uctea chamber of metallurgical & materials engineers

UCTEA Chamber of Metallurgical & Materials Engineers Proceedings Book

612 IMMC 2016 | 18th International Metallurgy & Materials Congress

Changes in Elasticity Modulus and Damping in Some Novel Magnesium Alloys at Room and High Temperatures

Çisem Çelik¹, S. Güneş¹, N. Sarıkaya¹, P. Karadayı¹, Y. Türe², A. Ataman³, E. Arkın4, Ç. Yalçın4, G. Palumbo5,6, D. Sorgente6,7, D. Turan³, A. Arslan Kaya¹

¹Muğla Sıtkı Koçman University, ²Şeyh Edabali University, ³Anadolu University, 4Assan Hanil Co., 5Polytechnico di Bari, 6CNR-IFN UOS Bari, 7Universita degli Studi della Basilicata - Türkiye, Italy

Abstract

Light weight magnesium alloys are also known to be advantageous due to their good damping characteristics. Although modulus of elasticity is a good indicator of sound/vibration absorption properties, direct measurements are better to characterize this property. Measurement of Young's modulus via measurements of sound velocity or damping in solid materials are known to give more accurate results. Resonance frequency damping analysis (RFDA) therefore is a highly reliable method to determine elasticity modulus as well as damping characteristics of solids. This study focused on room and high temperature (up to 350 C) measurements of Young's modulus and damping properties of a number of novel magnesium alloys. The same alloys have also been characterized in terms of microstructures and evaluated together with RFDA measurements.

1. Introduction Among other promising engineering features of magnesium, such as good strength-to-weight ratio, low-cost machinability, good heat dissipation characteristic, damping is considered to be the best among structural metals [1-10]. This property, being one of the relatively less focused-on concepts in literature [11], is also a difficult one to observe or accurately measure via conventional mechanical test techniques. Since it is difficult to discern through the modulus of elasticity from a simple stress-strain curve, a more careful evaluation requires measurement techniques using sound waves at a range of

frequencies and determination of the specific frequency(ies) at which the material shows noteworthy damping characteristics. Attenuation of wave propagation in a material occurs through independent and mostly concomitant atomistic mechanisms by which the wave looses energy. In effect, this is the dissipation of elastic strain energy and its conversion into heat. The better heat dissipation property of magnesium thus may be said to play an important role in imparting also good damping characteristic. By nature, this is a time dependant, i.e. anelastic, response of the material. Such an energy loss is also temperature and frequency-dependant and reaches a maximum at a critical frequency. This type of response of materials are related to a number of physical metallurgy processes such as precipitation, ordering, and defect generation / movement, that also collectively comprises all sorts of solute atom/defect interactions as well [12-13]. There also exist the hysteretic response which is independent of the frequency. This portion of the energy loss in damping process is also related to the movement of crystal defects, most notably dislocations [13]. In this case, much like the Bauschinger effect, energy loss is not equal during the movement of the traveling wave in the opposite directions (back and forth). This process, however, can eventually lead to fatigue failure. Stacking fault energy (SFE) concept can also be incorporated into the discussion of damping with a careful analysis by also taking into account the crystal system of the material. As a hypothetical example in case of magnesium, it may be said that lowering the SFE of specifically the prismatic and/or pyramidal

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planes may lead to higher damping via facilitation of birth of dislocations as an anelastic response mechanism. Whereas increasing the specific SFE value of the basal crystal planes should lead to better damping through facilitating the movement of dislocations both in terms of anelastic and hysteretic responses. Albeit an earlier fatigue failure may also be inevitable in the latter case. In order to tailor the damping properties based on SFE, a detailed knowledge regarding the selection of alloying element(s) or their necessary quantities is obviously needed. However, such knowledge unfortunately resides at an infant stage today. Damping for a particular alloy composition, on the other hand, can be discussed in terms of microstructural parameters such as defect density and/or grain size. When existing literature is reviewed

-

considered. In this respect, lower dislocation density and/or larger grain size that corresponds to relatively

-damping characteristics [11]. In general, microstructural parameters that give better strength seem not to favour improved damping capacity [14] [15]. However, as strengthening secondary phase particles LPSO (Long Period Stacking Order) phases have been reported to present an exception, improving both the strength and damping capacity [16-20] based on insufficient experimental evidence, some of these studies [16] are attributing the improved damping to the precipitate matrix interface structure, some [17] relates the phenomenon to the mobility of partial dislocations in the LPSO phases. In a study by Somekawa et. al. [21], although their interpretation was not validated, the importance of solute atoms as weak pinning points for dislocations was demonstrated. Their results indeed showed that mean-free path for dislocations, if longer the attenuation is greater and vice versa, explained the damping behavior of the magnesium alloys as compared to pure magnesium as well as its temperature dependency. In this study, a number of novel magnesium alloys were prepared and compared with each other in terms of resonance damping and modulus of elasticity in their as cast state. 2. Experimental Procedure Alloys were prepared via melting under protective argon atmosphere in steel crucibles and an electrical furnace. All alloying additions were made by using

