Strain amplitude-dependent internal friction of as-cast high damping magnesium alloy during cyclic vibration
Key Laboratory of Ministry of Education for Conveyance and Equipment, East China Jiaotong University
摘 要：
An investigation on strain amplitude-dependent internal friction (IF) of an as-cast high damping Mg-7 wt% Ni alloy was carried out. In the range of our tested strain amplitudes, whether the strain amplitude is increasing or decreasing, the strain amplitude-dependent IF curve can be pided into two stages: one is the strain amplitude weakly dependent part and the other is the strain amplitude strongly dependent part. However, after several cyclic vibrations, the IF values measured during the strain amplitude increasing are smaller than those obtained during the strain amplitude decreasing. The phenomenon is also observed at 100 ℃. Partial dislocations generate a short-range slip under the cyclic stress to be responsible for it.
作者简介：Di-Qing Wan e-mail: padwan@tom.com;
收稿日期：22 February 2012
基金：financially supported by the Natural Science Foundation of Jiangxi Province(No. 20114BAB216015);Natural Science Foundation of China(No. 50671083);
Strain amplitude-dependent internal friction of as-cast high damping magnesium alloy during cyclic vibration
Abstract：
An investigation on strain amplitude-dependent internal friction (IF) of an as-cast high damping Mg-7 wt% Ni alloy was carried out. In the range of our tested strain amplitudes, whether the strain amplitude is increasing or decreasing, the strain amplitude-dependent IF curve can be pided into two stages: one is the strain amplitude weakly dependent part and the other is the strain amplitude strongly dependent part. However, after several cyclic vibrations, the IF values measured during the strain amplitude increasing are smaller than those obtained during the strain amplitude decreasing. The phenomenon is also observed at 100℃. Partial dislocations generate a short-range slip under the cyclic stress to be responsible for it.
Keyword：
Magnesium alloys; Strain amplitude; Internal friction;
Received： 22 February 2012
1 Introduction
The low density of magnesium alloys combined with good mechanical properties has made them attractive to designers.Consequently,magnesium alloys have been used extensively in transportation,aerospace,defense,and other engineering applications[1–6].Magnesium alloy is also motivated by its high damping capacity[7,8].As we know,the damping material will service in the vibration condition.Hence,it is very important to know its strain amplitude-dependent damping properties[9].Moreover,it is well-accepted that the strain amplitude-dependent internal friction(IF)method is one of the most effective means of investigating the properties and defects of the alloys[10,11].
Generally,with respect to magnesium alloys,IF is weakly dependent on the strain amplitude at low strain amplitudes,whereas,it is strongly dependent on the strain amplitude at high strain amplitudes.Recently,a classic theory to illuminate the dislocations damping is the Granato–Lu¨cke(G–L)theory[12,13],in which the dislocation string vibration and breakaway are employed to illustrate the dislocation damping mechanism.As we know,when the strain reaches large strain amplitudes or the dislocations have a much higher mobility,the dislocation motion is beyond the string vibration and breakaway mode.However,up until recently,very little attention has been paid to this.The aim of this study is to study the strain amplitude dependence IF of a high damping magnesium alloy during several cyclic vibrations.It is very interesting to note that a hysteresis loop has been observed.Furthermore,the mechanism and effect of temperature on hysteresis loop were discussed in detail.
2 Experimental
The as-cast Mg-7 wt%Ni alloy was prepared by melting pure Mg and Mg-30 wt%Ni master alloy in a quartz crucible under the CO2/0.5 vol%SF6gas protection at 700°C.The melt was poured into a steel-mold also with the gas protection during the cooling.Rectangular bending beam specimens for IF measurements with dimensions of 50 mm 95 mm 9 1 mm were machined out using an electric spark cutting method.The IF was measured by using an inverted torsion pendulum(TPA-8)under room temperature and at100°C.The sample was gripped at its two ends and twisted around the longitudinal axis.The mechanical losses were given by the ratio tan u[14].The strain amplitude dependence of IF was tested at various maximum strain amplitudes from 5 9 10-6to 4 9 10-4,which has been continuously extended to higher strain amplitude range.The frequency was fixed to be 1 Hz.Furthermore,another test was also carried out at 100°C.The error of the calculated values was\2%.
Fig.1 Optical microstructure a,backscattered electron image b and EDX result of a-Mg dendrites in as-cast Mg–Ni alloy c
3 Results and discussion
Figure 1a shows the optical microstructure of the as-cast Mg-7 wt%Ni alloy.Two phases(a-Mg solid solution phase and Mg2Ni phase)comprise it.The laminar-like eutectic phase(a-Mg?Mg2Ni phase)is dispersive among the interdendritics.Figure 1b shows the X-ray energydispersive spectra of a-Mg grain in Mg–Ni alloy.The result indicates that it hardly contains Ni within a-Mg grains.The pure a-Mg matrix is the major reason for Mg–Ni alloys exhibiting high damping properties because the dislocation can easily move without solute atoms pinning.
