High-Temperature Creep Deformation and Fracture Behavior of a Directionally Solidified Ni-Base Superalloy DZ951

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INTRODUCTION

NICKEL-BASE superalloys are mainly strengthened by c¢ precipitates and solutioning of alloying elements that are produced in various forms suitable for application at high temperature, due to their excellent creep and fatigue strength and good corrosion resistance. In advanced gas turbine engines, some critical components, especially blades and vanes, have been widely fabricated in directionally solidified (DS) or single-crystal superalloys due to the marked reduction or elimination of the grain boundaries normal to the stress axis, which usually are the failure initiation sites in the conventional cast superalloys.[1–4] At high temperature, the life of turbine blades and vanes is controlled by creep deformation and damage processes of the materials.[5] Extensive research has been focused on investigating the creep behavior of some commercial Ni-base superalloys, including the effects of minor elements (such as C, B, and Zr),[6–8] environment,[9,10] c¢ rafting,[11,12] dislocation structures,[13–18] creep-fatigue behavior,[19,20] etc. DZ951 alloy is a DS Ni-base superalloy developed recently by the Institute of Metal Research, Chinese Academy of Sciences, for vanes in gas turbine engines, which possess high incipient melting temperature (1347 C), low density (8.17 g/cm3), and low cost due to the lack of costly elements. It has been reported that the alloy holds good ZHAOKUANG CHU, Assistant Professor, JINJIANG YU, Associate Professor, XIAOFENG SUN and HENGRONG GUAN, Professors, and ZHUANGQI HU, Academician are with the Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, P.R. China. Contact e-mail: [email protected] Manuscript submitted November 30, 2007. Article published online September 30, 2009 METALLURGICAL AND MATERIALS TRANSACTIONS A

high-temperature yield strength[21] and microstructure stability.[22] Investigating the creep deformation and rupture behavior can assist in understanding the effects on development of high-temperature alloys and in rationalizing the creep behavior of these alloys. The purpose of the present article is to explore in detail the creep deformation and rupture behavior under constant load at different temperatures and stresses and to gain a deeper understanding of the high-temperature creep deformation and rupture mechanism of DZ951 alloy.

II.

EXPERIMENTAL PROCEDURE

The nominal composition of DZ951 alloy used in this study is listed in Table I. The alloy was melted in a VZM-25F vacuum induction furnace. The directionally solidified specimens were made by the process of high rate solidification in a ZGD2 vacuum induction furnace. The temperature gradient was 80 C/cm and the withdrawal rate was 6 mm/min. The procedure of heat treatment was as follows: 1220 C/4 h AC + 1050 C/ 4 h AC + 870 C/24 h AC (AC: air cooling). The process of heat treatment produced a precipitation of regular cuboidal c¢ particles aligning along [100] in the c matrix. The size of c¢ was about 0.3 lm and the volume fraction of c¢ phase was about 65 pct. Creep specimens were machin

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