Dendritic growth of voids in irradiated nickelaluminum alloys

In a study of lon-irradmtion induced void formation m nickel-base alloys, a novel void morphology suggestive of dendritic growth has been observed. In this paper we describe our observations and discuss their imphcations The irradiations were carded out at the Argonne National Laboratories Dynamltron facility using 3.5 MeV s aNi + ions. The dose rates were typically about 4 x 10 -3 dpa/s, located approximately 6000 ,~ from the irradiated surface. The damage profile was calculated using the Naval Research Laboratories E-DEP-1 computer code (1). Most of the results m this note refer to disc-shaped samples of a well-annealed NI-2 at.% AI alloy, about I00/am thick and 3 mm in &a., irradiated at 550°C to a dose of 20 dpa. Pure nickel samples, as well as the 2% A1 alloy, were also trradiated to a dose of 10 dpa. Thin foils for transmission electron microscope examination were prepared by a jet pohshing method using a 30% nitric acid, 70% methanol solution at about 50V and -40°C. An X-ray absorption method (2) was used to measure the thickness removed from the irradiated surface. Transmission electron microscopy was performed using a JEOL model JEM-120 electron microscope operating at ! 20 kV. All the voids in pure mckel, and the overwhelming majority of those in the Ni-2% AI alloy were observed to be truncated octahedra with their largest faces parallel to { ! 11 } and truncated faces parallel to { 100 }. This morphology Is commonly observed in irradiated f.c.c, metals (3) and has also been observed m voids m quenched f.c.c, metals by Clarebrough, et al. (4). In the Ni-2% AI sample, a small frachon (<5%) of the voids contained small voids appended at thmr periphery. Several examples are shown in Fig 1. Examination oF thin foils in different orientations showed that s~x small voids were usually observed appended to the central void, and that these six appended vo]ds were in positions previously occupied by the truncated { 100~faces of the octahedron This morphology ]s strikingly similar to the dendritic growth forms that commonly result during the sohdffication ,of f.c c alloys. The most obvious s~mdanty is the preferred ( 100 ) growth direction of the appended voids, which is consmtent with the pre&ctlons of Weinberg and Chalmers (5) for the crystallography of dendrite arms m f.c.c crystals We therefore expect that dendritic void growth wdl result m the void shape dlustrated schemat]cally m Fig 2. Although this is a realistic model of the projected images observed, we have not been able to distinguish the true shape of the appended voids, the truncated octahedral shape depicted is an assumption Among hundreds of dendritic voids observed so far, almost all exhibit the kmd of symmetry shown m Fig 1, with the size of the appended voids about the same However, exceptions were observed, Fig. 3a shows a void with appendages of different size, while Fig. 3b shows an example with one of the appendages missing. Figure 3c shows a void w]th an additional appendage growing on the first, this is not far from being a full-fledged dendrite arm, although no other examples of this growth form were observed The relatwe scarcity of the growth forms observed m Fig. 3 compared to those m Fig. 1 suggests that the asymmetrical voids could be anomalies associated with an unusual local chemtcal environment, although thin is difficult to prove Evidently, symmetric dendritic voids have not been found and reported m the hterature before. Smldt, et al (6) observed elongated vo]ds, which they referred to as dendritic, m an irradmted Fe-0 1% C alloy. We have also seen electron rmcrographs of He bubbles in a paper by Bauer and Thomas (7) m which images s]milar in shape to those in Fig. la are evident. However, the general absence of such observations ~s significant, because there have been numerous investigations of void swelling m pure Nl under a wide variety of irradiation conditions. In fact, m our investigation dendritic voids were not found either in the NP2% AI alloy or m pure mckel irradiated to 10 dpa, containing an average void size (~800 .a,) of about twice that (~410 ,~) observed in the N~-2% AI samples ]rradmted to 20 dpa. The conditions governing the onset of the morphological mstabdity that affects the growth of a polyhedral void must therefore depend both on alloymg effects and on the dose The morphoiog]cal stabdity of a particle whose growth ~s dfffumon-controlled has been studied theoretically by many researchers (8-11). However, so far as we are aware, only Cahn (12) has treated the mstabil~ty problem of a particle undergoing growth by the lateral spreading of ledges of atomic dimensions on the polygonal faces of a polyhedral crystal. He showed that such a crystal will continue to grow as a polyhedron by thin kind of interface-reaction mechamsm until ~t reaches a critical size which approxtmately marks the tranmt~on from mterface-reachon control to diffusion control. Hm 1047