Abstract
The high brittle-to-ductile transition (BDT) temperature of conventionally produced tungsten (W), challenges the design of W-based structural components. Recent studies have demonstrated the potential of cold rolling to produce W sheets, which are ductile at room temperature and exhibit a BDT temperature of 208 K. In order to assess the thermal stability of these materials, we conducted isothermal heat treatments (at 1300 K, for annealing durations between 0.1 h and 210 h) combined with studies on the evolution of mechanical properties and microstructure of a severely deformed undoped W sheet. With this work, we demonstrate the need for a stabilized microstructure before utilization of cold-rolled W in high-temperature applications can take place successfully. After annealing at 1300 K for 6 h, the material properties changed remarkably: The BDT temperature increases from 208 K to 473 K and the sharp BDT of the as-rolled condition transforms into a wide transition regime spanning over more than 200 K. This means in fact, an endangered structural integrity at room temperature. We also address the so-called phenomenon of 45° embrittlement of W sheets. Here we show that cleavage fracture in strongly textured W sheets always takes place with an inclination angle of 45° to the rolling direction, independent of the studied material condition, whether as-rolled or annealed. An in-depth study of the microstructure indicates a correlation between an increased BDT temperature caused by annealing and microstructural coarsening presumably by extended recovery. We conclude that 45° embrittlement needs to be comprehended as a combined effect of an increased spacing between grain boundaries along the crack front, leading to an increased BDT, and a high orientation density of the rotated cube component or texture components close to that, which determine the preferred crack propagation of 45° to the rolling direction.
Highlights
The brittle-to-ductile transition (BDT) is a striking feature of bodycentered cubic metals [1]
Recent studies have demonstrated the potential of cold rolling to produce W sheets, which are ductile at room temperature and exhibit a BDT temperature of 208 K
We have demonstrated the enormous potential of cold rolling by decreasing the BDT temperature of technically pure W to 208 K [10], which means, stable crack propagation at room temperature
Summary
The brittle-to-ductile transition (BDT) is a striking feature of bodycentered cubic (bcc) metals [1]. The BDT temperature, bcc metals behave to face-centered cubic metals: they exhibit a high toughness and good formability [2]. Below the BDT temperature, the mechanical properties of bcc metals change funda mentally: they exhibit brittle material behavior with unstable crack propagation by cleavage fracture. This change in material behavior disqualifies bcc metals from being used as structural materials for ap plications below the respective BDT temperature [3]. Micro structural investigations have shown that a small grain boundary spa cing along the crack front and high dislocation densities are desirable
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