Effect of reinforcing shape on twinning in extruded magnesium matrix composites

Author’s Accepted Manuscript Effect of reinforcing shape on twinning in extruded magnesium matrix composites G. Garcés, K. Máthis, P. Pérez, J. Čapek, P. Adeva

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S0921-5093(16)30395-1 http://dx.doi.org/10.1016/j.msea.2016.04.028 MSA33557

To appear in: Materials Science & Engineering A Received date: 13 November 2015 Revised date: 25 February 2016 Accepted date: 8 April 2016 Cite this article as: G. Garcés, K. Máthis, P. Pérez, J. Čapek and P. Adeva, Effect of reinforcing shape on twinning in extruded magnesium matrix c o m p o s i t e s , Materials Science & Engineering A, http://dx.doi.org/10.1016/j.msea.2016.04.028 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Effect of reinforcing shape on twinning in extruded magnesium matrix composites G. Garcés1,*, K. Máthis2, P. Pérez1, J. Čapek2.P. Adeva1. 1

Department of Physical Metallurgy. National Centre for Metallurgical Research

CENIM-CSIC. Av. De Gregorio del Amo 8, 28040 Madrid, Spain. 2

Department of Physics of Materials, Faculty of Mathematics and Physics, Charles

University, Ke Karlovu 5, 121 16 Praha 2, Czech Republic [* corresponding author. Tel +34-91-553-8900; fax: +34-91-534-7425; E-mail address: [email protected]] Abstract The influence of the shape of the ceramic reinforcement, particles or whiskers, on the twinning mechanism in extruded magnesium matrix composites was investigated using in-situ synchrotron radiation diffraction and acoustic emission spectroscopy during compressive tests. The presence of the ceramic reinforcement hindered both twin nucleation and growth. Twinning in both composites is shifted towards higher applied stress, especially in that reinforced by whiskers. The load transfer capacity of SiC whiskers and the higher tensile residual stress developed in the matrix during the extrusion process in the AZ31-10%SiCw composite is superior to that of particles due to their higher aspect ratio.

Keywords: Magnesium alloys; Metal matrix composites; twinning; Synchrotron radiation diffraction; Acoustic Emission

1. Introduction

1

Twinning plays an important role in the deformation of textured magnesium alloys. Compositional and microstructural aspects as well as mechanical test parameters can affect its activation[1-8]. Within microstructural aspects, special attention has been devoted to the effect of the initial presence of precipitates within magnesium grains[914]. In general, precipitates promote twin nucleation but they reduce size, growth rate and volume fraction of twins. With respect to yield stress, ageing shifts twinning to higher stress level[12,14]. On contrary, the effect of incoherent reinforcing particles on twinning is less studied. Garces et al.[15,16] have investigated the evolution of internal strains during in-situ tensile and compressive tests of an extruded AZ31 alloy reinforced with SiC particles. The plasticity of the composite behaves similarly to the unreinforced alloy. In tension, deformation is controlled by the activation of basal slip and, in compression by the

 

activation of the 1011 1012 tensile twinning. However, in compression, the twin activity decreases as the volume fraction of reinforcement increases. During the mechanical test, SiC particles can bear an additional load transferred by the magnesium matrix and therefore, a higher applied stress is required to obtain the same deformation. It is well known that the load transfer capacity of the reinforcement increases with its aspect ratio[17]. Therefore, deformation by twinning must also be influenced by the change in the SiC aspect ratio. The present paper studies the compression behaviour of an extruded AZ31 alloy reinforced with SiC whiskers. For this purpose, the evolution of internal strain has been measured by synchrotron radiation diffraction (SRD) experiments during in-situ compression tests. Moreover, the acoustic emission (AE) response during in-situ compression has also been evaluated to investigate the influence of the reinforcement on the twin nucleation process. These values have been compared

2

with those obtained during in-situ compression tests in the same AZ31 alloy, unreinforced and reinforced with 10 vol.% of SiC particles [16].

