MICROSTRUCTURAL OBSERVATION ON THE γ’ PRECIPITATION AND GROWTH IN TANTALLUM CONTAINING NICKEL BASED SUPERALLOYS

Efendi Mabruri

Pusat Penelitian Metalurgi-LIPI
Kawasan Puspiptek Gd.470, Serpong 15314, Tlp. 021-7560991, Fax.021-7560553
Email: efendi_lipi@yahoo.com, Web: http: //efendimbr.wordpress.com

ABSTRACT

Superalloy of IN 718- the Ni-Cr-Fe based superalloys with Nb as one of the main strengthening elements- has been widely used in tubine gas components operating at medium temperature range (up to 650oC) because of its excellent mechanical properties, good workability and weldability and low cost. However, precipitation of incoherent  phase after long exposure limits the application temperature of the superalloys under 650oC. Our work is concerning to develop new γ’-strengthened alloys precipitation phase inherently has higher thermal stability than γ’’ γ’ phase precipitate phase. . This paper reports the preliminary results on the effect of Ta -the potent alloying element- on precipitation and coarsening of γ’ phase of Ni-Al-Ta ternary alloy as the basic system in superalloys. Experimental results show that the alloy containing Ta revealed the cuboidal γ’ precipitates after aging treatment. The increasing of Ta/Al ratio of the from 0.14 in the alloy 1 to 0.42 in the alloy 2 leads the increasing of γ’ precipitate size. The splitting phenomenon is observed on the aloy 2 after solutionizing at 1100oC for 3 h and subsequent aging at 750oC for 20 h.,

Keywords: tantallum, 718, superalloys, γ’ phase, microstructure


I. INTRODUCTION

Superalloy of IN 718- the Ni-Cr-Fe based superalloys with Nb as one of the main strengthening elements- has been widely used in tubine gas components operating at medium temperature range (up to 650oC) because of its excellent mechanical properties, good workability and weldability and low cost [1]. It is strengthened mainly by precipitation hardening of Ni3(Nb,Ti) γ’’ and Ni3(Al,Ti,Nb) γ’ phases which produce the coherency strains in the γ matrix [2]. The ordered metastable γ’’ which nucleates and coarsens on the is thermodynamically unstable at high temperature over than 650oC and transforms into the incoherent  phase after long exposure. This transformation deteriorates the mechanical properties and limits the application temperature of the superalloys under 650oC. This limitation defines the opportunity to design a new alloys, an improved 718 superalloys.. The improved superalloys should have higher thermal stability over than 650oC with comparable mechanical properties and lower production costs. There are two approaches to achieve this work. The first approach is to increase the strengthening effect and thermal stability of γ’’ phase in a predominantly γ’’ phase strengthening alloys. The second is to develop improved γ’phase strengthening alloys having better processing characteristics, better mechanical properties and lower cost. Our work is concerning to develop the new alloys based on the latter approach because the γ’ precipitate phase inherently has higher thermal stability than γ’’ precipitate phase. The characteristics of precipitate phases are strongly related to the chemical composition of the alloys, therefore it is important to look closely on the influence of the alloying elements that contributes the formation of the phase. This paper reports the preliminary results on the effect of Ta -the potent alloying element- on precipitation and coarsening of γ’ phase of Ni-Al-Ta ternary alloy as the basic system in superalloys.