Influence Of Structural Relaxation And Elemental Addition On The Microstructure And Properties Of The Rare Earth Based Metallic Glasses

In this paper, we have investigated (Ce-La-Pr-Sm-Gd)65Co25Al10bulk metallic glasses(BMG) and melt-spun Gd57.1-xAl42.9Lax(x=0,4,8and12at.%) alloys, respectively. The effectof structural relaxation and element addition on the microstructure, the corrosion behavior,thermodynamic and mechanics properties of metallic glass have been investigated by usingX-ray diffraction (XRD), differential scanning calorimetry (DSC), electrochemicalpolarization measurement, field emission scanning electron microscope (FESEM) andcomputer controlled electronic mechanical testing system.The effect of annealing on microstructure, corrosion resistance and mechanicalproperties of (Ce–La–Pr–Sm–Gd)65Co25Al10BMG:The results of XRD and DSC indicate that RE-BMG annealed below Tgand insuper-cooled liquid region still keeps fully amorphous phase and only results in theoccurrence of structural relaxation. The alloy atom just has location movement when annealedbelow Tg. The thermal stability and the crystallization behavior of the alloy do not changeobviously. While a large amount of atomic collaborative movement may make the clustersstructure and types corresponding to the onset crystallization peak destroyed when annealed inthe super-cooled liquid region of the alloy, which does not meet the requirement ofcomponent and structure for the growth of clusters. In the event, the crystallization issuppressed and a higher crystallization temperature is obtained.Moreover, the correlation between the values of recovery enthalpy and the densitychange of as-annealed alloy can be described by the linear relation during structural relaxation.Namely, the values of recovery enthalpy and the density change show that a trend offirst increased and then decreased with the increase of the annealing temperature. This result isin accord with the change of the inter-atomic distance during the alloy relaxation process.These results indicate that the relaxation process is mainly close-packing degree increasescaused by the disappearance of the free volume when annealed below Tg. Whereas, therelaxation process is mainly a small expansion caused by the change of the cluster structurewhen annealed in the super-cooled liquid region.The result of Cpcurve indicates that the values of Cpincrease with increasing annealing temperature, further indicating the amorphous nature of the relaxation sample. A distinctexothermic peak is observed for the sample annealed at373K for1h in Cpcurves whenheating temperature is come near Tg.However, the exothermic peak gradually disappears withthe prolonging of the annealing time. This may be attributed to that the(Ce-La-Pr-Sm-Gd)65Co25Al10systems have some heterogeneity structural region, which resultin the abnormal relaxation behavior.By comparing the corrosion behavior it is found that the relaxation can obviouslyimprove the corrosion resistance in3.5wt.%NaCl solution of the RE-based BMG. The highcorrosion resistance of the relaxation alloys could be attributed to the reduction of the atomicelectrochemical activity in chloride solution. Compared with the as-cast alloy, the surfacemorphologies of relaxation sample do not exhibit significant generalized corrosion or pitcorrosion phenomenon, the surface oxide film still keeps intact. However, the surfacemorphologies present some nanoscale porous structure when annealed in the super-cooledliquid region. The reasons may be caused by galvanic effects due to the chemicalinhomogeneity.The result of compression test shows that the relaxation state RE65Co25Al10BMGfracture strength is higher than the as-cast alloy. The fracture strength decreases firstly thenincreases with enhancing the annealing temperature for the relaxation sample.The FESEMresult indicated the shear fracture angle was measured to be about35°of the as-cast BMG. Avein-like pattern with a rather uniform arrangement can be observed on the fracture surface ofthe as-cast BMG. The fracture manner of the relaxation sample is caused by a brittle withmultiple pieces, and without obvious vein-like pattern can be observed on the fracture surface.Moreover, the fracture surface exhibited completely different morphology for differentrelaxation sampleThe hardness testing result indicate that the values of hardness increase with theannealing temperature when annealed below Tg. But the hardness has a small decline afterannealed in supercooled liquid region.Effects of La addition on microstructure and corrosion behavior of Gd-Al-based alloys:The results show that the La addition can obviously enhance the glass forming abilityand change the crystallization mode of the Gd-Al-based alloys. The inter-atomic distance increases with increasing La addition in the Gd-Al-based alloys, which may be related to theLa addition cause the lattice expansion and changed the topological disorder.Moreover, the Laaddition can affect the initial crystallization products of the Gd-Al-based alloys and inducethe precipitation of Gd2O3phase from the amorphous matrix.Potentiodynamic polarization measurements demonstrate that the corrosion resistance ofthe as-spun Gd-Al-based alloys is not sensitive to the La addition in3.5wt.%NaCl solution.However, the as-annealed Gd-Al-La alloys have a better corrosion resistance than the as-spunalloys. Simultaneously, the Gd2O3phase is beneficial to form oxide layer easily on the surfaceof the as-annealed samples.