Viscoelastic and Plastic Behaviour of Metallic and Other Glasses near the Glass Transition

Results about micromechanical properties (internal friction and creep) of several glasses (metallic, semi-conductor and oxydes glass) are presented : they seem more characteristic of the vitreous state itself than the chemical nature of the glass. The assumption about the formation of shear microdomains leads to the interpretation of the results which might be a contribution to a better knowledge of the "homogeneous" deformation of vitreous solids. In the large deformation range, it is well known that two modes of deformation can be observed in glassy materials, particularly in metallic glasses : an homogeneous modc at high temperature and inhomogeneous slip at low temperature. The present paper is concerned by the former, studied in the low deformation range by measurements of micromechanical properties (internal friction tgt, creep, stress relaxation...). Indeed, such measurements lead to informations about the mobility of structural units when a stress is .qpplied onto a vitreous solid. In these conditions, two relaxation phenomena are generally observed I I I : one (a relaxation)corresponds to the movement of nearly all the structural units leading to permanent strain (viscoplasticity) ; the other ( 6 re1axation)could be the consequence of limited ( localised) movements of some of &he s&ructural units leading to recoverable strain(viicoelastici ty) . In ttie first part, some results about both relaxation phenomena observed in the case of different vitreous solids will be shown ; in the second part, an interpretation of such a behaviour will be proposed. From three exemples (oxydes, semiconductor and metallic glasses), we intend to show how such materials can present a behaviour more characteristic of the vitrous state itself 11-1Oxydes glass The viscoelastic properties of an oxydes glass have been measured between 200 and 550°C 121 ; the glass transition observed by a dilatometric test is about 385 395°C. The curves tg@ in function of the temperature show in the whole frequency range (bet-3 3 ween 10 to 10 Hz) a monotonic increase. Figure I Relation between internal friction (tg# and frequency for different temperatures ; curves : experimental results and open triangles : calculated values obtained from simplified relation (3) : than of their precise chemical nature. Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19808209 A noticeable feature is shown in the figure I with the variation of tg@ versus frequency for several temperatures : for comparison, a straight lice of slcpe 1 corresponding to the MAXWELLTAN viscous flow, is added : it appears that the actual behaviour of the material is the more different from the MAXWELL model, the temperature is low and (or) the period is small : this means that there is not only a simple viscous flow but also a viscoelastic deformation as it has been shown previously : indeed, a recoverable strain does exist when the load is removed during a creep test [ 3,4] . 11-2Semi conductor glass Figure 2 tg+ of vitreous selenium versus measurement frequency for different temperatures. Points : experimental results and curves : values calculated with relation (3) with Y= 0,06 (from the COLE-COLE diagram) &/J,= 1 , l (from the creep curve) and T and T~ given by the low frequency and the high frequency part of the curve tg$(w) respectively Vitreous selenium has been studied in our laboratory 15.61. The figure 2 exhibits the curves tg$(w) : again the behaviour of the glass i: not simply viscous and the same explanations can be given : here again, a strain partially recoverable, partially permanent is obtained during a creep test (fig. 3). Futhermore the effect of the Fipure 3 : Cro-; C-f vi treeus seleniunl .,nd 4ef ormation recovery after unl qading (after quenching from 5Q°C ( b ) , aqe.1 state ( 2 ) and interme(!iery states (c ~ ~ n d d) . structure relaxation can be seen on the same figure : after the glass is aged at a temperature lower than 20°C, the mobility of the structural units is strongly decreased 161, modifying both viscoelastic and viscoplastic properties. 11-3Metallic glass Internal friction of metallic glasses is not yet well known : at temperatures lower than Tg, relaxation peaks are observed in the case of different alloys (see 171 for instance). In the glass transition temperature range a monotonic increase of internal friction is observed till the devitrification occurs, inducing a decrease of internal friction (see 181 for instance). Thus, the alloy Fe-P-C has been studied by YOSHIDA et a1 ( 9 1 : values of activation energy aye obtAined either directly from the curves tg@(T) (ET) or from the shift of these curves when the frequency is modified (Ef) : E is always higher than E f T and this feature is generally explained in terms of distribution of relaxation times 18;91 As with other glasses the viscoelastic behaviour is associated with viscous flow 110) : in the case of an alloy Fe-P-C (*) we have observed that both aspects are present at temperature hirh enough (figure 6). The transient part of the creep curves has been more particularly experimentally studied : the main following features can be put forward : i) at all temperatures, the strain rate at t = 0 9 varies linearly with stress till 1 , 2.10 Pa ; after the transient and chiefly at higher temperature, (*) This alloy has been elaborated by the "laboratoire central de SAINT GOBAIN-AUBERVILLIERS".