this alloy at roomtemperature is less than Mg-2.0Gd-0.60Y alloy is, but its strength at300°C exceedMg-2.0Gd-0.60Y alloy.In order to survey the effect of LPSO on the crack propagation, the in-situtransmission electron microscopy observation was carried out. The experiment resultsindicate that the crack prefer to bluntness near the LPSO. With the tension strainincreaseing, the LPSO is gradually thinning, and eventually penetrated by the crack.The low cycle fatigue tests were carried out for a Mg-2.1Gd-1.1Y-0.82Zn alloy at573K. The cyclic softening responses were observed under the diverse total strainamplitudes at573K. In general, the fatigue microcracks initiate at the interfacebetween the the long-period ordered phase and the magnesium matrix, and thenpropagate along the slip bands of Mg. When the fatigue crack extends to the LPSOnear grain boundary, either the crack bluntness or a cavity formation are observed.This shows that the massive LPSO can affect the fatigue crack growth process.The scale of Mg-2.1Gd-1.1Y-0.82Zn alloy was investigated employing scanningelectron microscopy, X-ray diffraction, electron probe microanalysis. The dense oxidefilms are believed to reduce the oxygen inward diffusion, and thus improve theoxidation resistance of the alloy at high temperatures. When the ingot of alloy is set ina box furnace at730℃, the alloy did not burn after melted, which show high safety atfire hazard.