1 ?? determine whether an application is possible No matter how good the electric, magnetic, chemical or other properties are, a material is of no use if it does not fulfill mechanical requirements hardness compressive & tensile strength deformability resistance Mechanical Properties cleavability elasticity brittleness ductibility Hardness Hardness is dependent upon the strength of the forces holding atoms together in a solid. F. Mohs in 1822 selected 10 minerals as standards and arranged them in order of hardness so that one mineral could scratch only those below it on scale. Intervals of hardness between the standard minerals are roughly equal except for that between corundum and diamond. Hardness Mineral 10 Diamond 9 Corundum 8 Topaz 7 Quartz 6 Feldspar 5 Apatite 4 Fluorite 3 Calcite 2 Gypsum 1 Talc Ionic crystals have moderate to medium hardness ( eg. NaCl hardness 2) Materials with Hardness less than 7 is lusterless in everyday use because they continually suffer from scratching from quartz particles in dust ( Which accounts for the advantage of glass over plexiglass) Hardness ?? This is found by scratching one mineral against another. The harder mineral leaves a scratch on the softer one. The Mohs hardness scale is a list of 10 minerals in order of hardness. 1.Talc 2.gypsum3. calcite4. fluorite 5. apatite 6. feldspar 7. quartz 8. topaz 9. corundum10. diamond Mohs￠ hardness scale is used when you are trying to identify the relative hardness of a mineral. The hardness of a mineral is useful when trying to identify an unknown mineral sample. Mohs Mineral Hardness Scale 1) Talc 2) Gypsum 3) Calcite 4) Flourite 5) Apatite 6) Feldspar 7) Quartz 8) Topaz 9) Corundum 10) Diamond Softest Hardest 1 2 3 4 5 6 7 8 9 10 Mohs￠ Hardness Scale is named after its creator, German mineralogist Friedrich Mohs (1773-1839). Finger Nail (H = 2.5) Penny (H = 3) Knife Blade (H = 5.5) You typically do not carry around a supply of the 10 minerals on the hardness scale: Talc (soapstone) Mg3(OH)2[Si4O10] Mg(OH)2 layer silicate sheet The silicate sheets are electrically neutral in talc, forces between them are weak, the crystal are soft and easy to cleave. The use of talc (also graphite and MoS2) as powder, lubricating agent, polishing materials and filling material for paper is due to these properties. 2 Mica Micas are cation-poor sheet silicate consisting of electrically charged sheets that are being held by intercalated, unhydrated cations. They cannot slip as Talc but can be cleaved. Electrical insulating Chemical resistance Thermal resistance Cation exchangers Stress vs. Strain Stress is force applied over area: load/original cross sectional area tensile stress compressive stress shear stress torsion stress Strain is a dimensional change due to an applied stress stress To describe how materials deform as a function of applied load. we need to discuss testing methods and language for mechanical properties of materials. Stress, s( MPa ) Strain, e (mm / mm) Stress, σ( MPa ) ε(mm / mm) Types of Loading Tensile Compressive Shear Torsion Stress vs. Strain: Units Stress σ=F/A0 (where A0 is the original cross-sectional area) psi(pounds force per square inch) MPa (Mega Pascals = 106 N/m2 ) Strain = L/L0 (where L0 is the original length) unitless ? sometimes expressed as a percentage Stress and Strain (tension and compression) To compare specimens of different sizes, the load is calculated per unit area. Stress: σ = F / A0 F: is load A0: cross-sectional area Perpendicular to F before application of the load. Strain: ε = ?l / l0 (× 100 %) ?l: change in length l0: original length. Stress / strain: + for tensile loads － for compressive loads 3 Shear and Torsion Shear stress: τ = F / Ao F is applied parallel to upper and lower faces each having area A0. Shear strain: γ = tanθ (× 100 %) θ is strain angle Torsion: is like shear. Load: applied torque T Strain:related to the angle of twist, φ. Shear Torsion Typical Mechanical Properties of Metals The yield strength and tensile strength vary with thermal and mechanical treatment, impurity levels, etc. Variability is related to the behavior of dislocations. Elastic moduli are relatively insensitive to these effects. Yield and tensile strengths and modulus of elasticity decrease with increasing temperature. Ductility increases with temperature. Metal Alloy Yield Strength MPa Tensile Strength MPa Ductility (%EL) [in 50mm] Aluminum 35 90 40 Copper 69 200 45 Brass(70Cu- 30Zn) 75 300 68 Iron 130 262 45 Nickel 138 480 40 Steel (1020) 180 380 25 Titanium 450 520 25 Molybdenum 565 655 35 Brittleness The brittleness may be modified by doping with impurities. Ceramic materials: oxides, silicates, and nitrides or carbides. They have strong chemical bonds and show high m.p. Substance m.p. (oC) M?O (?) MgO 2800 2.12 CaO 2580 2.40 SrO 2430 2.56 BaO 1923 2.76 SiO2 1700 SiC 2700 B4C 2350 BN 3000 Because of the short range of action of the chemical bonds, the material suffers a substantial loss of strength once a rupture has begun. The resulting brittleness is one of the most severe drawbacks of ceramic materials. Zirconia, ZrO2 > 2370oC cubic CaF2 (c.n. of Zr = 8) 1170 –2370oC tetragonal (c.n. of Zr = 4+4) < 1170oC baddeleyite, stable form (c.n. of Zr = 7) monoclinic tetragonal 5.56 6.10 gcm-3 On heating, 9% contraction in volume accompanies the transition Pure ZrO2 is not appropriate as a high T ceramic: it cracks during heating when the transition temperature of 1170°C is