The effect of zirconium content on the microstructue and fatigue crack growth behaviour of Al-Li alloys

Newman, John Michael (1989) The effect of zirconium content on the microstructue and fatigue crack growth behaviour of Al-Li alloys. University of Southampton, Doctoral Thesis.

Abstract

Aluminium-Lithium alloys based on the quaternary Al-Li-Cu-Mg system provide an attractive potential replacement material for conventional aluminium alloys in aerospace applications. Additions of zirconium as a grain refining element to the quaternary alloy system are made to produce an essentially unrecrystallised grain structure in commercial wrought material. The present research has involved a detailed microstructural characterisation of two 8091 alloys considering the influence of zirconium content on the development of grain structure and mechanical behaviour. The development of microstructure has been studied during alloy processing from the as-cast billet, through a semi-rolled stage, to the fully rolled plate material. A full range of metallographic techniques, using both optical and electron microscopy, have been employed to study the effect of zirconium content on features such as grain structure and crystallographic texture. Mechanical properties have also been determined at all stages of alloy processing, with both tensile and fatigue tests being conducted. Fatigue crack growth behaviour has been investigated on rolled plate material for both long and short cracks. Long crack growth rates have been determined using compact tension specimens. Short crack growth was studied using an acetate replica technique on cracks generated at fastener holes machined to an AGARD standard. Additional fatigue life tests were also performed on the AGARD specimen geometry to evaluate the performance of the Al-Li alloys in the presence of a fastener hole. Comparative tests were also made on 7010-T651 plate. The influence of specimen thickness and position within plate material has been considered for the long crack growth behaviour of a commercial 8091 alloy. Fractographic analysis has been used together with results from the microstructural characterisation to investigate the FCG behaviour of the 8091 alloys. Where appropriate, crack length was monitored using a four-probe, pulsed direct current potential drop system. The two zirconium levels produced differences in grain structure which were most marked in the rolled plate material: equiaxed at low zirconium content and unrecrystallised at higher levels. Both Al-Li alloys displayed superior fatigue properties (higher stress intensity thresholds and lower crack growth rates) to the 7010-T651 plate, with the equiaxed grain structure of the low zirconium alloy displaying optimum behaviour. The unrecrystallised grain structure was associated with superior fatigue lives in the AGARD tests where crack initiation was retarded by the finer grain structure which inhibited slip plane damage. Superior long crack growth properties are attributed to increased surface roughness and crack tip tortuosity and the presence of a plateau of almost constant crack growth rate was clearly associated with the formation of crystallographic faceting. Faceting was also seen in the commercial 8091 alloy with the onset of the plateau occurring at lower levels of ΔK with increasing specimen thickness. Formation of the crystallographic faceting is thought to result from the accumulation of damage ahead of the crack tip through slip planes and Stage I cracks and by the generation of crack closure. A model is proposed to explain the transition from structure sensitive to structure insensitive crack growth which also considers the effect of specimen thickness on the onset of a constant rate of crack growth.

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