Microstructure of a Two-phase γ′(Ni3Al)-γ(Ni(Al)) Intermetallic Alloy After Cold-rolling and Annealing

Abstract

The cold deformation, recrystallization and grain growth processes in the single-phase Ni3Al intermetallics have been extensively studied [1±8]. The two-phase a9(Ni3Al)ya(Ni(Al)) structure, as opposed to a single-phase Ni3Al intermetallic structure can be more suitable for high-temperature application because it exhibits more creep resistance than a singleor quasi-single-phase a9(Ni3Al) [9]. The creep resistance a9ya alloys shows an optimum at a a9 volume fraction of about 60±70% [10]. Additionally, a a9(Ni3Al) matrix dispersed with the Ni-rich disordered a precipitates, exhibiting muchimproved mechanical properties, can be obtained by choosing the composition at around the 4 : 1 Ni to Al atomic ratio followed by proper heat treatments [11]. However, there are no studies available on the cold deformation and recrystallization of two-phase, a9ya, alloys. Thus, the purpose of this letter is to show the in uence of the medium cold deformation and subsequent recrystallization annealing on the microstructure of a two-phase a9(Ni3Al)ya(Ni(Al)) alloy. An ingot of two-phase a9(Ni3Al)ya(Ni(Al)) alloy (78.8 at % Ni, 20.8 at % Al, 0.3 at % Zr and 0.1 at % B) was prepared by induction vacuum±melting and subsequently homogenized at 1273 K for 72 h in a vacuum furnace. The cast ingot exhibited a coarsegrained structure after homogenization. Five plates, 2.7 mm thick, sliced from the ingot, were cold-rolled by an approximately 30% reduction in thickness. These plates were isothermally annealed in a vacuum furnace (10y5 Torr) at 1273 K for 2, 10, 25 and 50 h. These temperatures and annealing times would seem to be suf®cient to obtain the fully recrystallized state. Metallographic specimens were prepared from the annealed plates. The samples were mechanically polished and then etched in a Marble's reagent (10 g CuSO4 ‡ 50 ml HCl ‡ 50 ml H2O). All microstructural observations were made in the transverse and longitudinal cross-section by optical and scanning electron microscopy (SEM). The Vickers hardness was measured using a 2000 g (VH2) load. Microtextures after rolling and annealing for 2 and 50 h, respectively, were measured in different regions of the specimens using the electron backscattering pattern (EBSP) system attached to the SEM Philips XL30 LaB6. The EBSP system is fully integrated with SEM and also with energy dispersive spectroscopy (EDS) and on image analyzer. A silicon intensi®ed target (SIT) ®ber-optic camera was used to capture the EBSP formed in SEM. Its spatial resolution is approximately 0.5 im, and the accuracy is on the order of 18. The {1 1 1}, {1 1 0}, {1 0 0} pole ®gures and grain boundary misorientation angles were analyzed using crystal orientation software (COS) integrated with SEM. All measurements were performed on the longitudinal sections of the specimens. The structure of the as-cast alloy was characterized by the dendrites with many eutectic cells and some cast shrinkage porosity. The three-dimensional microstructure view of the two-phase a9(Ni3Al)y a(Ni(Al)) alloy, which was 30% cold-rolled and subsequently annealed at 1273 K for 2 h, is shown in Fig. 1. The microsructure consists of dispersed recrystallized regions of equiaxed grains with many annealing twins and a large, partially recrystallized region through the whole thickness of the specimen. The cracks were not revealed in the investigated specimens, but may pores were observed inside the grains and on the grain boundaries during the examinations using optical and scanning electron microscopes. Nucleation of grains during annealing occured mainly at deformation bands (Fig. 2a).