FAILURE MECHANISM OF Cu-Al 2 O 3 MICRO AND NANOMATERIALS OBSERVED BY "IN-SITU TENSILE TEST IN SEM"

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1 Powder Metallurgy Progress, Vol.14 (2014), No FAILURE MECHANISM OF Cu-Al 2 O 3 MICRO AND NANOMATERIALS OBSERVED BY "IN-SITU TENSILE TEST IN SEM" M. Besterci, K. Sülleiová, B. Ballóková, O. Velgosová, J. Ivan Abstract The method of in situ tensile testing in SEM is suitable for investigations of fracture mechanisms because it enables us to observe and document deformation processes directly. Thus the initiation and development of plastic deformation and fracture can be reliably described. The deformation and fracture mechanisms of Glidcop Al-60 grade with 1.1 wt.% of Al 2 O 3 phase were analyzed before and after ECAP. Before ECAP, the deformation process causes an increasing number of pores and cracking. Decohesion of small Al 2 O 3 particles and clusters occurred and the final fracture path was influenced by the coalescence of cracks. After ECAP, initial cracks were formed in the middle of the specimen, first of all in triple nanograin junctions and, together with decohesion of Al 2 O 3 particles and clusters at small strains, led to failure. Based on the experimental observations, a model of damage and/or fracture mechanisms has been proposed. Keywords: Glidcop Al-60 grade system, mechanical alloying, fracture mechanism, in situ INTRODUCTION Damage mechanisms of composite materials are interesting from theoretical as well as practical point of view. Usually, the fracture micromechanisms of broken specimens after mechanical tests are described. Method of in-situ tensile test in SEM enables us to record and study the deformation process directly [1,2], which means that the initial and developing stages of the plastic deformation and fracture can be characterized more precisely. One of the leading candidates for practical application as an industrial material is called Glidcop, made by SCM Metal Products, Inc [3]. Glidcop is a metal matrix composite prepared by mixing copper primarily with aluminum oxide ceramic particles. We have analyzed, as reported in [4,5], the fracture of experimental Cu-Al 2 O 3 and Cu-TiC systems by direct monitoring of the strain and fracture in a scanning electron microscope by in-situ tensile test in SEM. The purpose of this paper is to describe the fracture mechanism in the Glidcop AL-60 grade system before and after equal-channel angular pressing, ECAP (micro and macro scale) and to propose fracture models. Michal Besterci, Katarína Sülleiová, Beáta Ballóková, Institute of Materials Research, Slovak Academy of Sciences, Košice, Slovak Republic Oksana Velgosová, Faculty of Metallurgy of the Technical University of Košice, Košice, Slovak Republic Jozef Ivan, Institute of Materials and Machine Mechanics, Slovak Academy of Sciences, Bratislava, Slovak Republic

2 Powder Metallurgy Progress, Vol.14 (2014), No EXPERIMENTAL MATERIAL AND METHODS A Glidcop AL-60 grade with 1.1 wt.% of Al 2 O 3 prepared by mechanical milling was used for all experiments, as the material before ECAP marked (A) and after ECAP marked (B). More details on the preparation and properties of experimental material are described in [3]. For the purposes of thi investigation, very small flat tensile test pieces (7x3 mm) were prepared, keeping the loading direction identical to the direction of extrusion. They were ground and polished mechanically. The final operation consisted of double-sided final polishing of specimens with an ion thinning machine to a thickness of approximately 0.1 mm. The test pieces were fitted into special deformation grips inside the scanning electron microscope JEM 100 C, which enables direct observation and measurement of the deformation by ASID-4D equipment. The experimental materials were deformed at 20 C at a strain rate of 6.6x10-4 s -1 in the elastic region. RESULTS AND DISCUSSION Status before ECAP material A Microstructure of the experimental material contains, besides the Al 2 O 3 dispersoid secondary phase, also a low volume fraction, approx. 0.5 vol.%, of closed sharp edged pores (Fig.1a). Al 2 O 3 particles size is less than 10 nm, the particles have a globular shape and occurred separately and also in clusters. Clusters of particles are randomly distributed in the matrix. The size of pores is significantly higher than of the particles, approximately ~1 μm. The approximate size of matrix grains was ~5 μm. During deformation of samples, the first cracks are created on particle clusters and then on pores inside of the sample. Further deformation of samples causes the propagation of a crack due to internal effect of the particle clusters (decohesion of small clusters of spheroid Al 2 O 3 particles with size < 100 nm) and pores. Initiation and crack propagation before ECAP is in the plane of maximum shear stresses e.i. cca at 45 to the direction of tensile loading, Fig.1b. Macroscopic deformation is too small. Resulting fracture is transcrystalline ductile with dimples with size < 1 μm. The dimples have a regular Poisson-type distribution, unlike the pores, which are distributed irregularly. Fig.1. Material before ECAP a) microstructure, b) fracture of the specimen.

