The effect of reduction ratio of hot rolling on texture and secondary recrystallization of MA 754 plate

Abstract

Ni-base oxide-dispersion-strengthened (ODS) superalloys produced by a mechanical alloying process have been used for high temperature applications because of their excellent high temperature strength, which can be useful up to a relatively high fraction of their melting points [1]. The high temperature strength of ODS superalloys is attributed to the presence of finely dispersed inert oxide particles, which gives dispersion strengthening effect by acting as a barrier to dislocation motion [2]. In order to develop high temperature strength, fine grains should be transformed into elongated coarse grains, which contributes to the retardation of the creep process and thus the prolongation of rupture life. The coarse elongated grains with a high grain aspect ratio (GAR), which is defined as the ratio of grain length (L) to grain width (W ), are possible with a high temperature annealing, during which secondary recrystallization occurs [3–6]. In particular, INCONEL alloy MA 754, which has been used as a turbine vane material in advanced jet engines, obtains the desired grain structure by isothermal annealing [7]. Although the exact mechanism of secondary recrystallization behavior is not fully understood yet, the importance of GAR has received considerable attention [3, 6, 8]. The general aim of thermomechanical processing (TMP) of ODS superalloys is to achieve a coarse grain morphology by means of secondary recrystallization [8, 9]. Another purpose of TMP is to increase the availability of the necessary product forms. However, subsequent secondary recrystallization behavior strongly depends on the temperature, strain rate, and strain of the prior TMP operation and also such compositional factors as yttria content and the amount of excess oxygen [10]. Bieler et al. have reported that the temperature and strain history during hot working have a large effect on texture, even though the same final sheet thickness was reached [11]. In a recrystallization study on a similar alloy, the recrystallization texture of sheet was sensitive to the heat treatment and rolling history [12].