Yielding and fracture in tungsten and tungsten-rhenium alloys

Abstract

A study was made of the mechanical properties of vacuum arc-melted tungsten and tungsten-rhenium alloys in the temperature range 77' to 810' K in order to elucidate the mechanism by which rhenium additions lower the ductile-brittle transition temperature of tungsten. The temperature and strain-rate dependency of the yield s t r e s s of tungsten is reduced by alloying with rhenium. This is shown to be because of a reduction in the Peierls s t r e s s . The reduction in the transition temperature is attributed to the reduced Peierls s t r e s s through its effect on the mobility and rate of multiplication of dislocations.ii I- ' YIELDING AND FRACTURE IN TUNGSTEN AND TUNGSTEN-RHENIUM ALLOYSby Peter L. Raffo Lewis Research Center SUMMARY A study was made of the mechanical properties of vacuum arc-melted tungsten and tungsten-rhenium alloys in the temperature range 7 7 O to 810' K. The ductile-brittle transition temperature of the unalloyed recrystallized tungsten prepared for this study was 490' K and was reduced to 430' and 350' K by additions of 2 and 25 percent rhenium (Re), respectively. The temperature and strain rate dependence of the yield stress of tungsten is decreased by alloying with rhenium. The work-hardening rate of l-percent plastic s t r a i n for unalloyed tungsten (W) increases rapidly with decreasing temperature, although it is relatively independent of temperature in the W-Re alloys. In unalloyed tungsten and dilute W-Re alloys, fracture was mainly by cleavage. In these materials, fracture is controlled by crack initiation; in the W-25-percent-Re alloy, failure is con trolled by crack propagation. The observations suggest that yielding of unalloyed tungsten is controlled by the nucleation of kink pairs over the Peierls b a r r i e r . Alloying with rhenium reduces the Peierls stress. The yielding of a W-25-percent-Re alloy is con trolled by a similar mechanism at low temperatures; at higher temperatures, an appar ent decrease in the s tacking-fault energy alters the yielding mechanism to one in which it is controlled by the recombination of dissociated screw dislocations. The reduction in the transition temperature in W-Re alloys is thought to be caused by the stress relax ation effect of an increased plastic-strain rate at the tip of a crack as a result of in creased dislocation mobilities and multiplication rates. Various theories have been advanced to explain the brittleness of these metals, with varying degrees of success. The fact that the transition temperature is sensitive to the presence of interstitial impurities has received the most attention and is supported by the fact that zone purification of molybdenum and tungsten has resulted i n significant decreases i n the transition temperature (refs. 3 and 4). The major effect of inter stitials appears to be related to their interaction with grain boundaries either in solid solution o r as Lxecipitate(ref. 5), since single crystals of the group VI metals are ductile at temperatures well below ambient. Other theories advanced t...

Ductilisation of tungsten (W) through cold-rolling: R-curve behaviour

“…The fact that rhenium (Re) additions increase the ductility of tungsten was first reported by Geach and Hughes [2] in 1955. In the 1960s and 1970s, several studies on the impact of rhenium on the ductility of tungsten were performed, including one by Raffo [3]. An explanation of the positive effect of rhenium on the ductility of tungsten was given by Romaner et al [4].…”

Influence of impurities on the fracture behaviour of tungsten

“…Over the last fifty years numerous investigations have been carried out with the aim of improving the understanding of the fracture behaviour. Most studies relating to the ductility of tungsten materials were done in the fifties, sixties and few in the seventies [2][3][4][5][6][7]. GB impurities, which were thought to be one of the main causes of intergranular fracture, were investigated to determine the effect of particularly carbon (C), oxygen (O), potassium (K) and phosphorous (P) [8][9][10].…”

Solid Solution Softening (Rhenium Ductilizing Effect and Bubble Strengthening in Tungsten-Rhenium Alloys

“…While there is some indication that: an extrinsic mechanism is responsible (42-48) in a couple Group V systems the overwhelming evidence is that the softening (weakening) is a result of a reduction in the Peierls stress (8,17,18,(28)(29)(30)(31)(32)(33)(34)(35)(36)(37)(38)(39)(40)(41) This is the same mechanism responsible for the rhenium ductilizing effect.…”