3 Powder Metallurgy Progress, Vol.14 (2014), No Status after ECAP material B Equal Channel Angular Pressing was realized at room temperature by two passes by route C [6]. Substructure obtained by TEM in Fig.2a shows Al 2 O 3 particles sized ~ 10 nm, as well as nanograins with size nm. It was found that ECAP refines the grain size of the matrix material. Electron diffraction confirmed the presence the both Cu and Al 2 O 3 phases, Fig.2a. Fig.2. Material after ECAP a) substructure and electron diffraction, b) fractured specimen. The experimental material (B) was deformed at the same conditions as the material before ECAP. Figure 2b shows a fractured specimen without significant plastic deformation. The fracture surface is perpendicular to the direction of loading and crack propagation, caused by normal stresses. Initiation of cracks is likely in triple junctions resp. in nanograin boundaries, what is in agreement with the work [7]. Decohesion of Al 2 O 3 particles and clusters also contributes to crack initiation. A detailed study of the deformation changes showed that the crack initiation was caused by decohesion of the particles, and occasionally also by coalescence of pores. Decohesion is a result of different physical properties of the different phases of the system. The Cu matrix has significantly higher thermal expansion coefficient and lower elastic modulus α = 17.0 x 10-6 K -1, E = GPa than Al 2 O 3 α = 8.3 x 10-6 K -1 and E = 393 GPa. Large differences in the thermal expansion coefficients result in high stress gradients, which arise on the interphase boundaries during hot extrusion. Since α matrix >α particle, high compressive stresses can be expected. However, because the stress gradients arise due to temperature changes, during cooling (which results in increase of the stress peaks) their partial relaxation can occur. The weak interface cannot withstand the external load and the internal stresses are large enough to cause microcracks. So microcracks are mainly initiated by interface decohesion and the failure mechanism of the composite is interface-controlled.

4 Powder Metallurgy Progress, Vol.14 (2014), No Fig.3. Model of the fracture mechanism before ECAP (a, b, c) and after ECAP (d, e, f) Model of the fracture mechanism Before ECAP Based on the microstructural changes observed in the process of deformation, the following model of fracture mechanism before ECAP is proposed, Fig.3a, b, c: a) the microstructure in the initial state is characterized by Al 2 O 3 particles, their clusters and pores in the matrix, b) with increasing tensile load, local cracks on pores occurred, there are cracks formed by decohesion of Al 2 O 3 particles and clusters, c) with further increase of deformation of the material, a crack propagates preferentially along the pores and decohesed particles and clusters at a 45 angle. After ECAP The following fracture mechanism of the material after ECAP was proposed, Fig.3d, e, f: a) initiation of fracture is caused by cracks at triple junctions, as well as by decohesion of Al 2 O 3 particles and clusters in the middle of the specimen, b) fracture is propagated very quickly with minimum plastic deformation, c) fracture is perpendicular to the tensile loading of the specimen. CONCLUSIONS The aim of the study was to analyze fracture mechanism before and after ECAP in the Glidcop AL-60 grade (with 1.1 wt.% of Al 2 O 3 ) system.

5 Powder Metallurgy Progress, Vol.14 (2014), No Based on microstructural changes during the process of deformation, models of fracture mechanism of materials before and after ECAP were proposed. In the material before ECAP, with increasing tensile load, local cracks formed predominantly on pores and decohesion of smaller Al 2 O 3 particles and clusters occurred. Fracture after ECAP is uniform, transcrystalline ductile with dimple morphology. Dimples on the fracture surfaces can be divided into two categories: small (~ 0.1μm) initiated by the discrete Al 2 O 3 particles, and large (~2.5 μm), initiated by clusters of Al 2 O 3 particles. The fracture process takes place in three stages: initiation, growth and coalescence of dimples. Acknowledgement The work was supported by the Slovak National Grant Agency under the Project VEGA 2/0118/14. REFERENCES [1] Velísek, R., Ivan, J.: Metal Mater., vol. 32, 1994, p. 531 [2] Broutman, LV., Krock, RH.: Compos. Mater., vol. 5, 1974, p. 27 [3] and Maintenance/Miscellaneous/tech_info/Glidcop/SCM_Glidcop_product_info.pdf: "GLIDCOP (SCM Product Literature, 1994)", SCM Metal Products. Retrieved [4] Besterci, M., Ivan, J., Kováč, L.: Materials Science and Engineering A, vol , 2001, p. 667 [5] Besterci, M., Ivan, J., Kováč, L., Weissgaerber, T., Sauer, C.: Mat. Letters., vol. 38, 1999, p. 270 [6] Besterci, M., Sülleiová, K., Kvačkaj, T., Kočisko, R.: Int. J. of Materials and Product Technology, vol. 40, 2011, no. 1/2, p. 36 [7] Vratnica, M.: J. Mater. Research., vol. 5, 2012, p. 624