reached. By doping with 10 to 20 percent of CaO, MgO or Y 2O3 (these oxides form solid solutions with the high T, cubic polymorph of ZrO2 and these cubic solid solutions are stabilized to much lower T) the tetragonal form can be stabilized down to room temperature. Use volume effect to reduce the brittleness 1170oC Toughness Toughness: ability to absorb energy up to fracture Area under the strain-stress curve up to fracture Units: the energy per unit volume, e.g. J/m3 Material deformed plastically and stress is released, the material ends up with a permanent strain. If stress is reapplied, the material again responds elastically at the beginning up to a new yield point that is higher than the original yield point. The amount of elastic strain that it will take before reaching the yield point is called elastic strain recovery. Elastic Recovery During Plastic Deformation 4 Plastic Deformation of a Single Crystal When the applied force increases, it may break the bonds of the atoms to produce a fracture, or cause the atoms to slide over one another to produce a permanent shift. For brittle materials the first mechanism requires lower forces, but for ductile metals the second one occurs more easily. Plastic slip occurs most easily in densely packed planes that are widely spaced apart and along directions of closest packing. The specific combination of plane and direction is called a slip system. Each lattice structure has a specific number of slip systems which determines the ease with which a crystal can be deformed. Slip System Preferred planes for dislocation movement (slip planes) Preferred crystallographic directions (slip directions) Slip planes + directions (slip systems) àhighest packing density. Distance between atoms shorter than average; distance perpendicular to plane longer than average. Far apart planes can slip more easily. BCC and FCC have more slip systems compared to HCP: more ways for dislocation to propagate ? FCC and BCC are more ductile than HCP. Superplasticity of Nanoscaled Copper Cleavage Some minerals split along flat surfaces when struck hard ? this is called mineral cleavage. A mineral cleavage is the way it breaks. Minerals tend to break where the bonds holding the atoms together in the crystal are the weakest. When they break, a series of surfaces parallel to these bonds may form; the surfaces are called cleavage planes. Other minerals break unevenly along rough or curved surfaces ? this is called fracture A few minerals have both cleavage and fracture Mineral Cleavage This is the tendency of minerals to break along smooth planes. Cleavage in one direction Cleavage in two directions at right angles Cleavage in three directions at right angles Cleavage in three directions not at right angles Cleavage in four directions Some minerals lack cleavage ( like quartz) Cleavage is related to the atomic structure of the mineral Mineral Cleavage and Crystal Form 5 Mineral Cleavage Cleavability Ionic crystals can be cleaved in certain directions Shearing force Shearing force The displacement is easiest along planes which have the fewest cation-anion contacts. A NaCl crystal is most easily cleaved perpendicular to a cell edge Cleavage & Fracture ? Cleavage: reflects the tendency of a mineral to break along preferred planes of weakness (weak bonds). Not all minerals have cleavage. ? Cleavage may conform to crystal faces, but does not have to. Cleavage is a result of atomic structure. Crystal faces result from growth. ? Fracture: mineral breakage along irregular surfaces. All minerals can fracture. Mineral Fracture This is how a mineral breaks on an uneven surface. Some break in a conchoidal fracture like glass & quartz Some break in a fibrous fracture Others in a splintery fracture Fracture is a breakage unrelated to the atomic structure of the mineral Important Materials related to Mechanical Properties Ceramic Engine Memory Metal Ceramic Engine Engines have long life Allow engines to run at higher temperature More powerful Improve gas mileage by 18% Allow engines to run at more than 18000rpm’s Valves require no lubrication No loss of oil through leakage of burning Advantages: 6 Ceramic Engine Ceramic motor under commercial production materials: Sintered silicon nitride 40 kW power swept volume of 225 cc weight of 14 kg would not need a liquid cooling system (i.e. no water- jacket, no radiator, no pump). Possibly priced at $2300 That is, it is much smaller than a reciprocating internal combustion engine with similar power (e.g. 39.5 kW, 3990 cc, 427 kg). The attributes of Ceramic Engine are expected to be: High power and high torque, small size and minimal vibration levels (normal attributes of rotary engines) Fuel efficient with low emissions (the result of more efficient combustion at higher temperatures) Minimal Maintenance (due to unique design characteristics) Multi-fuel capability e.g. using diesel fuel, kerosene, gasoline, Very few engine components minimize build costs Minimal cooling requirements simplifies installation Low running costs (owing to its anticipated fuel efficiency and multi-fuel capability). The ceramic engine could achieve the following: its better fuel efficiencies and multi-fuel capabilities would reduce dependence on foreign oil and improve the economic security of industrial nations more efficient combustion of fuel in the ceramic confines of the engine would result in less pollutants being discharged into the environment a new