There is a complex relationship between the grain size of tungsten wire and its fracture toughness. The following is a detailed analysis of this relationship:
- The mechanism of the influence of grain size on fracture toughness
Dislocation density and hardness:
The smaller the grain size, the higher the dislocation density of tungsten wire is generally, which helps to increase the hardness of the material. The increase in dislocation density hinders the propagation of cracks, thereby improving fracture toughness to a certain extent.
Grain boundary effect:
Fine grains mean more grain boundaries. Grain boundaries play a role in hindering crack propagation in materials because they can provide additional paths to disperse and absorb energy during crack propagation. However, small grains may also be more likely to cause fracture when subjected to external stress because the strength of the grain boundaries may be relatively low.
Ductile-brittle transition temperature:
Grain refinement can reduce the ductile-brittle transition temperature (DBTT) of the material, which means that the material can also exhibit ductile behavior at lower temperatures. This is beneficial for tungsten wire that needs to work in a low temperature environment.
- Experimental observation of grain size and fracture toughness
Experimental studies have shown that for pure tungsten materials that have undergone specific deformation treatments (such as ECAP deformation), grain refinement can significantly improve the toughness of the material. For example, after 8 passes of ECAP deformation, the grain size of pure tungsten materials with an original grain size of about 50 microns can be refined to about 1 micron, while showing obvious toughness fracture characteristics.
- Relationship between grain size and other properties
It should be noted that grain size not only affects fracture toughness, but also has a significant impact on other properties of tungsten wire (such as hardness, tensile strength, etc.). Generally, small grains can increase hardness but may reduce toughness (although there is the beneficial effect of the reduction of the ductile-brittle transition temperature mentioned above), while larger grains may increase tensile strength.
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