the relevant master alloys. Alloys were homogenized via very slow cooling in the furnace following solidification. Samples for metallographic inspection were mechanically cut, grinded and polished by employing the conventional metallographic preparation techniques. Use of water was avoided in the last stage of polishing. Etchings were done by treating the polished surfaces with dilute picric acid and ethanol solutions for 2 seconds. Samples were then washed with unhydrous ethanol and immediately dried by blowing air. Hardness measurements were made using standard HV and Brinell testers. Modulus of elasticity and damping characteristics were measured using RFDA (Resonance Frequency Damping Analysis) system. This system displays the damping of a mechanically introduced resonance by hitting the sample (a ping), and calculates the E-

dimensions or weight. The system can take

certain intervals as the temperature is increased. The loss angle, , is the phase angle between stress and strain. The damping may be expressed either by this parameter, or the quality factor Q which represents how sharp and intense a resonance peak is. If the damping is not too large, the relationship is [22]: tan = Q-1. The damping of the material in this study is measured as the internal friction parameter, Q-1, that can be expresses as: Q-1= k fr) where k is the exponential decay parameter of the vibration component of the mechanical resonant frequency fr. Q 1

high damping standard. The precision of Q-1 depends on the suspension of the specimen and on its size. As the external energy losses are relatively smaller for larger specimens, the lower limit of measurable Q-1 starts from 10-3 for small specimens (e.g., <1g) down to 10-5 as the sample size increases. The precision of fr depends on the size and stiffness of the specimen, varying between 10-3 to 10-5 (i.e. 1Hz at 1kHz and

0.1Hz at 10kHz) [23]. 3. Results and Discussion The nominal compositions of the alloys investigated and their mean hardness values are given in Table 1. As-cast microstructures of all the alloys are given in Fig.1. It was observed that almost all grain boundaries were decorated by a second phase particles in Mg-Zn-Ce system (Fig. 1b) as compared to the other two alloys that contained noticeably much less such precipitates along their grain boundaries. On the other hand, the Sn and Pb containing systems (Fig. 1a and b) showed much greater grain sizes and greater densities



in the annealing twins. These annealing twins are seen in all three alloys in all favorably oriented grains as shown in Fig.1. Although it is not discernible in this work Sn containing alloy is very likely to contain densely populated intragranular small sized intermetallic precipitates [24, 25].

Figure 1. As-cast microstructures of alloys: a) Mg-2Sn-2Y, b) Mg-2Zn-2Ce, c) Mg-1Pb-2Y.

Table 1. The nominal compositions of the alloys and their Vickers and Brinell hardness values.

ALLOYS HV (500g-10s)

HB 1/5 3 sn

Mg-2Y-2Sn 41 40.5 Mg-2Zn-2Ce 59 45.2 Mg-2Y-1Pb 52 39.6

It should be noted for comparison of the mechanical properties of the alloys studied, the yield strength of as-cast pure magnesium is about 20 MPa with UTS value of 80 MPa, elongation of 6%, E-modulus of 45GPa, and Brinell hardness of 30. Thus, it can be seen that all the alloying element additions led to considerable increase in hardness with the minimum contribution being in the Sn or Pb containing systems despite their tendency to increase E-modulus. Table. 2 gives the E-modulus values, damping and damping related properties of the alloys for the room temperature. The Sn or Pb containing alloys showed an increase in E-modulus as compared to as-cast pure Mg. It is interesting to note that Mg-Zn-Ce alloy, while having the highest hardness, almost maintained the modulus value of pure Mg. All three alloys displayed the expected trend, i.e. decrease, in terms of E-modulus with temperature as seen in Fig. 2. Table 2. Frequency, loss rate, damping and E-Modulus values at room temperature of the alloys.

Alloy E-

Modulus (GPa)

Freq. (Hz)

Loss rate (1/s)

Damping (x10-6)

Mg-2Y-2Sn 47.5 7291.95 13.30 581 Mg-2Zn-2Ce 44.7 6921.19 13.30 612 Mg-2Y-1Pb 46.2 6040.99 20.00 1052 The damping value of pure magnesium has been reported as 0.074 (Q-1). As a rule of thumb none of its alloys has better damping property than pure magnesium [26]. The changes in damping with temperature and E-modulus in the alloys studied are given in Figs. 3 and 4. These changes showed corresponding trends to each other. The Mg-Zn-Ce system mainly followed the trend of the Mg-Sn-Y system. The relatively small deviations seen in the general trend of Mg-Zn-Ce system in the 100-25 temperature range was attributed to measurement errors and therefore neglected. The changes in damping with temperature and E-modulus were observed to be different for Mg-Pb-Y system than those in the other alloys. While a continuous decrease with temperature was observed in the damping behavior of Mg-Pb-Y system, the other two alloys showed, first, a mild decrease and then a steep

a

b

c



. Changes in damping versus E-modulus showed a corresponding trend for all three alloys as compared to damping versus temperature changes as seen in Fig. 3. Note that the temperature decreases in the same manner to the abscissa of the damping versus E-modulus graph.

Figure 2. Change in E-modulus with temperature.

