| ABSTRACT: |
DETECTION OF CENTERLINE SIGMA PHASE IN AL-6XN PLATES VIA NONDESTRUCTIVE
METHODS
K. Lipetzky, M. Posada, J. Liu, and J. Warren
Naval Surface Warfare Center Carderock Division, Welding and Nondestructive Evaluation Branch, West
Bethesda, Maryland, USA
The Navy is evaluating AL-6XN for use in the construction of the next class of surface combatant. As part
of the Navy’s ongoing assessment, several studies have focused on mechanical property characterization,
corrosion susceptibility, and welding process development. In each of these studies, investigators have
observed a secondary phase located at the approximate center of the plate thickness. The secondary phase,
identified as centerline sigma, is a hard, brittle, non-magnetic (at service temperatures) intermetallic
compound that solidifies last during the continuous cast process. The deleterious effects of thermally
formed sigma phase are well documented (reduced corrosion resistance, tensile ductility, and toughness) and
while a concern, can be controlled by proper heat treatment. The effect centerline sigma has on the
mechanical and corrosion resistant properties of AL-6XN is yet unknown. The current study is identifying
and evaluating nondestructive methods to detect centerline sigma phase. A priori detection of centerline
sigma would aid in understanding its influence on mechanical properties and corrosion resistance.
Furthermore, an effective nondestructive method would allow the Navy to quantify and specify acceptable
limits for the amount and/or distribution of sigma phase. This paper will discuss the candidate
nondestructive methods that were identified, the results of the evaluation process, and the challenges that
remain for the detection of centerline sigma in AL-6XN plate.
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