2. Experimental procedure AZ31 powders of less than 100 μm in size were blended with 10% volume of SiC whiskers. The silicon carbide, SiC, single crystal whiskers, supplied by Sumitomo (SiC Tokai b, cubic, whiskers; grade TWS-100), were originally 0.3–0.6 m in diameter and 5–15 m in length. The aspect ratio (length/diameter) was between 10 and 40. They are commonly long cylinders with smooth superficial aspect. AZ31 and SiC whiskers were mixed in a planetary ball milling at 120 rpm for 8 hours. After homogenisation, the powders were uniaxial cold compacted by slowly increasing the pressure up to 350 MPa. Compacts of 40 mm in diameter were extruded at 350 ºC employing an extrusion ratio of 25:1 and ram rate of 0.5 mm s-1. Microstructures were examined by optical (OM) and transmission electron (TEM) microscopy. Samples for OM were prepared by mechanical polishing and finishing with an etching solution of 5 ml acetic acid, 20 ml water and 25 ml picric acid in methanol. The size of magnesium grains were measured counting a minimum of 500 grains from images taken using OM. Statistical analyses were carried out with the software Sigma Scan Pro and taking the grain size as the average value obtained. Specimens for TEM were prepared by twin jet electrolytic polishing using a mixture of 25% nitric acid in methanol at -20ºC and a voltage of 20 V, followed by ion milling at liquid nitrogen temperature to remove a remnant fine oxide film. Tensile and compression tests were carried out to evaluate the mechanical properties of the composite reinforced with whiskers. Cylindrical samples (3 mm in diameter and gauge length of 10 mm) for tensile tests and cylinders (6 mm in diameter and 11 mm

3

length) for compression tests were machined from the extruded bar with its longer dimension parallel to the extrusion direction. Tensile and compressive tests were performed at room temperature in a universal tensile machine under a constant crosshead speed corresponding to an initial strain rate of 10-4 s-1. Lubricant was used in compression test to avoid barrelling. In-situ SRD experiments were carried out on the beamline EDDI at BESSY, Berlin, Germany. The specimens were compressed uniaxially using a servo-hydraulic rig capable of a 20 kN maximum load. The rig was placed in a cradle that can rotate 90º. Cylindrical samples of 6 mm in diameter and 11 mm in length were used for compression tests, which were performed in load control in the elastic regime and in strain control in the plastic regime. The energy range of the synchrotron white beam was from 10 to 135 keV. The diffraction angle was 2 = 10º. Regarding the slit settings, the resulting gauge was a rhomboid prism of 0.5x0.5x5.6 mm. The gauge volume was always positioned in the centre of the cylinder. The sample was tilted within the scattering plane between  = 0º (axial direction) and  = 90º (radial direction), where  is defined as the angle between the scattering vector and the extrusion axis. A cylindrical co-ordinate system (axial and radial directions) was adopted due to the symmetry of the extrusion process. The principal stress system coincided with the sample geometrical system. When the extrusion axis was parallel to the scattering vector q, the axial strain component was measured, and when the extrusion axis was perpendicular to q, the radial component was measured. The use of a white beam allowed the entire diffraction pattern to be collected for each

angle. The fibre texture with the basal plane parallel to the extrusion direction resulted in a low signal of the {0002} peak in the longitudinal direction. Individual diffraction

4

peaks were obtained for each diffraction pattern and fitted with a Gaussian curve to determine the peak position and peak intensity. This analysis was carried out using the provided software package at the EDDI beamline[18]. The resulting information allowed lattice spacing values, d, to be obtained using Bragg's law: d

hc 2sin  E

(1)

