Effect of solute atoms on the motion of a low angle tilt boundary

Abstract

IntroductionThe stress-induced movement of low-angle tilt boundaries has bee9 observed in single • crystals of zinc by several investigators (1-6). The motion o,f the. boundary under the action of a sheer stress in the direction of the Burgers vector of the dislocations comprising the boundary is well established. However the reported modes of the boundary motion are different from one invest'igation to another. Bainbridge et al. (3) have obse.rved that the movement at room temperature was jerky and during each jlUIIp the boundary moved through an appreciable volume of the material while Vreeland (4) reports a smooth and continuous motion at the same temperature. One would probably attribute this discrepancy to the different levels of impurity content. Zinc· crystals of 99.99 wt% p_uri_ty were used in the former investigation while crystals of higher purity (99.999 wt%) and zone refined zinc were used in the latter. Thus we decided to examine this point further by introducing much higher doses of substitutional solute atoms into the crystals. We used alloy crystals containing controlled amounts of silver as an alloying element in addition to high purity zinc crystals. ·Experimental Single crystals of high purity zinc (99.999+ wt%) and alloys containing 0.05, 0.10 and 0. 21 wt% silver. were grown in a graphite mold under helium atmosphere by a modified Bridgman technique at a rate of 0.4 em/hr.· The crystals we!e rectangular 1.3 em x 0.7 em and 8 em long and were orient-ed in the direction [1120] with their basal planes parailel to the wider faces. The side faces were parallel to the prismatic planes. In the case of alloy crystals,· a reservoir was designed in the upper portion of the mold (7) to reduce the concentration gradient along the 8 em length to about ±3%. The top parts were removed by spark cutting and the samples were cut to 2.5 em length by an acid sawing technique. Then the fol.lowing sequence of preparation and annealing under He atmosphere was applied: i) 1-hr. anneal at 400°C and furnace cool; ii) side surfaces were acid polished; iii) cleaved along the basal plane to reveal the natural surface of this plane; iv) 1-hr. anneal at 400°C and furnace cool; v) ~2° angle tilt boundary (FIG. 1) introduced by bending at room temperature; vi) 1-hr. anneal at 400°C and furnace cooled to 200°C; 2. vii) 48..:hr. aging at 200°C followed by air quench and then stored in liquid nitrogen for testing.The cleaved specimens were 2.5 em long, "-0~8 em wide and 0.2 to 0.4 em ~hick. The width and thickness could not be controlled very closely due to the difficulty of cleaving the acid polishing.The observation of the boundary motion was performed by using essentially the same r technique and stressing apparatus as used by Bainbridge et al. (3). The displacements of the Further movement starts with a jumping motion at a discontinuously increased stress level.However the motion at room temperature is different and the original position does not impose an increased stress. This phenomenon was not obserlled in the earlie...