Dislocation Substructures in Deformed and Recovered Chromium

Abstract

SummaryIn an attempt to correlate the degree of plastic deformation of chromium with the physical properties associated with its antiferromagnetism, a study has been made by thin-foil electron microscopy of the dislocation substructures in polycrystalline chromium sheet after deformation by rolling and subsequent annealing. The original hot-rolled sheet exhibited a typical cell structure. Annealing at 900°C produced regular hexagonal networks and simple tilt boundaries. Complete recrystallization was achieved by annealing at 1250°C. 1· 5% defor=ation of this material at room temperature produced a non-uniform substructure of sharply kinked dislocation lines and irregularly shaped closed loops. Annealing at temperatures below 650°C had little effect. However, at that temperature there was some recovery and large circular loops lying in the slip planes appeared. The beginnings of regular networks appeared and the closed loops annealed out at 900°C. Comparison with the annealing characteristics of dislocation loops in iron allowed an estimate of the activation energy for the process in chromium to be made. It is shown that the elastic stress induced by plastic deformation, which critically affects the physical properties associated with the antiferromagnetism, is consistent with the experimental observations of the influence of an applied magnetic field on the elastic properties of chromium. I. INTRODUOTIONPrevious work (Bacon 1961;Street 1963) has shown that the physical properties of chromium associated with its antiferromagnetism are strongly dependent upon its state of plastic deformation. In an attempt to correlate the degree of plastic deformation with the observed antiferromagnetic properties, the dislocation substructures in annealed and plastically deformed chromium have been studied by thin-foil electron microscopy. II. EXPERIMENTAL PROOEDUREThe specimens were made from 99·97% pure chromium supplied in the form of hot-rolled sheet 0 ·002 in. thick. All annealing treatments were carried out in a vacuum of approximately 10-5 mmHg. The specimens were furnace cooled.Thin foils suitable for transmission electron microscopy were prepared by electropolishing in a solution of 65 ml perchloric acid (sp. gr. 1·54), 300 ml acetic acid, and 300 ml butyl cellosolve at about 1°C, in an apparatus similar to that described by Presland (1961). The cathodes were stainless steel and a current of 1·5 A from a current-stabilized rectified a.c. power supply was used. The thinned foils were examined in a JEM-6A electron microscope operating at 100 kV.

Impact of magnetism on screw dislocations in body-centered cubic chromium

“…With an anisotropy coefficient A = C 44 /C ′ equal to 0.59 and 0.52 respectively in the NM and AF phases, this is true for both magnetic phases. Although our ab initio calculations lead to larger core energies for 100 than for 1/2{ } < 111 > screw dislocations, both dislocations appear relevant to rationalize plasticity in BCC Cr, as confirmed by experimental observations [11][12][13] which report activity for both 1/2[111] {110} and [100] {110} slip systems. We now focus on the competition between these two slip systems, comparing the lattice friction opposing glide of 1/2 111 and 100 screw dislocations.…”

“…TEM observation using the extinction method, i.e. g • b contrast, which concluded to 1/2 111 Burgers vector in AF Cr [11][12][13] are also compatible with 111 dislocations. This is also true when the Burgers vector is determined from the screw orientation defined as the intersection of cross-slipped planes.…”

“…Using transmission electron microscopy (TEM), 1/2 111 dislocations were found in strained Cr polycrystals [11][12][13] at temperatures where magnetic order prevails, below the Néel temperature of 311 K. Slip traces analysis confirm that these dislocations are gliding in {110} planes [11,12,14], with glide in the {112} and {123} planes also observed at high temperature [11]. Regarding the disruption of the magnetic order, both Ravlic et al [15] and Kleiber et al [16] show the existence of AF domains at {100} surfaces separated by walls using spin-polarized scanning tunneling microscopy at room temperature.…”

Effect of tip radius on the incipient plasticity of chromium studied by nanoindentation

“…The surface of the Cr sample was ground using 1200-and 2400-grit SiC, then mechanically polished using alumina of 0.06 lm grit size. To reduce existing dislocations and minimize possible surface damage resulting from the mechanical polish, the Cr specimen was initially sealed in a fused silica tube filled with pure argon gas and then annealed at 1250°C for 4 h [23]. As the final procedure, the annealed Cr was additionally electropolished in a solution (H 3 PO 4 :H 2 SO 4 = 4:1) to remove a possible oxide layer resulting from annealing.…”

A note on the effect of strain upon the heat capacity of gadolinium near the curie temperatureTc = 293 K

“…Measurements have for long been hampered by the difficulty in obtaining pure samples, It is only recently that sophisticated purification techniques such as solid state electrolysis [ 2 , 31 have allowed samples with residual resistivity ratios (RKR) Il,,,/R4.2 2 100 to be produced. h'evertheless, for many purposes, samples with RRR 1 : 20 have been widely used. The importance of purity has been investigated [4,51 and shifts or spreading of the Curie temperature observed in these samples.…”