Structure and Strength of Rapidly Quenched Cu-Al 2 O 3 Composites
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pure Cu and Alz0
3
and their condensation on glass ceramic substrate at a temperature in the
range of 170-360 T in vacuum 10-3 Pa. Film structure was investigated by X-ray diffractometry and transmission electron microscopy of thinned samples. The composition of films was determined by X-ray microanalyser with an error not more than 5 % for Al. Tensile tests of specimens with working part sizes 15 x 0.04 x 3 mm were carried out under ambient conditions using original micromechanical testing machine of rigid type [2] with relative strain rate k = 6.7 x 104 c1 . RESULTS According to X-ray and electron microscopy analysis, the films in as-deposited state consist of small grain copper matrix with homogeneously distributed amorphous particles Al 201 (Fig. la). With increasing substrate temperature from 170 to 360 TC mean size of copper grains grows from 0.6 to 1.2 ltm whereas mean diameter of A120 3 particles rises from 5 to 16 nm. High temperature annealing of Cu-A120 3 films at 900 TC for 2 hours results in crystallization of amorphous A120 3 particles without noticeable change of their size (Fig. l b) and with some increasing of the equiaxity of their form. At the same time, strength of the films decreases by 10-15 %, while their plasticity grows by a factor of 3-4. It is important to note that electric conductivity of composites in as-deposited state remains at the high level with increasing A120 3 contents and increases after annealing reaching 80-87% of that for pure copper. Ultimate strength of annealed films Cu-2.5 vol. % A120 3 is shown to be up to 0.9 GPa. 279 Mat. Res. Soc. Symp. Proc. Vol. 555 ©1999 Materials Research Society
Fig. 1. Electron microscopy images of Cu+0.5 vol.% A120 3 film in asdeposited state (a) and after annealing at 900'C for two hours (b).
b
pm
a
Itt Analysis of structure parameters and values of yield stress of Cu-A120 3 filns shows that high level of their strength is caused by both dispersion (Orowan mechanism) and grain boundary hardening. 0 In process of tensile tests Cu-2.5 vol.%A A0 , films deposited at higher substrate temperatures (300-360°C) demonstrate strain-stress curves (curve 3 in Fig.2) with fast hardening followed by stationary strain state without any hardening or with slight hardening less than 200 MPa in both as-deposited and annealed states. Films Cu-2.5 vol.%A120 3 deposited at substrate temperature 170 TC have low plasticity, not exceeding strain value of 1-1,5 %, and their tensile diagrams do not have the stationary strain part (curve I in Fig.2). Taking into account the approximation c - ;, + K,", it is convenient for analysis of hardening mechanisms to convert the tensile curves in the coordinates ln(c - ,(T) -lnri, where 7m.5 is elasticity limit and Ed is plastic strain. Figure 2 shows three distinct stages of film deformation: the first stage (1) is linear with index nr0,8-1, the second one (II) is parabolic with index n•0,40,5, and the third stage (Ill) with low strengthening, decreasing with growth of substrate temperature.
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