where h is Planck's constant, c is the speed of light,  is the diffraction angle and E the peak energy. The elastic strain in the principal direction i (axial or radial) is given by: (2) where d0 is the lattice spacing of the stress-free value and di is the lattice spacing measured when the diffraction vector is parallel to the principal direction i (axial or radial). The stress-free value was measured using powders of the AZ31 alloy heattreated at 350 ºC for 1 h. The AE measurements were performed during separate deformation tests, realized at a constant cross head speed, which gives an initial strain rate of 10-3 s-1. The specimens had a dimension of 6x4x4 mm3. The DAKEL-XEDO-3 facility incorporated a highly sensitive Dakel Microtransducer (DAKEL-ZD Rpety, Czech Republic) with a flat response between 50and 650 kHz and a built-in preamplifier giving a gain of ~30 dB. The total gain was about 94 dB. Vacuum grease was applied between the sensor and the specimen in order to ensure good acoustic contact.

3. Results and Discussion Figure 1a shows the microstructure of the AZ31-10vol.%SiCwcomposite. The SiC whiskers are broken during the extrusion process and their aspect ratio comprised between 2 and 5. The whiskers are homogenously distributed in the magnesium matrix

5

and oriented with their longer length along the extrusion direction. The interface between the magnesium matrix and the SiC whiskers shows good adhesion and the absence of pores (Fig. 1b). Figure 2a and b show the grain structure of the composite and the grain histogram. The matrix grain size, evaluated using image analysis, is 2.16±0.04 m. Grains size is slightly smaller compared to the other two materials[15]. It is worthy to note that grain size can be smaller in the vicinities of the SiC whiskers as is observed in the TEM image of Figure 1b. The composite shows the typical extrusion texture observed in extruded magnesium alloys with the basal plane parallel to the extrusion direction (Fig. 3). The texture intensity is similar for the unreinforced AZ31 alloy and for the composite reinforced with SiC, disregarding the shape of the reinforcing particles. Figure 4 shows the compressive and tensile tests for the composite reinforced with SiC whiskers. Tensile and compressive tests for the unreinforced alloy and the composite reinforced with particles are also plotted for comparison [15,16]. Tensile and compressive stress-strain curves are similar for the three materials. Although the tension/compression anisotropy is small for all materials, t/c ratio becomes 1 for the composite reinforced with whiskers. Under compression, when twinning is normally activated in extruded magnesium alloys, the AZ31 alloy shows elastic behaviour up to 250 MPa. After yielding, the stress is kept almost constant for around 3% of the strain, after which both the stress and the work hardening increase again. For composites, the yield stress and work hardening increase with respect to the unreinforced alloy, higher in the case of the SiC whiskers. Figure 5a shows the evolution of internal elastic strains (obtained from Eq. 2) during the compressive test, parallel to the applied stress (axial direction),for the AZ31 alloy reinforced

with

SiC

whiskers.

The

{

̅ },

(0002),

{

̅ },

{

̅ }and

6

{

̅ }diffraction peaks of the magnesium phase and the {

} diffraction peak for the

SiC phase are shown. In previous studies[15,16], it has been demonstrated that axial and radial elastic strains give analogous information. Therefore, for clarity, only elastic internal strains in the axial direction are plotted in this figure. For comparison, the evolution of internal elastic strains during compression for the unreinforced alloy as well as the composite reinforced with particles is also plotted (Figs. 5b and c). In the as-extruded state, residuals strains(RS)in both composites are positive (tensile) in the matrix and negative (compressive) in the reinforcing particles while no RS are observed in the unreinforced alloy. RS are generated during the extrusion process due to differences in CTE values between the metal matrix and the SiC phase[17]. Moreover, it is also reported that RS in magnesium matrix are developed not only for the presence of the SiC particles, but also for its intrinsic plastic anisotropy and they are different for each family of grains[19]. In previous work[15], the SiC phase was used to calculate an average RS in the matrix assuming that this phase born a mean stress, which is generated by all magnesium grain orientations. Then, the total mean stress in the matrix is calculated assuming the stress balance between the matrix and the ceramic phase. Figure 6 plots the residuals strains for the SiC phase as a function of sin2 for both composites. While residual strains in the composite reinforced with particles are almost hydrostatic, the composite reinforced with whiskers also shows a deviatoric component. The total axial stress component as a function of the axial and radial strains in the SiC phase is given by [20]: ̅

