“The Structure of Metals”Group # 32/06/2006Keith DagerDavid FullerGrant ThomasAustin WeddingtonJared Martinez
Introduction • Many questions about metal can be answered by knowing their ATOMIC STRUCTURE(the arrangement of the atoms within the metals).
The Crystal Structure of Metals • Metals and Crystals • What determines the strength of a specific metal. • Three basic atomic arrangements
1of 3 basic atomic arrangements 1. Body-centered cubic (bcc) 1. A portion of the structure of a body-centered cubic metal (b.c.c.)
2 of 3 basic atomic arrangements 2. Face-centered cubic (fcc) also known as Cubic close packing
3 of 3 basic atomic arrangements 3. Hexagonal close-packed (hcp)
Review of the three basic Atomic Structures B.C.C. F.C.C H.C.P
Basic terminology • Hard-ball / Hard-sphere - the small spheres used to display the unit cell and show the individual atomic arrangement. • Basal planes- This is the orientation/layout of the atomic arrangement specifically in the h.c.p. layout of the ABAB pattern. • Alloying- this is formed by adding atoms of one metal/metals to some other metal/metals. • Allotropism/Polymorphism (meaning many shapes)- the appearance of more than one type of crystals.
Deformation and Strength of Single Crystals • When a single crystal is subjected to an external force, it goes through Elastic Deformation • If the force on the crystal is increased sufficiently, the crystal goes through Plastic/Permanent Deformation • The amount of stress required for a crystal to permanently deform is the called the Critical Shear Stress
Shear Stress Cross-Section F2 • Shear Stress is the ratio of the applied shearing force to the cross-sectional area being sheared • When this occurs, one plane of atoms slips across an adjacent plane of atoms F1 Tensile Force2 Tensile Force1
Deformation Video here
Shear Stress • b/a ratio – Proportional to the amount of shear stress needed to cause slip in single crystals Atoms b is inversely proportional to the atomic density in the atomic plane • a is the spacing • of atomic planes
Shear Stress • Anisotropic – The different properties of a single crystal when tested in different directions • Examples : Plywood & Cloth • Twinning – The crystal forms a mirror image of itself across the plane of twinning Tensile Force2 Tensile Force1
Slip Systems • A slip system is the combination of a slip plane and its direction of slip • Body-Centered Cubic structure • 48 slip systems • Highly probable for any shear stress to act on one of these systems, but because of a high b/a ratio, the shear stress required must be high • Metals with these structures have good strength and moderate ductility (flexibility)
Slip Systems • Face-Centered Cubic structure • 12 slip systems • Moderate probability for a shear stress to act on one of these systems • Low b/a ratio, the shear stress required is low • Metals with these structures have moderate strength and good ductility
Slip Systems • Hexagonal Close-Packed structure • 3 slip systems • Low probability for a shear stress to act on one of these systems • More systems become active at elevated temperatures • Metals with these structures are usually brittle at room temperature
Imperfections in metal crystal structures • The actual strengths of metals are lower than theoretical calculations because of defects and imperfections in crystal structures • This includes: • Grain/Phase Boundaries (Next Section) • Volume/Bulk Imperfections • Voids, Cracks, Inclusions ( nonmetallic elements) • Point Defect • Dislocations
Point Defects • Vacancy - a missing atom • Interstitial Atom - an extra atom in the structure • Impurity – a foreign atom that has replaced an atom of metal
Dislocations • Dislocations are defects in the orderly arrangement of the atomic structure including: Screw Dislocations Edge Dislocations
Dislocations • Dislocations can lower the required shear stress to cause slip • They can also interfere with each other and be impeded by barriers ( grain boundaries, impurities, inclusions ) which can cause the required shear stress for slip to go up. • This is referred to as Work/Strain Hardening • Increases strength of metal • Increases hardness of metal
Grains and Grain Boundariesand their effects on a metal Grain size has a significant effect on the strength of metals. Grain boundaries have a major influence on metal behavior.
What are grains? • Grains- individual, randomly oriented crystals within a metal Grain Structure of metal alloys
How are grains formed? • Molten metal begins to solidify • Crystals begin to form independently of each other • Each crystal has random unrelated orientation to the other • Each of these crystals grows into a crystalline structure (a grain)
What difference does Grain Size make? • Significantly influences mechanical properties of the metal • Large grain size is generally associated with - Low strength - Low hardness - Low ductility (extent of deformation before fracture) • Small grain size is generally the opposite
What determines Grain size? Three factors that influence the median size of developed grains • Rate of nucleation (initial rate of formation of the individual crystals) • Number of sites where crystals begin to form • Rate crystals grow Calcite crystal (~50 µm) grown at T = 800 °C and p = 300 MPa within 10 hours
What determines Grain number? • High rate of nucleation (comparatively) - Number of grains per unit volume of metal will be high - Grain size will be small • High growth rate (comparatively) - Fewer grains per unit volume of metal - Grain size will be large The grain size of a nanocrystalline metal, right, is about 1,000 times smaller than conventional metal, above. When grain size is cut in half, the company says, hardness quadruples. Wear-resistant nanometals could make sporting goods more durable.
