Introduction to Engineering Materials. Eng. Yousef Shatnawi

Transcription

1 Introduction to Engineering Materials

2 Historical Perspective Historically, the level of development of any society is tied to the ability to produce and manipulate materials to fill the needs. Stone age - Bronze age -Iron age

3 Historical Perspective Materials are everywhere in our life: Transportation Housing Clothing Communication Recreation Food Production

4 Historical Perspective In early stages, most materials are found naturally. Later, human discovered different ways to mix two or more materials or to treat (heat treatment) a material in order to get artificial materials (usually superior to naturally available). However, the relation between structure of any material with performance was not established in early stages. Recently, this relation is being understood. This enables to tail or fashion so many materials that meet the complex modern needs

5 Materials Science and Engineering Material Science: concerned with investigation of relation between structure of materials and the resulting properties. Materials Engineering: on the basis of material science (structure-properties correlation), materials engineering is concerned with design (tail or fashion) a new material that meet predetermined set of properties. This is achieved by material processing.

6 Materials Science and Engineering Structure of a material means the arrangement of its internal components; and this is studied in levels: Subatomic level: nuclei and surrounding electrons Atomic level: the organization of atoms relative to each other Molecular level: the organization of molecules relative to each other Microscopic level: how the material looks like under a microscope. Macroscopic level: how the material appears for a naked eye.

7 Materials Science and Engineering Property of a material is defined as the kind and degree of response to an external stimulus. Examples: How a specimen deform under a load. How the surface of material reflect light. How a specimen responds to a magnetic field. Property of a material is independent of size and shape.

8 Materials Science and Engineering Categories of materials properties: 1) Mechanical : e.g. deformation 2) Electrical: e.g. conductivity 3) Thermal: e.g. heat capacity & thermal conductivity 4) Magnetic: response to magnetic field 5) Optical: the stimulus here is the light, e.g. index of refraction and reflectivity. 6) Deteriorative: chemical reactivity of materials.

9 Materials Science and Engineering Material s performance is a function of properties. Material s property is a function of structure. Material structure is a function of processing. In design, production and utilization of materials, it is essential to understand these relationships.

10 Crystallization Process of formation of solid crystals from the liquid state. Two stages (Does not happen instantly): 1) Nucleation : Cluster formation 2) Crystal growth: clusters are growing up and ultimately join together to form the final solid material.

11 Crystallization Basically, crystallization starts just below the melting temperature T m and the final resulting solid is determined to be either crystalline or glass (vitreous) based on the viscosity of the substance in the molten state. Viscosity depends on: 1) Size and complexity of the molecules: viscosity of liquid metals are low whereas it is high for polymeric. 2) Cooling rate: crystallization is prevented if cooling is quick.

12 Crystallization T g : glass transition temperature

13 Crystal Crystal is the unique arrangement (pattern) and special order of atoms, molecules, or ions to form the a solid. (Miller Notation)

14 Polymorphism Polymorphism refers to the ability of a solid to exist in more than one crystalline form or structure. This is because of different crystallization conditions (Temperature and pressure)

15 Amorphous Material Amorphous (non-crystalline solid or glass) is a solid that lacks the long-range order characteristic of a crystal. For example; glass and polymers.

16 Example Aluminum Oxide Material Single Crystal (mono-crystalline): transparent So many crystals connected by grain boundaries: translucent So many crystals but with holes and cavities: Opaque

17 Why Study Materials Science and Engineering How to select a material from several choices: I. Required properties are established based on the in-service conditions. o Rarely, a material can combine the best properties. Trade-off or compromised solution is required. For example: strength vs. ductility. II. Deterioration III. Economics: what is the cost of the final product.

18 Classification of Materials Solid materials have been grouped into THREE main categories: Metals Ceramics Polymers This classification is based on atomic structure and chemical makeup. Most materials fall into one of the 3 groups. However, some materials are intermediate. Moreover, there are composites which are combination of two or more of basic material classes.

19 Metals Definition: any material composed of one or more metallic elements (such as iron, aluminum, copper, titanium, gold, nickel) and often a non-metallic element (such as carbon, nitrogen, oxygen) but in relatively small amount. For those composed of two or more metallic elements, they called alloys.

