Effects of Magnesium on Niobium Segregation and Impact Toughness in Cast Alloy 718

Abstract

A program has been conducted on the effects of Mg in cast alloy 718. The results show that small amounts of Mg improves impact toughness and decreases Nb segregation by decreasing secondary dendrite arm spacing which results in less and smaller interdendritic Laves and MC eutectics. Small amounts of Mg produce a more spheroidal as well as a more dispersive MC phase. Impact toughness was found to be related to the refinement of interdendritic segregation of Nb which affects the size, quantity and morphology of Laves and MC phases. Introduction The unique effects of Mg have been determined for many wrought superalloys. Mg has been shown to improve the creep properties and particularly the high temperature ductility due to refinement of the grain boundary carbides and equilibrium segregation. Mg also reduces the detrimental effect of S. Effects of Mg in cast superalloys have recently been investigated and results indicate that Mg improves solidification behaviors and structures. Mg segregates to phase boundaries and refines the interdendritic MC carbides and 1/ eutectic as well as decreasing the quantity of ?( eutectic. A program to study the effect of Mg in cast alloy 718 was conducted. The purpose of this study was to study the effects of Mg on the segregaton behavior of Nb and the morphology distribution of primary Laves and MC carbides. This paper presents the effects of Mg on impact toughness and segregation of Nb in cast alloy 7 18. Experimental Procedure Seven heats of alloy 718 with various contents of Mg were prepared. The materials used for first cycle tests were commercial alloy 718 without Mg (Alloy A) and with Mg of 0.0026% (Alloy B). The content of Nb is as high as 5.25%. The 718 materials used for the second cycle testing have a lower Nb content (4.75%) and varying amounts of Mg (0.000%. 0.0016%, 0.0084% and 0.01 l%, respectively. Investment cast specimens were used for testing. The pour temperature was 1420-1450°C and the mold temperature was 820-840°C. The heat treatment for first cycle tests was llOO”C/l hr/AC + 980°C/1 hr/AC + 720°C/16 hr/AC. The heat treatment for the second cycle tests was 1 lOO’C/l hrs/AC + 97O”C/2 hrs/AC + 72O”C/8 hrs/AC to 62O”C/18 hrs/AC. The morphology of Laves and MC eutectic in alloys with various contents of Mg was characterized. Fracture surface were investigated by SEM to determine the effects of Mg. Quantitative measurements of Laves and MC particles were carried out. Both electron microprobe and EDS analyses were also used in this program. Impact tests were premanely used to follow the resultant structural changes on toughness. Superalloy 718--Metallurgy and Applications Edited by E.A. Loria The Minerals, Metals & Materials Society, 1989 545 Experimental Results and Discussion The results of the impact tests show that Mg increases significantly the impact toughness as shown in Table I. Impact toughness increases with Mg content as illustrated in Figure 1. Microstructural studies show that Mg additions have no influence on grain size, but decreases the secondary dendritic arm spacing as shown in Figure 2. Table I. Impact Toughness of Cast alloy 718 with Various