(

where E and

)(

)

[(

) ̅ ̅

]

[3]

are the Young´s Modulus and Poisson´s ratio, respectively. Table 1

shows the experimental RS in the axial direction for the magnesium matrix and the SiC

7

phase. These values are compared with those obtained using an Eshelby approach obtained from[16]: ̅

(

)[(

)(

(

))

]

(

)

[4]

where f is the reinforcement volume fraction, S is the Eshelby matrix, CI and CM are the reinforcement and matrix stiffness matrices, and I the identity matrix in the 6-D Voigt space,  m and  f are the coefficients of thermal expansion of the matrix and the inclusions, respectively, and T is the “effective” temperature drop during the cooling process in the composite production, step that generates the misfit strain. For the theoretical calculation, the average thermal expansion coefficients of AZ31 alloy and SiC are 26.8 10-6 K-1 and 210-6 K-1, respectively, ΔT = 200 K and the aspect ratio s=5.Experimental and calculated RS of both phases for composite reinforced with particles and whiskers are in good agreement. RS in the magnesium matrix and the SiC phase are higher in absolute values in the composite reinforced with whiskers. Returning to figure 5, grains oriented with the {

̅ } and {

̅ } planes perpendicular

to the extrusion direction exhibit micro-yielding at low stresses, below the macroscopic yield stress (250-290 MPa), at as noticed by the deviation from linearity at stresses above about 100-175 MPa depending on the material. These grains, which are commonly defined as “soft grains”, are favourable oriented for the activation of basal slip system[14]. However, due to their low volume fraction, the overall macroscopic response shows little evidence of yielding at this low stresses. The applied stress for the activation of the basal slip in “soft grains” increases in composites with respect to the unreinforced alloy especially when the matrix is reinforced with whiskers (around 175 MPa, marked in Fig 5). The SiC phase born an additional load transferred by the magnesium matrix. Therefore, the magnesium matrix in the composites required a higher applied load to achieve the same stress of the unreinforced alloy.

8

On the other hand, in figure 5, grains oriented with { 1010 } and { 1120 } planes perpendicular to the extrusion direction, which are defined as “hard grains”, behave almost elastically up to the macroscopic yield stress. These “hard grains”, which are the majority, are not well oriented for the activation of basal slip. However, their

 

macroscopic yield stress and their deformation are controlled by the tensile 1011 1012

twinning system[15,16,21-24]. Twinning reorients the (0002) planes of twin areas 86º towards the compression axis which results in a continuous increase in the intensity of the (0002) diffraction peak at expense of the decrease in the intensity of the { 1010 } and { 1120 } peaks (Figure 7).Therefore, after yield stress, the internal strain of the (0002) diffraction peak can be measured accurately giving the stress state information of twins. It is expected that (0002) plane within twins should be initially under tension just after rotation and it is relaxed rapidly. After yield stress, the internal strain of twins, oriented with the (0002) plane perpendicular to the extrusion direction, increases rapidly because the crystallographic orientation of twins inhibits the deformation by dislocations in the

 

basal system as well as by twinning in the tensile 1011 1012 system and harden the magnesium matrix. As it was commented above, the increase in the integrated intensity of (0002) grains in the axial direction represents twins generated in grains oriented with the { 1010 } and {

1120 } planes perpendicular to the extrusion directions (Fig. 7). However, with the same macroscopic engineering strain, the volume fraction of twins decreases in the composites, especially in the alloy reinforced with whiskers. Moreover, the volume fraction of twins tends to saturate rapidly in the composite, while the unreinforced matrix can continue deforming by twinning (Fig. 7). In other words, it seems that the maximum volume fraction of twins (twin growth) decreases with SiC particles.