breed of engine would enhance the marketability of the automobile. the combination of minimal maintenance, low running costs, long engine life and multi-fuel capability is ideally suited for the rural countryside and less-developed nations to power, for example, water supply pumps or electrical generators. Memory Metal Memory metal is a nickel-titanium alloy. This piece has been formed into the letters ICE, heat-treated, and cooled. When the memory metal is pulled apart, it deforms. When placed into hot water, the metal "remembers" its original shape, and again forms the letters ICE. How the Memory Metal Rivet Automatically Soldered The steel spring easily compresses the memory metal spring when it is in the low-T martensite phase. Fluid transmission As the temperature increases, the NiTi spring remembers its extended shape (it transforms into the austenite phase) and becomes rigid, and thus forces the steel spring to compress so that the pathway for transmission fluid through the valve is opened. 7 Despair Cool weather Warm weather What′s Shaped Memory Alloys? Shape Memory Alloys (SMA's) are novel materials that have the ability to return to a predetermined shape when heated. When a SMA is cold, or below its transformation temperature, it has a very low yield strength and can be deformed quite easily into any new shape-which it will retain. However, when the material is heated above its transformation temperature it undergoes a change in crystal structure, which causes it to return to its original shape. Shaped Memory Effect At a low temperature, a SMA can be seemingly “plastically” deformed, but this “plastic” strain can be recovered by increasing the temperature. This is called the Shape Memory Effect (SME). At a high temperature, a large deformation can be recovered simply by releasing the applied force. This behavior is known as Superelasticity (SE). (a) Shape Memory Effect and (b) Superelasticity Definition of a Shape Memory Alloy Shape Memory Alloys (SMAs) are a unique class of metal alloys that can recover apparent permanent strains when they are heated above a certain temperature. Different phases of SMA The SMAs have two stable phases - the high-temperature phase, called austenite and the low-temperature phase, called martensite. In addition, the martensite can be in one of two forms: twinned and detwinned. A phase transformation which occurs between these two phases upon heating/cooling is the basis for the unique properties of the SMAs. The key effects of SMAs associated with the phase transformation are pseudoelasticity and shape memory effect. Upon cooling in the absence of applied load the material transforms from austenite into twinned (self-accommodated) martensite. As a result of this phase transformation no observable macroscopic shape change occurs. Upon heating the material in the martensitic phase, a reverse phase transformation takes place and as a result the material transforms to austenite. Definition of a Shape Memory Alloy Typical CsCl structure The size of the spheres indicates depth. 2D projections of one of the rectangles. Series of stacked planes The arrows indicate one component of the sliding of the planes that leads to changes in the atomic positions during the phase (A→M) transition. 8 The plane may shift along a keeping the 90 degrees angles. The plane may shear along b or c. These motions will destroy the 90o angles of the plane. Four variants The six equivalent planes pass through face diagonals in the CsCl structure. Thus, a total of 6 x 2 x 2 = 24 variants may grow from the planes. There are 24 different ways to carry out phase transformation. The Structural Cycle of Shape Memory The cycle starts with the NiTi in the austenite phase. When bent, the variants can re-orient from left to right or vise versa to relieve the stress. high density low volume low density high volume Reaction Energy vs Structure (gray tin, diamond structure, stable below 13oC) tetrahedral white tin Pressure vs Structure pressure + P ? P The transformation of sodium chloride from the rock salt to the cesium chloride structure can be accomplished at 298 K at high P (~105 atm). pressure 9 From the View of Crystal Chemistry: From Austenite to Martensite a2/a 0 → ca0 → Expand~12% Expand~12% Change of the Chemical Composition (i) Nickel concentration effect: Preparation of NiTi: heating Ni and Ti elements together above m.p. (1200?1300oC). NiTi can tolerate small deviations in chemical composition around 1:1 stoichiometry before the shape memory effect is lost. Change of the Chemical Composition (ii) Adding impurity metal A small quantity of impurity atoms the may be present may influence the transition temperature. Example: By substitution of Ni with Pd or Pt will tune the transition temperature to as high as several hundred degrees. Ti-Ni-based alloys SMAs Although there are many SMAs, such as Ti-Ni, Cu-Al- Ni, Cu-Zn-Al, Au-Cd, Mn-Cu, Ni-Mn-Ga, and Fe-based alloys， most of the practical SMAs are Ti-Ni-based alloys, since other SMAs are usually not ductile (or not ductile enough) or are of low strength and exhibit grain-boundary fracture. Ti-Ni-based alloys are superior to other SMAs in many respects. They exhibit 50~60% elongation and tensile strength as high as 1000 MPa. To our knowledge, they possess the best mechanical properties among intermetallics and can be used as structural materials as well. They also have a very high resistance to corrosion and abrasion.