Controlling grain size In general: Rapid cooling produces smaller grains. Slow cooling produces larger grains. The smaller the grain size, the stronger the metal
How is grain size measured? • Counting the grains in a given area • Counting number of grains that intersect a length of a line (microscopic) • Determined by comparing to a standard chart (ASTM Chart)
ASTM Chart • American Society for Testing and Materials • Grain number is determined by formula: N = 2^(n-1) N- Number of grains n- grain size number Per square inch at 100x magnification
Examples of grain size • Grain sizes between 5 and 8 are generally considered fine grains • Grain size of 7 is acceptable for sheet metal of car bodies, kitchen utensils, and appliances
Grain size can also be so large as to be seen with the naked eye as in zinc on the surface of galvanized sheet steels
What are grain boundaries? • Surfaces that separate individual grains 1. Influence strain hardening (boundaries interfere with the movement of dislocations) 2. More reactive than the grains themselves (atoms along grain boundaries are more disordered and packed less efficiently) 3. Great influence on the strength and ductility of the metal Cast Iron grain boundaries
What can happen at grain boundaries? • Grain boundary sliding • Grain boundary embrittlement 3. Hot shortness
Grain boundary sliding • Process by which grains will begin to slide along one another at the boundary Possible Effects on Metal: • Plastic deformation • Creep mechanism (elongation under stress over period of time) Note: These types of deformation are usually accompanied with high temperatures as well
Grain Boundary embrittlement • Generally: weakening of the grain boundaries by embrittling elements • Liquid-metal embrittlement (elements are in liquid state) • Solid-Metal embrittlement (elements are in the solid state) • Temper embrittlement (in alloy steels -caused by segregation (movement) of impurities to grain boundaries)
Hot Shortness • Generally: softening and/ or melting of metal along grain boundaries • Cause: local melting of impurity in the boundary at a temperature below the melting point of the metal itself • Effect: when subjected to plastic deformation at elevated temperatures, (hot working) the piece of metal crumbles and disintegrates along the metal boundary • Prevention: metal is worked at a lower temperature
Plastic Deformation of Polycrystalline Metals • Polycrystalline Metals • Equiaxed grains • Plastic deformation • Strain (deformation) • Anisotropy • Preferred Orientation • Mechanical Fibering
Polycrystalline metals What is Plastic Deformation of Polycrystalline Metals and all that goes with it? polycrystalline materials: metals, alloys, intermetallic compounds, ceramic materials, compound materials, polymers, semiconductors, nanocrystals, supraconductors, rocks; • Metals commonly used in manufacturing various products • Consist of many individual, randomly oriented crystals (grains); • Thus, metal structures typically are not single crystals but • Polycrystals. (“many crystals”) • The most natural and artificial solids (rocks, ceramics, metal alloys or polymers) are polycrystalline. They contain many crystallites of different size, shape and different orientations.
Equiaxed grains Equiaxed grains Having equal dimensions in all Directions, as shown in the in Fig.1.12a Reduction operation resulting in directionality or anisotropy
GrainSizes Equiaxed grains (having equal dimensions in all Directions, as shown in the in Fig.1.12a)
Cold Working • If a polycrystalline metal with uniform equiaxed grains is subjected to plastic deformation at room temperature(cold working), the grains become deformed and elongated. Homologous Temperature Ranges for Various Processes
Deformation Process • Compressing the metal: • Compression stresses develop within a material when forces compress or crush the material. A column that supports an overhead beam is in compression, and the internal stresses that develop within the column are compression. • As is done in forging to make a turbine disk • "Stretching" • Stretching is a process where sheet metal is clamped around its edges and stretched over a die or form block. This process is mainly used for the manufacture of aircraft wings, automotive door and window panels. • As is done in stretching sheet metal to make a car body.
ANISOTROPY (Texture) As result of Plastic deformation, the grains have elongated in one direction and contracted in the other. Consequently, this piece of metal has become anisotropic. Anisotropic surface will change in appearance as it is rotated about its geometric normal, as is the case with velvet. anisotropic properties: plasticity, elasticity, hardness, strength, cleavability, thermal expansion and conductivity, electric conductivity, magnetization, corrosion resistance, optical properties.
ANISOTROPY • Texture and Anisotropy of Crystalline Materials • A crystal is characterized by the periodic arrangement of its elements (atoms, ions) in space. This always generates a dependence of the crystal properties on the chosen direction, which is called anisotropy. Thus, the modulus of elasticity can vary by the factor 22 in a graphite crystal depending on the direction.
Anisotropy (b) Figure 1.13 (a) Schematic illustration of a crack in sheet metal that has been subjected to bulging (caused by, for example, pushing a steel ball against the sheet). Note the orientation of the crack with respect to the rolling direction of the sheet; this sheet is anisotropic. (b) Aluminum sheet with a crack (vertical dark line at the center) developed in a bulge test; the rolling direction of the sheet was vertical. Source: J.S. Kallend, Illinois Institute of Technology.
Anisotropy The most important parameter describing the anisotropy of polycrystalline materials is their texture. Via the anisotropy of physical properties due to the lattice structure, a regular texture in which the crystallites of one phase have only a few preferred orientations produces anisotropy of the polycrystalline material as well. .