Heterogeneous Nucleation of Theta Prime Precipitates in plastically deformed Al-Cu-Si-Ge Alloys.

M. V. CASTRO RIGLOS; A. TOLLEY

Rio de Janeiro

Congreso; 17th International Microscopy Congress; 2010

Sociedade Brasileira de Microscopía y Microanálise

The precipitation of the metastable θ’ phase in Al-Cu based alloys is a process of interest because ithas been demonstrated to play a very significant role on hardening of these materials. In Al-Cu-Si-Ge alloys, the peak hardness microstructure has been shown to contain a high density of θ’precipitates formed by heterogeneous nucleation on Si-Ge precipitates [1]. The aim of this work is toanalyze in detail the heterogeneous precipitation process of the θ’ precipitates. For this purpose, anAl-2%Cu-0.5%Si-0.5%Ge (at.%) alloy was prepared by arc melting, homogenized at 500ºC andquenched into iced water. A pre-deformation of 8% in tension was applied before artificial annealingat 190ºC in order to accelerate the precipitation process [2]. Peak aged specimens with 1h annealingwere examined using transmission electron microscopy.Figure 1 shows a bright field image obtained close to the [001]Al matrix zone axis. The inset showsthe diffraction pattern with characteristic reflections of the θ´ phase. The most commonly observedfeatures are edge-on plate shaped θ´ precipitates connected to end-on rod-shaped SiGe precipitates,with a rectangular cross section, that are indicated with letters A. These plate-shaped θ´ precipitatesare parallel to the (100)Al and (010)Al planes, while the end-on Si-Ge rods are parallel to [001]Al.Also indicated (as B) are examples of the same kind of precipitate arrangement but in a differentorientation, rotated 90º with respect to features named A. Rod shaped SiGe precipites showcharacteristic Moire fringes that arise from the near coincidence of the (020) matrix and (220) SiGeinterplanar spacings. Following reference [3], the average fringe spacing d, was measured todetermine the lattice parameter a of the SiGe diamond cubic phase. The results were d = (5,40 ±0,05) nm and a = (0.552 ± 0,001) nm. Figure 2 shows a high resolution image of a feature of kind Ain figure 1. The end-on SiGe precipitate is composed of two twin-related variants. The zone axis ofboth twins is [011]SiGe, parallel to the [001]Al, and the twin plane is (1⎯1 1)SiGe, parallel to the (010)Al plane. The long side of the rectangular cross section is parallel to the [2 1⎯1]SiGe direction, in turnparallel to [100]Al. Both variants have the expected orientation relationship for rod-shapedprecipitates in Al-Cu-Si-Ge [1]. The lattice misfits of the SiGe rods with the matrix the along thethree <100> directions were calculated using the measured value of a (Table I). The lowest misfit isalong [011]SiGe (parallel to the long direction of the rod), while the largest misfit is between the(111)SiGe || (010)Al planes that correspond with the shortest dimensions of the rods. Heterogeneousnucleation for the θ´ phase is favorable on the (1⎯1 1 )Si-Ge || (0 1 0)Al interface, that corresponds tothe intermediate misfit between the matrix and the Si-Ge precipitate. The θ´ precipitate is formedperpendicular to the common (1⎯1 1)SiGe twin plane, at one end of the SiGe particle.