9

Unfortunately, SRD cannot give information about twin nucleation. Then, in-situ AE measurements have been used to study twin nucleation.The evaluation of the root main square (RMS) of the AE signal of particular specimens with the applied stress and the corresponding stress-strain curves are depicted in figure 8. It is obvious that the AE response strongly depends on the reinforcement content of specimens. For all studied materials, there is a weak AE activity already in the elastic regime. This feature is in good agreement with the diffraction data and, as mentioned above, it is a consequence of the microplasticity caused by local basal slip in soft grains[25-28]. The “elastic AE activity” is more marked for the non-reinforced alloy. For reinforced materials it is less significant, most likely owing to the internal stresses, which hinder local deformation mechanisms. A synergic effect of dislocation slip and twin nucleation is in the background of the AE peak, perceptible in the vicinity of the macroscopic yield stress[25,27]. The “peak size” again depends on the reinforcement content. The recent statistical analysis of AE signal, published by Vinogradov et al.[26], indicates that the extension twinning contributes to this peak with the higher extent. Thus it seems reasonable to assume that the twinning activity is the largest in the pure alloy, whereas in reinforced samples it is less pronounced. The AE data characterize only the twin nucleation, which is a result of collective motion of several hundred dislocations. In contrast, the extent of twin growth, which is not detectable with AE owing to the small amount of energy released during this process, is included in the diffraction data. So if we summing-up the results of the diffraction and AE experiments we can conclude that the presence of reinforcement hindered both twin nucleation and growth. The decrease of the RMS at later stage of deformation can be ascribed to the both “inaudible” twin growth and reduced mean free path of the dislocations caused by increasing dislocation

10

density. Nevertheless, some burst-type signal are still present indicating that at higher stresses twinning can take place also in less favorable oriented grains. The mechanism that reduces twin activity (nucleation and growth) in magnesium matrix composite has therefore two origins. On one hand, since RS in the matrix in both composites are under tension, a higher applied stress is required to reach the same deformation by twinning compared with the unreinforced alloys. Moreover, comparing both composite, the curve for the composite reinforced with whisker is similar in shape but shifted to higher applied stress due to the higher initial tensile RS. On the other hand, the magnesium matrix transfers part of each load to the reinforcement phase. Therefore, the magnesium matrix required again an additional compressive stress for twin nucleation and growth in both composites. AE curves evidence continuous bursting associated with hardening stage in compression curves. However, the intensity is much higher in the case of the alloy reinforced with particles that in the case of the whisker-reinforced alloy. This could be attributed to a more effective load transfer from the magnesium matrix to the SiC whiskers than towards the SiC particles. Consequently higher stresses should be required to nucleate new twins in the material whiskerreinforced AZ31 alloy. Moreover, the smaller grain size of the particle-reinforced alloy compared to the whisker reinforced alloy, 4.8 versus 3 m, should assist twin nucleation [3], as experimentally observed by the high intensity of bursts detected by AE.

4. Conclusions The influence of the reinforcement morphology on twinning in extruded magnesium matrix composites has been measured using synchrotron radiation diffraction and acoustic emission during in-situ compressive tests. The presence of reinforcement significantly hinders both nucleation and growth of twins. This effect is more efficient

11

in the case of the composite reinforced with whiskers since their reinforcement effect is higher compared to particles. Moreover, the composite reinforced with whiskers also shows a higher tensile residuals stress. These two factors delay the onset of twinning until higher applied stress in the reinforced alloys.

