The effect of grain boundaries on the athermal stress of tantalum and tantalum-tungsten alloys
- PDF / 2,552,014 Bytes
- 8 Pages / 612 x 792 pts (letter) Page_size
- 36 Downloads / 269 Views
DUCTION
THE mechanical behavior and mechanisms involved in the deformation of bcc metals and their alloys have been extensively investigated, and many excellent reviews are in the literature.[1–6] The main distinguishing feature of bcc metals and alloys is their strong temperature and strain-rate dependence of both yield and flow stresses at temperatures below about 300 to 400 K, indicating significant contributions of thermal activation to dislocation movement over short-range barriers. This strong temperature dependence is attributed to the high Peierls barriers associated with the complex nature and movement of screw dislocations. The mobility of the 具111典 screw dislocations is reduced due to the threefold splitting of these dislocations along three distinct {110} planes, resulting in somewhat ill-defined slip planes at elevated temperatures, as discussed in a review by Duesbery and Vitek.[7] At increasing temperatures, the temperature dependence drops rapidly as the resistance to screw-dislocation motion due to short-range obstacles (i.e., Peierls barriers) becomes small relative to that of other dislocation processes, such as interactions of edge dislocations with the lattice, solutes, interstitials, or other shortrange obstacles. In the case of polycrystalline materials, the athermal regime is often characterized by dislocation interactions with grain boundaries, which represent longrange obstacles. An example illustrating the thermal and athermal regimes for two different strain rates is shown schematically in Figure 1. The point at which the stress becomes independent of temperature is usually referred to as the critical temperature (Tc). As illustrated, the curves obtained at different strain rates tend to merge early in the athermal region, which is indicative of a strain-rate independence of stress in this DAVID H. LASSILA, Materials Technology Leader, ALFRED GOLDBERG, Metallurgist, and RICHARD BECKER, Engineer, are with the Lawrence Livermore National Laboratory, Livermore, CA 945510808. Contact e-mail: [email protected] Manuscript submitted March 21, 2001. METALLURGICAL AND MATERIALS TRANSACTIONS A
region. Although, theoretically, the athermal stress should be independent of temperature and strain rate, data reported in the literature sometimes exhibit a relatively slight decrease in stress with increasing temperature and decreasing strain rate. This departure from a constant stress generally becomes more prevalent with substitutional alloying, therefore making it difficult to define Tc and a value for the athermal stress for many alloys.[8,9] Our fundamental knowledge about the deformation behavior in bcc metals and alloys, as summarized previously, has been gained mostly from studies on single crystals. Some of the early work on yield behavior of Ta single crystals can be seen in publications by Mitchell and Spitzig[10,11] and Mordike et al.;[8] some of the more recent studies have been performed by Wasserba¨ch.[12,13] In addition to work on single crystals, studies on the yield behavior of polycryst
Data Loading...
