#ductility
Researchers present new strategy for extending ductility in a single-phase alloy
Simultaneous high strength and large ductility are always desirable for metallic materials. However, while the strength of metals and alloys can be easily increased by five to 15 times through simple plastic deformation or grain refinement down to the nano-scale, the gain in strength is usually accompanied by a drastic loss of uniform ductility. Ductility depends strongly on the work hardening ability, which becomes weak in materials with high strength, especially in a single-phase material.
Publishing online in PNAS, the research group of Prof. WU Xiaolei at the Chinese Academy of Sciences, in collaboration with Prof. En Ma at Johns Hopkins University, U.S., have demonstrated a strategy for exploiting a dynamically reinforced multilevel heterogeneous grain structure (HGS). They demonstrated the behavior of such an HGS using the face-centered-cubic CrCoNi medium-entropy alloy (MEA) as a model system.
Back stress hardening is usually not obvious in single-phase homogeneous grains. To overcome this, the scientists purposely created an unusually heterogeneous grain structure. They took advantage of the low stacking fault energy of the MEA, which facilitates the generation of twinned nano-grains and stacking faults during tensile straining, dynamically reinforcing the heterogeneity on the fly.
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Materials Testing: Tensile Tests
Extremely useful for determining various mechanical properties, the tensile test (or tension test, as it is also called) is one of the most common methods of materials testing.
A tensile test is carried out by taking a sample of a material and subjecting it to increasing tension, stretching the sample until failure. This simple test results in the graph shown above (or something similar, depending on the material) called a stress-strain diagram. The elongation of the sample is determined with relation to the force required to produce such stretching and, knowing the cross-sectional area of the sample, the resulting stress and strain can be calculated and graphed.
This simple diagram gives a surprising number of mechanical properties, including the Modulus of Elasticity (or Young’s Modulus),yield strength,ultimate strength (also called ultimate tensile strength or tensile strength), and ductility (depending on the elongation before failure). Though less common, the stress-strain diagram can also be used to calculate the modulus of toughness and of resilience.
Along with the many properties that can be determined, another reason that tensile tests are so popular is that they can be used for almost any material. Depending on said material, the specimen is usually shaped similar to those in the bottom right image in what is called a ‘dog bone’ sample. The top left and bottom left images show brittle and ductile failure, respectively.
Image sources: top left,top right,bottom left,bottom middle,bottom right
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