Acknowledgements Magnesium Elektron is kindly acknowledged for supplying the AZ31 cast alloy. BESSY (Berlin, Germany) is kindly acknowledged for beamtime on the beamline EDDI. We kindly acknowledge the support of the EU during the measurements at BESSY. We should like to acknowledge financial support of the Spanish Ministry of Economy and Competitiveness under project number MAT2012-34135. The K.M. are grateful for the financial support of the Czech Science Foundation under the contract 14-36566G. References [1] N. Stanford, M.R. Barnett, Inter. J. Plast. 47 (2013) 165–181. [2] O. Muransky, M.R. Barnett, D.G. Carr, S.C. Vogel, E.C. Oliver, Acta Mater. 58 (2010) 1503–1517. [3] M.R. Barnett, Z. Keshavarz, A.G. Beer, D. Atwell, Acta Mater. 52 (2004) 5093– 5103. [4] N.V. Dudamell, I. Ulacia, F. Gálvez, S. Yi, J. Bohlen, D. Letzig, I. Hurtado, M.T. Perez-Prado, Acta Mater. 59 (2011) 6949-6962. [5] N.V. Dudamell, P. Hidalgo-Manrique, A. Chakkedath, Z. Chen, C.J. Boehlert, F. Gálvez, S. Yi, J. Bohlen, D. Letzig, M.T. Pérez-Prado, Mat. Sci. Eng. A 583 (2013) 220-231. [6] N. Stanford, R.K.W. Marceau, M.R. Barnett, Acta Mater.82 (2015) 447-456.

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[7] A. Jain, S.R. Agnew, Mat. Sci. Eng. A 462 (2007) 29-36. [8] D. Sarker, D.L. Chen, Mat. Sci. Eng. A 596 (2014) 134-144. [9] N. Stanford,M.R. Barnett Mater. Sci. Eng. A 516 (2009) 226–234 [10] J.D. Robson, N. Stanford b, M.R. Barnett, Scripta Mater. 63 (2010) 823–826. [11] J.D. Robson, N. Stanford, M.R. Barnett, Acta Mater. 59 (2011) 1945–1956. [12] N. Stanford, A.S. Taylor, P. Cizek, F. Siska, M. Ramajayam, M.R. Barnett, Scripta Mater. 67 (2012) 704–707. [13] N. Stanford, J. Geng, Y.B. Chun, C.H.J. Davies, J.F. Nie, M.R. Barnett, Acta Mater. 60 (2012) 218–228. [14] S.R. Agnew, R.P. Mulay, F.J. Polesak III, C.A. Calhoun, J.J. Bhattacharyya, B. Clausen, Acta Mater. 61 (2013) 3769–3780. [15] G. Garcés, E. Oñorbe, P. Pérez, I.A. Denks, P. Adeva, Mat. Sci. Eng. A 523 (2009) 21-26. [16] G. Garcés, E. Oñorbe, P. Pérez, M. Klaus, C. Genzel, P. Adeva, Mat. Sci. Eng. A 533(2012) 119-123. [17] T.W. Clyne and P.J. Withers, An Introduction to Metal Matrix Composites. Cambridge University Press 1993 Cambridge. [18] C. Genzel, I.A. Denks, J. Gibmeier, M. Klaus, G. Wagenes, Nucl. Instr. Meth. Phys. Res. A 578 (2007) 23-33. [19] G. Garces, G. Bruno, Comp. Sci. Tech.66 (2006) 2664-2670. [20] A.D. Krawitz, in: M.T. Hutchings, A.D. Krawitz (Eds.), Conferences ProceedingsBoston, Kluwer Academic Publishers, 1992, p. 405. [21]

S.R. Agnew, D.W. Brown, C.N. Tome, Acta Mater. 54 (2006) 4841-4852.

[22]

O. Muránsky, D.G. Carr, M.R. Barnett, E.C. Oliver, P. Sittner, Mat. Sci. Eng. A

496 (2008) 14-24.

13

[23]

B. Clausen, C.N. Tomé, D.W. Brown, S.R. Agnew, Acta Mater. 56 (2008) 2456-

2468. [24]

D.W. Brown, S.R. Agnew, M.A.M. Bourke, T.M. Holden, S.C. Vogel, C.N.