20 Properties of Metals Atoms in metals and their alloys are arranged in a very orderly manner more dense. Relatively stiff, strong, and ductile. That is why metals are widely used in structural applications. Metals have large number of non-localized electrons (free electrons). Good conductors of electricity and heat. Non-transparent. Some metals have desirable magnetic properties (ferrous metals).

21 Metals

22 Ceramics Definition: a material composed of metallic and non-metallic elements; like oxides, nitrides, and carbides. Examples: Aluminum oxides Al2O3 Silicon dioxide SiO2 Silicon carbide SiC Silicon nitride Si3N4 Cement Glass Clay minerals

23 Properties of Ceramics 1) Stiff and strong (comparable to metals). 2) Very hard. 3) Extremely brittle (lack of ductility). 4) Bad conductors of electricity and heat. 5) Maybe transparent, translucent, or opaque. 6) Some like Fe3O4 exhibit magnetic behaviors.

24 Ceramics Microstructure of typical ceramic material Examples of ceramic products

25 Polymers Definition: organic compounds that are chemically based on carbon, hydrogen, and any other non-metallic elements like O, N, and Si. Have very large molecular structure (chain-like molecules that based on carbon). Examples: Poly-ethylene (PE) Nylon Poly-styrene (PS) Poly vinyl chloride (PVC) Poly-carbonate (PC) Silicon rubber.

26 Properties of Polymers 1) Low density 2) Non stiff, nor strong However, if taken on a per-mass basis, their stiffness and strength are comparable with metals and ceramics. 3) Extremely ductile and pliable easily shaped into complex shapes. 4) Chemically inert (unreactive) 5) Soften and decompose at modest temperature 6) Bad conductors 7) Non-magnetic.

27 Polymers

28 Composites Definition: composed of two or more individual materials which come from the main THREE categories. The main idea is to design a material that exhibit combination of properties. Two main types of composites: 1) Natural: e.g. wood and bone. 2) Synthetic (Man-made): e.g. fiberglass.

29 Examples-Composites 1) Fiberglass: small glass fibers are embedded within a polymeric material (epoxy or polyester). Glass is strong and stiff but brittle. Polymer is ductile but weak. Thus, the resulting fiberglass is stiff, strong, and ductile (flexible).

30 Examples-Composites 2) Carbon Fiber Reinforced Polymer (CFRP) : carbon fibers embedded in polymer. More expensive that fiber-glass. Used in aerospace crafts. Used in sport equipment (bicycle). 3) Natural composite: Wood

31 Advanced Materials Definition: materials that are utilized in high-tech. High-tech: any device that functions using relatively intricate or sophisticated principle, e.g. aerospace, electronics, computers, mechatronics, and biomedical systems. Advanced materials: 1) Semiconductors 2) Biomaterials 3) Smart materials 4) Nano-engineering materials

32 Semiconductors Definition: a material that has electrical properties that are intermediate between conductors (metals and metal alloys) and insulators (ceramics and polymers). Controlled impurities over very small spatial regions. Example: Doping process of Silicon chip with phosphorous. Electronics and computer revolution are the prominent consequence.

33 Biomaterials Biomaterials are materials employed in components implanted into the human body for replacement of diseased or damaged body part. Special properties have to be satisfied: 1) Not toxic 2) Compatible with body tissues Example: materials used for hip replacement.

34 Smart Materials Definition: a material that can sense changes in environment and response in predetermined manner, just like living organism. Smart material is composed of: 1) Sensor: that detect a signal like: a) Temperature b) Electric field c) Magnetic field 2) Actuator: that responds in someway like: a) Change in shape b) Change in position c) Change in natural frequency or any mechanical property.

35 Smart Material-Sensor Materials that are used as sensors: 1) Optical fibers 2) Piezoelectric 3) Micro-electromechanical devices (MEMS): inkjet printers, accelerometer, micro-pumps.

36 Smart Materials-Actuator Materials that are used as actuators: 1) Shape memory alloys: after having been deformed, revert back to its original shape when temperature is changed (normally when heated). 2) Piezoelectric ceramics: expand or retract in response to externally applied electric field (voltage) and also operate in reverse manner. i.e. generate voltage when squeezed. 3) Magnetostrictive materials: same as piezoelectric but in response to magnetic field. 4) Electorheological material: a fluid (liquid) that experience dramatic changes in viscosity upon the application of electric field. 5) Magnetorheological material: same as electorheological but in response to magnetic field.

