Why would metals behave as brittle materials without dislocations?

Why no single theory, can be applied to a specific material at all times?

Help me please

The minimum yield strength for iron with an average grain size of 6x10-²mm is 135 MPa, this increases to 260 MPa when the average grain size is reduced to 8x10-³ mm. What must the average grain size be to achieve a yield strength of 205 MPa.

In order to find out the effects of the grain sizes on the mechanical strength of a metal, three different grain sizes of 10 µm, 500 nm and 10 nm were produced. What will be the yield stresses of these same metals at different average grain sizes? The yield strength for a grain or reference point is 78 MPa, while the material proportional constant is 29.2 MPa-mm1/2.

When does fatigue occur in metals?

an alloy with an avg grain diameter of 0.053 mm is l16 MPa. When the grain diameter is 0.008Amm, the yeild strength goesupto 231 MPa.lf the yeild strength is 207MPA, what would the grain diameter be?

Calculate the approximate total dislocation line length(or range of lengths) expected in a cubic cm of a very highlycold-worked metal.

The yield strength for an alloy that has an average grain diameter of 5.2 x 102 mm is 110 MPa. At a grain diameter of 6.3 x 103 mm, the yield strength increases to 251 MPa. At what grain diameter, in mm, will the yield strength be 222 MPa? d = mm

What are slipbands and slip lines? What causes the formation of slipbands on a metal surface? What are the principal slip planes and slip directions for FCC, BCC and HCP metals?