机械制造工程 开卷习题答案.doc

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1、QUESTIONS1.1.Explain.the.difference.between.a.unit.cell.and.a.single.crystal..Answer.(from.page.#.40):.The.Smallest.group.of.atoms.showing.the.characteristic.lattice.structure.of.a.particular.metal.is.known.as.a.Unit.Cell...It.is.the.building.block.of

2、.a.crystal,.and.a.single.crystal.can.have.many.unit.cells...1.2.In.tables.on.crystal.structures,.iron.is.listed.as.having.both.a.bcc.and.a.fee.structure..Why?..Answer.(from.page.#.41):.Same.metal.may.form.different.structures,.because.of.a.lower.energ

3、y.requirement.at.that.temperature...For.example,.iron.forms.a.bcc.structure.(alpha.iron).below.912.°C.(1674.°F).and.above.1394..°C.(2541.°F),.but.if.forms.an.fcc.structure.(gamma.iron).between.912.°C.and.1394.°C.1.3a..Define.anisotropy,.and.what.mater

4、ials.can.you.think.of.other.than.metals.that.exhibit.anisotropic.behavior?..Answer.(from.page.#.42):.Because.the.b/a.ratio.is.different.for.different.directions.within.the.crystal,.a.single.crystal.has.different.properties.when.tested.in.different.dir

5、ections...We.say.that.a.single.crystal.is.anisotropic...Where.“a”.is.the.spacing.of.the.atomic.planes.and.“b”.is.inversely.proportional.to.the.atomic.density.in.the.atomic.planes.86A.common.example.of.anisotropy.is.woven.cloth,.which.stretches.differe

6、ntly.when.we.pull.it.in.different.directions,.or.plywood,.which.is.much.stronger.in.the.planar.direction.than.along.its.thickness.direction.(it.splits.easily).............................................................................................

7、...........................................................................................................................1.4.What.effects.does.recrystallization.have.on.properties.of.metals?..Answer.(from.page.#.50):.Recrystallization.decreases.the.

8、density.of.dislocations,.lowers.the.strength,.and.raises.the.ductility.of.the.metal...1.5.What.is.strain.hardening?.And.what.effects.does.it.have.on.the.properties.of.metals?..Answer.(from.page.#.45):.The.effect.o9.fan.increase.in.shear.stress