Figure 3. Changes in damping: a) with temperature and, b) E-modulus. The yielding in magnesium starts with slip in the basal planes and, almost immediately after, followed by twinning. Additional slip systems become active

7]. Since hexagonal crystal system of magnesium possesses a low symmetry and very characteristic sequential behavior in terms of activation of its slip systems and twinning, a detailed discussion on the observed changes in properties with

reference to specific crystal planes may be given as follows: Among the alloying elements used only Zn has some known solubility in Mg. The solubilities of the other alloying elements may be given in decreasing order as Pb, Sn, Y and reaching possibly to zero in the case of Ce. Yytrium addition to magnesium has been known to increase both strength and ductility, while Ce addition seem to have contradicting effects depending on its amount in the alloy while increasing the SFE [28]. There also exist reports showing that combined addition of Zn-Ce to magnesium resulted in mainly strengthening effect [29,30]. Thus, in the alloys studied here Zn and Pb are the most likely elements to create dilute solutions. While Zn is likely to provide only weak pinning points in solution, with its strong effect on SFE, Pb is likely to be influencial on damping due to an additional effect. Although Sn is stronger than Pb in decreasing the SFE, it is more likely to provide strong pinning points by the intermetalic precipitates it forms. Higher hardness of Mg-Zn-Ce system can be explained due to its smaller grain size, greater solute solutioning by Zn, and precipitation due mostly to Ce. The lower hardness of the other two alloys can also be attributed to the lowering of SFE by the alloying elements Pb and Sn [31]. Lower SFE facilitates deformation by facilitating formation of dislocations as well as twinning as an extra deformation mechanism in Mg. Microstructures of Sn and Pb containing systems attest to this phenomenon with the presence of ample amount of annealing twins in the microstructures. Modulus of elasticity, being dependant on the interatomic bonding showed an expected decrease with temperature for all alloy systems. It should be noted that the changes in damping are not reflected by the smooth change in modulus of elasticity with temperature for any of the alloys studied. The variation in damping with E-modulus found not to be smooth and simple. The initial slight decrease in damping up to about 250 C in Mg-2Y-2Sn and Mg-2Zn-2Ce systems may be attributed to interaction of dislocations with weak pinning points, becoming easier with increasing temperature, scavenging the wave energy by local movement of dislocations. The phonon movement in this temperature range also consumes the energy of the vibration wave by convertion into heat, but with a lower energy cost as the modulus drops. On the other hand, the following steep increase in damping after the temperature corresponding to a minima may be explained by activation of non-basal dislocations.

a

b



Such dislocations are known to be activated after -alloys

[27]. Thus, the passing waves can be evisaged to cause movements on some segments of those dislocations which were, until about those temperatures, completely inactive due to their high critical resolved shear stress (CRSS) values. The difference in the temperature of inflection between the Mg-2Y-2Sn and Mg-2Zn-2Ce systems has been interpreted to be a result of the strong effect of Sn in lowering the SFE. Lower SFE leads to difficulty in dislocation movement as their core size together with their strain field increases. Due to this effect, movement of the dislocations on prismatic and pyramidal hexagonal planes is somewhat delayed in Sn-containing alloy due to larger core size of the dislocations compared to that in Mg-Zn-Ce system. The damping characteristics of the Mg-2Y-1Pb alloy need be explained for its much higher room temperature damping value, and for the continuous decrease in damping with temperature. The room temperature damping has to be explained in relation to the relative ease of dislocation movement compared to the other alloys. Such an explanation would also account for the much higher loss rate observed with this alloy system. It is known that both Sn and Pb, Sn being more effective, have a strong reduction effect on SFE [31]. However, SFE should be more critically considered with regard to specific planes. With such a consideration Sn and Pb requires detailed evaluations. Even if some Sn is still in solution after it forms precipitates, the different effects observed in Sn- and Pb-containing alloys indicated that their preferential positions in dilute solid solutions may not be the same, leading to different influences on the SFE values of different slip planes they preferentially reside. It may thus be envisaged that Pb possibly facilitated creation of dislocations more effectively on the basal planes, thus led to much higher damping values starting right from the room temperature. The absence of an inflection point in the damping graph for Pb containing system may only be due to the selected temperature range of measurement, i.e. at a higher temperature an inflection may yet to be encountered. Our study will continue in search of such a temperature threshold. 4. Conclusions A direct correlation between the elastic modulus of an alloy with its damping characteristics does not seem to be possible. Damping must be evaluated in terms of atomistic mechanism by taking into account SFE, presence of weak pinning points such as solute atoms, and the strong pinning points such as precipitates.

It has been observed that Sn as an alloying element, despite its known common, and in fact more effective influence in decreasing SFE does not impart the same damping level to the alloy as Pb does. Both Sn and Pb appear to be effective in increasing the modulus of elasticity. Acknowledgements The authors acknowledge and thank for the financial support provided for this work towards the payment of scholarships to the students involved, for purchasing the necessary materials and equipment, and the travelling of researchers to the following sources:

i) i) TUBITAK under the project number 213M535, within the framework of bilateral agreement with CNR of Italy;

ii) ii) CNR of Italy; and iii) iii) Turkish Ministry Science, Industry and

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