37 Smart Materials-Example Thermo-responsive materials: change of properties in response to applied heat. Electro-chromic materials: Change of color in response to applied voltage

38 Smart Materials-Example Shape memory alloys: Change shape in response to temperature. Quantum Tunneling Composites: Change of conductivity in response to applied pressure.

39 Smart Materials-Example Noise canceling in a helicopter.

40 Stress Stress is defined as the applied load over a certain area. Stress is actually represents the internal forces that adjacent particles inside material exert on each other. Mathematically; σ stress = F (Force) A (Area)

41 Types of Stresses 1) Normal Stress a) Tension b) Compression 2) Shear Stress 3) Bearing Stress

42 Strain Engineering Strain: is the deformation (change in length) over the original length. Mathematically, ε E strain = L L o True Strain: provides the correct measure of the final strain, taking it in increments: δε = δl L ε T = ln L 1 L 0 = ln(1 + ε E )

43 Stress-Strain Diagram (Ductile Material)

44 Stress-strain Diagram Brittle Material Suddenly breaks before any significant deformation.

45 Plastic Flow in Metals Block-slip theory: an early theory states that when the yield strength is exceeded, plastic deformation takes place by movement of large blocks of atoms sliding relative to one another across planes called slip planes inside the crystal. Block-slip theory can interpret so many phenomena, but has significant drawbacks. One of that it fails to predict the shear strength (it over-estimate the strength of metals in shear).

46 Slip occurs on the plane of the highest degree of atomic packing within the crystal structure. The direction of slip within the slip plane occur on the greatest atomic line density. Slip Planes

47 Isotropic Materials Strength of a single- crystal material is not isotropic, i.e. depends on the direction of loading. Strength of a glass or polycrystalline material is isotropic, i.e. does not depend on the direction of loading. If the grains of a polycrystalline material are textured, then it is not isotropic

48 Single-Crystal Material Anisotropic

49 Dislocations Dislocation is defined as a defect within a crystalline lattice, i.e. areas where atoms are out of position. Usually, dislocation exists in some combination of all types, i.e. rarely existence as pure of single type. When dislocations move inside the crystal lattice, plastic deformation takes place. In polycrystalline materials, grain boundaries hinder the motion of dislocations and as a result the material is harder but more brittle.

50 Dislocations Point Dislocation: presence of an impurity like a strange atom or vacancy. Edge Dislocation: an additional half-row of atoms

51 Dislocation Dislocation motion is analogous to movement of a caterpillar.

52 Dislocations In screw dislocation, defect line movement is perpendicular to direction of the stress and the atom displacement, rather than parallel like edge type.

53 Twinning Deformation In Twinning, large number of atoms undergo movement inside the lattice all at once, usually in response to impact loading. Clear change of orientation occurs inside the lattice in the region about the twinning plane. Structure of the twinned atoms is the mirror image of the original ones around the twinning plane.

54 Dislocations Number of dislocation in a 1 mm 2 is called dislocation density. In cold-worked materials, dislocation density is /mm 2, whereas it is much less in annealed materials; about /mm 2 Annealing is a process of heating the material above recrystallization temperature (but below melting temperature) in order to be more workable (more ductility).

55 Dislocations-Frank-Read Source Line of dislocations are firmly attached together between A and B. Shear stress causes the AB line to bow. Kidney-shaped curve develops. Two opposing ends meet to form a loop A new line starts to develop and the process is repeated. Cold work hardening is easily explained in view of this theorem as more dislocations are encountered for more plastic deformation flow.

56 Dislocation-Failure Void If more dislocation lines of the same sign meet each other (especially at grain boundary), a void will develop as a starting sign for imminent failure.

57 Recrystallization If the material is cold worked, high density dislocation regions are developed. As a result the material becomes harder but more brittle. If the material is heated, the stored (residual)strain inside the material can be relaxed and released gradually: 1) Firstly the vibrational energy of atoms is raised minor atomic movement 2) If the material is further heated beyond the recrystallization limit ( of melting temperature), new nucleates are formed and new grains are developed around (grain migration).

58 Recrystallization Recrystallization temperature is affected by: 1) Degree of plastic deformation: for more dislocation density, recrystallization can be started at lower temperature. 2) Composition: for more impurities inside the material, higher temperature is needed to initiate recrystallization.

59 Recrystallization