| Compared with traditional alloys,metallic glasses display excellent properties due to their unique disordered structures,which makes them become one good candidate as the engineering materials.However,some shortcomings also can be found in metallic glasses,such as the brittleness at room temperature.In the process of exploring improvement method it is found that ?-relaxation and?-relaxation in metallic glasses are closely related to the mechanical properties and the glass transition.Specially,amorphous materials generally display better ductility with an obvious ?-relaxation behavior.However,not all metallic glasses show such a characteristic feature.Therefore,it is important to clarify the relationship between the compositions and ?-relaxation sbehaviors in metallic glasses.In this work,La64CO20Al16,La60Co20Al20,La55Co20Al25,(La0.78Al0.22)xaCo1-x(x=0.63,0.73 and 0.83),Zr65Co30Al5,Zr55Co40Al5,Zr12.5Co82.5Al5,Zr10Co85Al5,(ZrxLa1-x)57Co27Al16(x=0,0.8,0.3,0.4,and 1)metallic glasses were selected to study the compositional sensitivity of?-relaxation behavior and the corresponding mechanical performance by dynamic mechanical Analysis(DMA).differential scanning calorimetry(DSC).x-ray diffraction(XRD).scanning electron microscopy(SEM)and so on.For La-based metallic glasses:(1)La64Co20Al16,La60Co20Al20,and La55Co20Al25 metallic glasses were prepared by rapid solidification to investigate the effect of the element Al addition on the ?-relaxation behaviors in La-Co-Al metallic glasses.According to DMA and DSC measurments.it was found that the increase of Al content can promote the-peak temperature of ?-relaxation(T?iop)tO shift to high temperatures.These observations are similar to the change of the glass transition temperature(Tg)with the addition of Al.However,the Al addition only shows little effect on the separation degree of ?-and ?-relaxation behaviors.Besides,the measured T?top of the investigated Zr-Co-Al metallic glasses shifts to high temperatures with the increase of the frequency.On the other hand,the activation energy E? calculated by Ahinnius equation increases with the increase of Al content and the variation of E? with A1 element content is similar to that of mixing enthalpy(?Hmix),it is the highest when the content of Al element reaches 25 at.%.(2)Three kinds of amorphous alloy ribbons(La0.78Al0.22)xCo1-x(x=0.6,0.7 and 0.8)were prepared by rapid solidification method,and the effect of Co addition on the ?-relaxation in La-Co-Al metallic glasses was investigated.The experimental results show that the ?-relaxation behaviors are not obvious at a low frequency of 0.3 Hz.When the applied frenquency is enhanced to 0.5,1,2,and 4 Hz,the ?-relaxation peak becomes more obvious.Moreover,the ?-relaxation peak is the most obvious change with 37 at.%Co addition in La-Co-Al metallic glasses.The frequencies of?-relaxation peak are more concentrated when the Co element reaches 37 at.%.The corresponding activation energy(E?)calculated by Ahinnius equation slightly increases with the increase of Co content,whose change is similar to that of the mixing enthalpy of the whole systems.Accoring to both experiments mentioned above,it can be inferred that the ?-relaxation behaviors may be related to the mixing enthalpy of La-baed glass-forming system.For Zr-based metallic glasses:(1)Zr65Co30Al5,Zr55Co4oAl5,Zr12.5Co82.5Al5 and Zr10Co85Al5 amorphous ribbons were prepared by rapid solidification method.The DMA results exhibit that the increase of Co content will promote the separation of ?-and a-relaxation behaviors to some extent.During experiments,the frequency is low,the ?-relaxation behavior appears as a relaxation wing,while the relaxation shoulder can be found with the increase of frequency.(2)Based on the DSC results,it was found that the glass transition activation energy[Eg)of Zr65Co30Al5 metallic glass was a little lower than others.With the increase of Co content,the slight increase of the Eg shows a change tendency similar to the change of ?-relaxation.These results indicated the reistance to glass transition has a large influence on the occurrence of ?-relaxation in Zr-based metallic glasses.Finally,(ZrxLa1-x)57Co27Al16(x=0,0.8,0.3,0.4,and 1)metallic glasses were prepared by rapid solidification method to explore the ?-relaxation behaviors and mechanical properties.It is found that:(1)Dynamic mechanical analysis(DMA)shows that the weak ?-relaxation excess wing occur's in Zr57Co27Al16 amorphous alloy at about 0.7Tg together with an obvious a-relaxation peak.The weak?-relaxation excess wing becomes more obvious with the increase of applied frequency.On the contrary,there exists an obvious ?-relaxation peak near 0.7Tg and the corresponding relaxation peak frequency gradually moves to a high value with the increase of temperature in La57Co27Al16 metallic glasses.(2)The mechanical performances of two kinds of Zr57Co27Al16 and La57Co27Al16 metallic glasses show that when the ?-relaxation exists in the form of obvious peak or excess wing,obvious plastic strain can be obtained near about 0.7Tg.(3)An obvious ?-relaxation peak can also be observed in micro-scale phase-seperating(ZrxLa1-x)57Co27Al16(x=0.8,0.3,0.4)metallic glasses.The loss factor tan ? which is closely related to ?-relaxation can be well fitted by Cole-Cole function in isothermal multi-frequency dynamic mechanical test results.(4)Although the(ZrxLa1-x)57Co27Al16,(x=0.8,0.3,and 0.4)metallicg lasses near 0.7Tg exhibit obvious ?-relaxation,they are still brittle at this temperature region.This may be due to the double glass transition during heating.The measured ?-relaxation is only contributed by La-rich amorphous components but not Zr-rich amorphous components.Therefore,it can be inferred that the weak bonding region adjacent to the flow unit can not be fully activated around 0.7Tg in the whole matrix.which results in the intrinsic brittleness of micron-scale phase-separating metallic glasses.The current investigations suggest that the micro-scale phase-separating is not an effective way to tune the ductility of MGs. |
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