The curves of strain amplitude versus IF are shown in Fig.2,where the IF values are above 0.01,which indicates that this material possesses high damping property.Basically,each curve can be pided into two parts:one is the strain amplitude weakly dependent part and the other is the strain amplitude strongly dependent part.Furthermore at the first certain cycle measurement,it is also found that the IF value is completely reversible,and no amplitude hysteresis loop(the difference between the dependences measured with an increase and a subsequent decrease in the amplitude)appears.During the cycle number increasing however,it is found that the IF value tested while increasing the strain amplitude is smaller than that obtained while decreasing the strain amplitude.It demonstrates that the motion of dislocations becomes complex in this material.
Fig.2 Strain amplitude-dependent IF of a high damping magnesium alloy
Granato–Lu¨cke dislocation damping theory[12,13]is widely accepted to explain the dislocation damping mechanism.The theory adopts a vibration string model to illustrate the dynamic loss and breakaway loss during the increasing stress process.The strain amplitude-dependent IF behavior at the low amplitude stage may be ascribed to continuous pinning–depinning of mobile dislocations by mobile point obstacles.The breakaway of dislocations from pinning agents such as impurities or vacancies results in a strain amplitude-dependent IF.This process could be clearly illustrated in Fig.3.In the case of free applied stress,the length is pinned down by the impurity particles(Fig.3a);for a very small stress(Fig.3b),the loops(Lc)bow out until the breakaway stress is reached.During the breakaway process,a large increase in the dislocation strain occurs(Fig.3c,d).Further increase of the applied stress leads to the creation of forest dislocations and their multiplication(Fig.3f,g)[12,13].
Fig.3 G–L dislocation model illustration
From the G–L theory,it is known that the dislocation movement determined by the applied stress and pinners are the direct factors impacting dislocation damping.Similar results were also reported by Nishiyama and Asano[15]in the high damping magnesium alloys.The authors considered that the increased mobility dislocation density is responsible for it.As we know,the dislocation is of high mobility in the high damping material because the solute has very low solubility in the matrix;hence,the dislocation is very easy to slip even under the small applied stress.In other words,the dislocation cannot only vibrate stringily but also generate short-range slip under the cyclic stress.Figure 4 shows transmission electron microscopy(TEM)images of the alloy with or without cyclic vibration tested.It can be found that the dislocation density is obviously smaller than that in the tested alloy.Furthermore,it can be seen from Fig.4b that the dislocation line is parallel to each other.It finally confirms that the dislocation movement occurs.Therefore,the dislocation generated a shortrange slip under the cyclic stress,which would also be an important reason for the hysteresis loop.
The dislocation movement is also controlled by thermally activated process.With the aid of thermal fluctuations,the dislocation can easily overcome obstacles dispersed in the matrix.Figure 5 shows the results measured at 100°C.At a higher temperature,IF value is obviously smaller than that measured in the decreasing of strain amplitude.
In Fig.5,it can clearly be seen that,at a higher strain amplitude range,IF deviation is less than that of the lower strain amplitude compared to the result in Fig.2.This may be due to the effects of both heat and stress,which cause the mobile dislocation length increasing at low strain amplitude.However,at high strain amplitude,littledeviation occurs in the IF value,which may be due to the weaker pinners that are almost breakaway.The IF was only determined by the mean length of a dislocation between two strong pinning points and by the short-range disloca tion slip which is basically identical both at room tem perature and at 100°C.
Fig.4 TEM of as-cast Mg-7 wt%Ni with or without cyclic vibration testing
Fig.5 Strain amplitude-dependent IF curves at 100°C
4 Conclusion
In summary,the damping versus strain amplitude curve o a high damping Mg-7 wt%Ni alloy mainly exhibits two stages:the first is the strain amplitude weakly dependen part due to pinning–depinning of mobile dislocations by point obstacles and the second is the strain amplitude strongly dependent part due to the breakaway loss.When IF was measured with an increase and a subsequen decrease in the amplitude,it is found that IF values tested while increasing the strain amplitude are smaller than those obtained while decreasing the strain amplitude.Because the dislocation is of high mobility in the high damping magnesium alloys,the phenomenon is attributed to the short range slip of dislocations.At higher temperature,the dislocation generating the short range slip also occupies its mechanism.
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