Texture and properties - II

Transcription

1 Texture and properties - II

2 Texture and Hall-Petch strength The Hall-Petch equation 0 k d - ½ where, 0 = k = d = lattice frictional stress locking parameter average grain size modifies for textured polycrystals to m ( 0 k s d - ½ ) Armstrong equation where, m = 0 = k s = d = Taylor orientation factor lattice frictional stress stress concentration required to propagate slip across a grain boundary average grain size

3 Dependence of Ductility on texture Orientation of slip plane and slip direction with respect to the tensile axis decides the ductility For example, In FCC materials, slip system {111}<110>, maximum ductility is achieved when maximum number of crystallites with {111}<110> are oriented at 45 to the axis of tension.

4 Texture dependence of impact and fracture toughness Texture leads to variation in impact toughness. This is governed by the direction of crack propagation. Charpy energy Here the notch is made on the edge. In this case, crack plane is (10 10), and crack direction is [1 210] Here the notch is made on the surface. In this case, crack plane is (10 10), but crack direction is [0001] Temperature Texture influence of fracture toughness is by selection of preferred sites for (i) crack nucleation, and (ii) propensity of crack propagation along a particular crystallographic orientation

5 Texture and Fatigue Texture affects fatigue properties in two ways: The elastic modulus is influenced by texture. Therefore, the anisotropy in elastic modulus influences fatigue life. See Baskin s formula: Influence of local texture on fatigue damage -micro-crack initiation -crack-growth mechanism

6 Fatigue of titanium alloy as a function of texture Effect of specimen orientation on fatigue life is clear from the following figure. It is to be noted that by changing the direction of extraction of specimen from a textured plate, the texture along the loading direction can be changed. When tested along RD Stress When tested along TD Number of cycles

7 Influence of anisotropy in modulus on fatigue life Young s modulus ranges from GPa for single crystal titanium. In textured material, this variation influences elastic strain amplitude, and hence fatigue life. Young s modulus Orientation of the single crystal In random polycrystals, grain to grain anisotropy is more, and it may be leads to easy crack initiation with facet formation. In highly textured material this may not be the case.

8 Influence of local or micro-texture Large facets observed in the left figure is due to local texture. Local texture is detrimental for fatigue resistance. Lindemann, Wagner, Mater Sci Engg A, 263, (1999),

9 Can texture be utilized to improve fatigue resistance? Knowing the influence of texture on Cyclic stress strain curve Anisotropy in mechanical response Preferred sites for crack nucleation Preferred orientation for propagation Effect of micro-texture on early crack growth...one can definitely improve the efficiency of design against fatigue.

10 Orientation dependent properties of Titanium Orientation dependence of mechanical properties is very clear from the experimental data obtained on titanium. The following table of mechanical properties is obtained for a titanium plate, whose texture is described as figure. The specimens were extracted from all three directions of the plate. TD ND (0002)//TD RD Test direction Tensile Strength (MPa) Yield Strength (MPa) Elongation (%) Reduction in area (%) Elastic modulus (GPa) K 1c (MPa.m 1/2 ) K 1c *Specimen Orientation L L-T T L-T S S-T *45 0 from the longitudinal (rolling) direction and about 40 0 from the plate. L = longitudinal; T = transverse; S = short transverse

11 Textures dependent properties of some magnetic materials Soft magnetic materials: Iron silicon alloys Permalloy (Fe-50Ni) <100> : easy magnetisation direction Permanent magnets: Alnico Barium hexaferrite Samarium-cobalt <100> : easy magnetisation direction <0001> magnetic field

12 Electrical steels Fe-3%Si is used as a core for power transformers. The desired texture for this application is {110}<001>. Alignment of easy direction of magnetisation [001] along the rolling direction a very high permeability reduces the mangnitude of field necessary to operate transformer Variation of magnetic power loss with angle around the sheet for for Goss textured silicon iron a (110)[1 0 0] a a This direction also corresponds to the minimum power loss during alternating magnetisation By a controlled schedule of processing, it is possible to orient ~98% of the grains to (110)[001] orientation.

13 Important magnetic properties of some important magnetic materials in their isotropic and textured conditions Material Remanence (Tesla) Coercive field (amp/cm) Alnico (Isotropic) Alnico (oriented) Lodex (isotropic) Lodex (oriented) Hexaferrite (isotropic) Hexaferrite (oriented) SmCo 5 (isotropic) SmCo 5 (oriented) Energy product (kj/m 3 )

14 Questions 1. Fatigue properties of a polycrystalline material depends on texture because (a) yield strength of polycrystalline materials depend on texture (b) elastic modulus of polycrystalline materials is anisotropic (c) direction of loading during fatigue testing is reversed during each cycle (d) crack propagation depends on local orientation of the grains 2. What is good texture component in steel for electric application? 3. List five properties that depend on texture. 4. List any two industrial applications where texture control has extensively been practiced. 5. Briefly explain texture dependent properties in HCP metal and alloys.