Tomé, Mater. Sci. .Eng. A 399 (2005) 1-12. [25] P. Dobroň, J. Bohlen, F. Chmelík, P. Lukáč, D. Letzig, K.U. Kainer, Mat. Sci. Eng. A 462 (2007) 307-310. [26] A. Vinogradov, D. Orlov, A. Danyuk, Y. Estrin, Acta Mater. 61 (2013) 2044-2056. [27] J. Čapek, K. Máthis, B. Clausen, J. Stráská, P. Beran, P. Lukáš, Mat. Sci. Eng.A 602 (2014) 25-32. [28] M. Friesel, S.H. Carpenter, J. Acoust. Em. 6 (1984) 11-18.

FIGURE CAPTIONS Figure 1(a,b). Microstructure of extruded AZ31-10%SiCw composite. a) optical and b) TEM. The arrow shows the extrusion direction. Figure 2(a,b). Optical image showing magnesium grains and b) histogram showing the distributions of grain size measured in the AZ31-10%SiCw composite. Figure 3. Intensity of the { 1010 } planes as function of the polar angle for the unreinforced AZ31 alloy and both composites[15]. Figure 4. Tension and compression tests for the AZ31-10%SiCw composite. Data from the AZ31 alloy and AZ31-10%SiCp composite are also included[15,16]. Figure 5. Axial elastic internal strains in the Mg matrix and in the SiC phase for the AZ31 alloy, AZ31-10%SiCp composite and the AZ31-10%SiCw composite. Figure 6. Internal residual strains vs. sin2 ψ plot for the SiC reinforcement (particles and whiskers) for the AZ31-10%SiCw composite.

14

Figure 7. Evolution of the diffraction peak intensity as a function of the applied stress in ̅ }, {

the axial direction for the {

}, {

̅ }and {

̅ } planes of the magnesium

phase and the {111} planes for the SiC phase. Figure 8. Evolution of the root mean square (RMS) of AE signal with applied stress a) for AZ31 alloy; b) AZ31 + 10% SiC particles; c) AZ31 + 10% SiC whiskers. The dotted lines plot the corresponding experimental stress-strain curves.

Table 1.Measured and calculated residual strains for the SiC particles and the matrix for composites reinforced with particles and whiskers.

Experimental

Theoretical

SiC (MPa)

Matrix (MPa)

SiC (MPa)

Matrix (MPa)

Particles

- 266

30

- 261

29

Whiskers

- 711

79

- 549

61

a)

ED b) 15

ED

Figure 1(a,b)Microstructure of extruded AZ31-10%SiCwcomposite. a) optical and b) TEM The arrow shows the extrusion direction.

16

ED

25

Frecuency (%)

20 15 10 5 0 0

2

4

6

Grain sie (m)

8

10

Figure 2 (a,b). OM image showing magnesium grains and b) histogram showing the distributions of grain size measured in the AZ31-10%SiCw composite.

17

Figure 3. Intensity of the { 1010 } planes as function of the polar angle for the unreinforced AZ31 alloy and both composites [15].

Figure4. Tension and compression tests for the AZ31-10%SiCw composite. Data from the AZ31 alloy and AZ31-10%SiCpcomposite are also included[16].

a) MicroY

b) MicroY

c) MicroY

18

Figure 5. Axial elastic internal strains in the Mg matrix and in the SiC phase for the AZ31 alloy, AZ31-10%SiCp composite and the AZ31-10%SiCw composite in the axial directions.

Figure 6. Internal residual strains vs. sin2ψ plot for the SiC reinforcement (particles and whiskers) for theAZ31-10%SiCw composite.

19

Figure 7. Evolution of the diffraction peak intensity as a function of the applied stress in the axial direction for the { 1010 }, {0002}, { 1011 }and { 1120 } planes of the magnesium phase and the {111} planes for the SiC phase.

a)

b)

c)

Figure 8. Evolution of the root mean square (RMS) of AE signal with applied stress a) for AZ31 alloy; b) AZ31 + 10% SiC particles; c) AZ31 + 10% SiC whiskers. The dotted lines plot the corresponding experimental stress-strain curves.

20