Single Crystal Deformation

Transcription

1 Single Crystal Deformation To make the connection between dislocation behavior and yield strength as measured in tension, consider the deformation of a single crystal. Given an orientation for single slip, i.e. the resolved shear stress reaches the critical value on one system ahead of all others, then one obtains a pack-ofcards straining.

2 Resolved Shear Stress Geometry of slip: how much of an applied stress is required for slip? To obtain the resolved shear stress based on an applied tensile stress, P, take the component of = b the stress along the slip direction which is given by Fcosl, and divide by the area over which the (shear) force is applied, A/cosf. Note that the two angles are not complementary unless the slip direction, slip plane normal and tensile direction happen to be co-planar. t = (F/A) coslcosf = s coslcosf = s * m = n In tensor (index) form: t = b i s ij n j Schmid factor := m

3 Schmid postulated that: Schmid s Law Initial yield stress varies from sample to sample depending on, among several factors, the position of the crystal lattice relative to the loading axis. It is the shear stress resolved along the slip direction on the slip plane that initiates plastic deformation. Yield will begin on a slip system when the shear stress on this system first reaches a critical value (critical resolved shear stress, crss), independent of the tensile stress or any other normal stress on the lattice plane. E. Schmid & W. Boas (1950), Plasticity of Crystals, Hughes & Co., London. 3

4 Schmid s Law Resolved Shear Stress t = s c s = s n = s cosf cosl = s m t c t c = cosf cosl m

5 Critical Resolved Shear Stress The experimental evidence of Schmid s Law is that there is a critical resolved shear stress. This is verified by measuring the yield stress of single crystals as a function of orientation. The example below is for Mg which is hexagonal and slips most readily on the basal plane (all other t crss are much larger). s = t/coslcosf Exercise: draw a series of diagrams that illustrate where the tensile axis points in relation to the basal plane normal for different points along this curve Soft orientation, with slip plane at 45 to tensile axis Hard orientation, with slip plane at ~90 to tensile axis

6 Schmid s Law Using Schmid s law 6

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8 Rotation of the Crystal Lattice The slip direction rotates towards the tensile axis

9 Slip Systems in fcc materials The combination of slip plane {a,b,c,d} and slip direction {1,2,3} that operates within each unit triangle is shown in the figure For FCC materials there are 12 slip systems (with + and - shear directions: Four {111} planes, each with three <011> directions

10 Geometry of Single Slip For tensile stress applied in the [100]- [110]-[111] unit triangle, the most highly stressed slip system (highest Schmid factor) has a (11-1) slip plane and a [101] slip direction (the indices of both plane and direction are the negative of those shown on the previous page). Caution: this diagram places [100] in the center, not [001].

11 Schmid factors (a) (b) The Schmid factors, m, vary markedly within the unit triangle (a). One can also (b) locate the position of the maximum (=0.5) as being equidistant between the slip plane and slip direction.

12 Names of Slip Systems In addition to the primary slip system in a given triangle, there are systems with smaller resolved shear stresses. Particular names are given to some of these. For example the system that shares the same Burgers vector allows for cross-slip of screws and so is known as the cross slip system. The system in the triangle across the [100]-[111] boundary is the conjugate slip system.

13 Rotation of the Crystal Lattice in Tensile Test of an fcc Single Crystal The tensile axis rotates in tension towards the [100]-[111] line. If the tensile axis is in the conjugate triangle, then it rotates to the same line so there is convergence on this symmetry line. Once on the line, the tensile axis will rotate towards [211] which is a stable orientation. Note: the behavior in multiple slip is similar but there are significant differences.

14 Latent Hardening and Overshooting

15 Rotation of the Crystal Lattice in Compression Test of an fcc Single Crystal The slip plane normal rotates towards the compression axis

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24 Typical deformation textures in metals and alloys FCC Metals and alloys Texture of wires and rods:

25 Compression textures

26 Rolling textures Characteristic metal / Copper type textures develop in metals with high / medium SFE

27 Rolling textures With increase in alloying which lower the SFE / increase strength etc. Characteristic alloy / Brass type textures develop in metals with high / medium SFE

28 Rolling reduction ~ 50% Combined Effect: Amount of Deformation + Composition Rolling reduction ~ 75% Texture transition with increasing Zn content from Metal / Copper type texture to alloy / Brass type texture. Rolling reduction ~ 95% 0% Zn 5% Zn 30% Zn

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32 B. BCC Metals and alloys Texture after uniaxial deformation: A simple [110] fibre texture after cold drawing In general, no additional component forms, major or even minor (studied metals and alloys include Fe, Mo, Nb, Ta, V, W, Fe-4.6%Si and Beta Brass) No alterations by solid solution formation

33 Rollling Textures

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35 C. HCP Metals and alloys More than ideal Major criterian: c/a ratio Ideal, Less than ideal

36 Rolling textures In addition to c/a ratio, strain rate, temperature and chemical composition (e.g. oxygen content) plays important role Texture variation is overall dictated by relative ease of slip and twinning