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Are ductility and strength always opposite?

  1. Dec 15, 2015 #1
    1) Is it always true for materials that if its ductility increase, its strength must decrease (and vice versa)?

    For example, the ductility of FCC > BCC > HCP. So does this entail that the strength of HCP > BBC > FCC?

    According to my (entry-level) understanding, ductility increases when:
    - Number of slip systems increase
    - The shear force required for slip decreases (such as on close-packed panes or close-packed directions).
    - Number of dislocations decrease (ie. less grain boundries AKA bigger grains), since dislocations impede slip.

    2) It seems to me all the characteristics that increase ductility also decrease strength. Is there anything I'm missing out above which may increase ductility without decreasing strength (or even, helps increase strength)?

    3) I guess I'll also ask this question: is it possible that material A has both better ductility and better strength than material B? Comparing a copper sheet and a piece of tissue paper, I'm inclined to think the answer is 'yes'.
     
  2. jcsd
  3. Dec 16, 2015 #2

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    Ductility is the ability of material to be stretched into a wire.
    Strength is the ability of material to resist deformation.

    1) Is it always true for materials that if its ductility increase, its strength must decrease (and vice versa)?
    I would say with alloy and other composite technology, that statement is not always true.

    In general speaking(which is so wide) there're are material with better strength and also ductility (look into refined alloy) compared to another material. I'm not an expert in materials but hope there's a bit of idea that i can give.

    Maybe you could provide more details about the materials then the experts here can answer you. =)
     
  4. Dec 16, 2015 #3
    That is more the definition of hardness in material. Not strength.

    Strength is usually used in conjunction with an adjective in front, and means a particular value of stress defined by the aforementioned adjective (https://en.wikipedia.org/wiki/Strength_of_materials#Strength_terms).

    In other news, I have found that there are ways in which ductility and strength of a material can be improved simultaneously. So I guess I've kind of answered my own question XD. Details in spoiler below.
    Nanocrystalline copper typically possess high yield strength but poor ductility (thanks to their extremely small size, there is an absence of dislocation activity), while microcrystalline copper has low yield strength but high ductility. Fortunately scientists have recently been able to produce nanocrystalline copper with ductility comparable to its microcrystalline counterpart.

    This is accomplished through a complex thermomechanical-forming process that includes a series of cold-rolling passes of the sample at liquid nitrogen temperature with additional cooling after each pass followed by a highly controlled annealing process. The resulting material contains about 25% micrometer-sized grains in a matrix of nanometer and ultrafine grains.

    The cold-rolling process at liquid nitrogen temperature allows for the formation of a large density of dislocations. The cold temperature does not allow the recovery of dislocations and in turn causes the density (of dislocations) to increase beyond normal levels achieveable at room temperature. This sample is then annealed under highly controlled conditions. The annealing process allows recrystallization and growth of some grains to a range of 1 to 3 micrometer (called abnormal grain growth). The existance of the large micrometer-sized grains allows an elevated level of dislocation and twinning activity and hence ductility, while the predominantly nanosized and ultrafine grains still maintain high yield strength.

    - Paraphrased from Foundations of Materials Science and Engineering 5th edition, by Smith/Hashemi, page 315
     
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