TEM Characterization of oxide dispersions in Al compacts from powder metallurgy.

M. V. CASTRO RIGLOS; MARIO MORENO; MARTIN BALOG; PETER KRIZIK; A. TOLLEY

PRAGA

Congreso; 18TH INTERNATIONAL MICROSCOPY CONGRESS IMC 2014; 2014

SOCIEDAD CHECA DE MICROSCOPIA

Due to their high specific strength and their relatively low cost, Al alloys are of great interest in applications related to the automotive and aerospace industries. Because pure aluminium is not hard enough for many of its target applications, different methods have been applied to enhance its mechanical properties [1,2]. Among such approaches, powder sintering has gainedattention as an alternative method to obtain Al alloys that can provide the benefits of conventional Al alloys but offering superior mechanical properties even at elevated temperature exposures. Over the past years, increasing reliability has made it possible for Al powder metallurgy to expand its implementation scope [3]. In this kind of alloys the mechanical properties are related to the distribution of the in situ introduced Al2O3 dispersoids from the Al powder native oxide skin. The properties of the resulting powder compact can vary significantly depending on the process by which the compact is prepared and the original powder quality. Thus, microstructure must be studied in order to understand the properties variations.The aim of the present research was to study how oxide dispersoids distribute into an Al alloy obtained by forging from ultra fine powders, and the oxide crystalline structure. In addition,the evolution of the microstructure after annealing over 24 h at 520ºC was addressed. Microstructural characterization was carried out with Transmission Electron Microscopy (TEM) in a Philips CM200 microscope. Figures 1 and 2 illustrate the microstructure of the ?as forged? alloy. It can be observed that the oxide distributes over the grain boundaries in the form of an oxide ?skeleton? (Fig.1). High resolution images of the oxide indicate that its structure is amorphous (Fig.2). Figures 3 and 4 display the microstructure after annealing. Whereas the mean grain size was not appreciablyaltered, the structure and morphology of the oxide was significantly modified. Distinct oxide particles are observed mainly along grain boundaries, but also inside grains (Fig.3), and their structure is shown to be crystalline. Figure 4 shows a high resolution image and its corresponding diffractogram. Analysis of such images indicates that the reflections are compatible with the structure of γ-Al2O3.Annealing was found to be detrimental to the strength of the alloy. Such changes are related to the transformation of the oxides from amorphous to crystalline and the modification of their distribution, leading to a loss in efficiency of grain boundary strengthening.[1] I.Polmear. ?Light Alloys: Metallurgy of the Light Alloys.? Butterworth-Heinemann (1995).[2] J.Gilbert Kaufman. ?Introduction to Aluminum Alloys and Tempers.? ASM International, 2000.[3] M.Balog et al. Mat.Sci.Eng.A 504(2009)1-7.