Effect of Trace Additions of Magnesium on Creep Crack Growth of a Superalloy

Abstract

The present work has shown that the addition of Mg in superalloy can significantly influence its creep crack growth. The creep crack growth could be divided into four stages. An optimum addition of Mg to superalloys can prolong the initial stage of creep crack growth, decrease the crack growth rate and prolong the creep rupture lives. The optimum amount of Mg addition can also increase the transient point of crack growth C* in lodga/dt-log C* curve. Superalloys 1992 Edited by S.D. Antolovich, R.W. Stusrud, R.A. MacKay, D.L. Anton, T. Khan, R.D. Kissinger, D.L. Klarstrom The Minerals, Metals & Materials Society, 1992 629 Introduction Many investigations 11,2*31 over the last ten years have shown that the mechanical properties, including hot workability, tensile ductility and creep resistance, can be markedly improved by the proper addition of trace amounts of Mg to nickel-base superalloys. Studies have shown that the segragation of Mg to grain boundaries is the main cause for the improvement of superalloy properties. The segregation of Mg to grain boundaries can decrease the grain boundary energy, enhance the cohesion of the grain boundary, promote spherodizing of the carbides and change the distribution and morphology of secondary phases at grain boundaries. The nucleation and growth of creep cracks are important aspects in creep studies, however until now, there have been few studies on the effect of Mg on the behavior of creep crack growth. Therefore, this subject was studied in this work. Materials and Procedures A nickel-base superalloy GH33 with a composition of Ni-20Cr-2STi-0.7Al, strengthed by the Ni3(Al,Ti) phase, was chosen as test material. The alloy was melted in vacuum induction furnace as primary alloy, then remelted into ingots with various additions of Mg. The ingots were hot-rolled into @ 25 mm rods. The specimens were heat treated with the heat treatment procedures, 1080”C-8h, AC ; 700”C-16h, AC. The tested alloys contain 0, 0.001, 0.008 and 0.013 wt-% Mg. Single edge notch specimens were used in the test of creep crack growth, the notches were cut with a line cutting machine. Tests were conducted on the creep testing machine and crack length was measured by the direct current potential method. Test data were recorded and stored by computer. Results Results of creep crack growth The specimen with 2 mm deep edge, was tested at 700°C and 343 MPa, nominal stress. The curves of creep crack length as a function of the time for the specimen with differents amounts of Mg are shown in Fig. 1. The curves could be divided into four stages : (1) In the initial stage of crack growth, new creep cracks nucleate and propagate at an extremely slow rate at the tip of fatigue pre-crack. It can be seen from Fig. 1 that the crack growth behavior in this stage is a function of Mg content. (2) In the steady-state region of creep crack growth microcracks formed at the tip of main cracks start to grow. The crack growth developed steadily with a very small acceleration, it is linear in the a-t curves. (3) Accelerated propagation of creep cracks occured in this stage. The growth rate of cracks is accelerated markedly, the a-t curves becomes bent. (4) Fast propagation of creep occurs in the last stage. The cracks propagate with very high rate until rupture, the a-t curves are perpendicular to the coordinate axis. The influence of Mg on the creep crack growth are described as follows : (1) Mg has an obvious effect on the time of the initial stage as show in Table I. The specimen with the optimum addition of Mg has the longest initial stage, but the cracks in the specimen without or with excessive Mg started to propagate following shorter initial stage. (2) Mg also markedly influenced the growth rate of creep cracking in the steady-state region. The highest rate of crack growth was found in the specimen without Mg and the lowest growth rate in the specimen with the optimum amount of Mg, and higher rate of crack growth was found with excessive additions of Mg. 630 0 I I 1 i (a) NoMg 8-