Materials with high strength generally have low fracture toughness,and vice versa.
Material failure types include elastic, plastic and creep deformation. Other types of material failure involve cracking, such as brittle or ductile fracture, environmental cracking, creep rupture and fatigue.
Primary bonds: ionic, covalent and metallic. Primary bonds are strong and stiff and do not easily melt with increasing temperature. They provide high elastic modulus in metals and ceramics.
Secondary bonds: Van der vaals and hydrogen bonds, which are relatively weak. Theses are important in determining the behavior of liquids and as bonds between the carbon-chain molecules in polymers.
#Ionic bonding involves the transfer of one or more electrons between atoms of different types to fulfill empty outer shell or stable outer shell of eight electrons (e.g. NaCl).
#metallic bonding: for metals, the outer shell of electrons is in most cases less than half full; each atom donates its outer shell electrons to a "cloud" of electrons. These electrons are shared by all of the metal atoms, the positively charged metal ions are thus held together by their mutual attraction to the electron cloud.
#Covalent bonding involves the sharing of electrons and occurs where the outer shells are half full or more than half full.
*The tight covalent bonds make simple molecules(e.g. H20,Cl2) relatively independent of each other, so that collections of them tend to form liquids or gases at ambient temperature.
*Covalent bonds (2.2.2) have the unique property of being strongly directional--not shared by the other two types of primary bonds. This arises from covalent bonds being dependent on the sharing of electrons with specific neighboring ions. A continuous arrangement of covalent bonds can form a three-dimensional network to make a solid. One example is carbon in the form of diamond. Each atoms are arranged at equal angel in three-dimensional space, which make the crystal very hard and stiff. Another important continuous arrangement of covalent bonds is the carbon chain.
Mixed bonding is more common.
Second bonds:Van der Waals and hydrogen bonds.
Secondary bonds occur due to the presence of an electrostatic dipole, which can be induced by a primary bond. One strong dipole bond is called a hydrogen bond.
Van der Waals bonds can also be called a fluctuating dipole bond-distinguished from a permanent dipole bond because the dipole is not fixed in direction as it is in a water molecule.
In polymers, covalent bonds form the chain molecules and attach hydrogen and other atoms to the carbon backbone. Hydrogen bonds and other secondary bonds occur between the chain molecules and tend to prevent them from sliding past one another. The relative weakness of the secondary bonds accounts for the low melting temperatures, and the low strengths and stiffness.
A given metal or other material may change its crystal sructure with temperature or pressure, or with the addition of alloying elements. E.g. iron: BCC- (910 degree) - FCC- (1390 degree) - BCC
Point defects: substitutional impurity, self interstitial, interstitial impurity and vacancy.
Line defects: edge dislocation and screw dislocation.
glass transition temperature Tg, is the temperature where the rapid decrease in E occurs.
Plastic deformation occurs by motion of dislocations under the influence of a shear stress. As a dislocation moves through the crystal, plastic deformation is, in fact, proceeding one atom at a time, rather than occurring simultaneously over an entire plane. This incremental process can occur much more easily than simultaneous breaking of all the bonds, as assumed in the theoretical shear strength calculation for a perfect crystal.
slip plane; slip step; slip bands
The result of plastic deformation (yielding) is that atoms change neighbors and return to a stable
configuration with new neighbors after the dislocation has passed. Note that this is a fundamentally different process than elastic deformation,which is merely the stretching of chemical bonds.
*Ways to impede dislocation in metals:
obstacles (the strength may be increased by a factor of 10);
second phase of hard particles;
Alloying (different-size atoms have better effect)
*In nonmetals and compounds where the chemical bonding is covalent or partially covalent, the directional nature of the bonds makes dislocation motion difficult. (the crystals of carbon, boron, silicon and metal carbides, borides, nitrides, oxides and other ceramics) At ambient temperature, these materials are hard and brittle. However, some dislocation does occur, especially for temperatures above about half of the (usually high) melting temperature.
In crystalline materials--that is , in metals and ceramics--one important mechanism of creep is diffusional flow of vacancies. Creep behavior in crystalline materials is strongly temperature dependent.
Different creep mechanisms operate in amorphous (noncrystalline) glasses and in polymers: one is viscous flow in the manner of a very thick liquid.(Temperature is above Tg and approaching Tm).
Around and below Tg, more complex behavior involving segments of chains and obstacles to chain sliding become important.
(第二章终于搞完了，累死爹爹了，第三章待补 = = )
For a given alloy composition, the properties are further affected by the particular processing used. Processing includes 1.heat treatment, 2.deformation and 3.casting.
Some methods of forming metals into useful shapes (deformation):(1)forging;(2)rolling;(3)extrusion;(4)drawing.
Strengthening methods for metals and alloys: cold work; grain refinement; solid solution strengthening; precipitation hardening; multiple phases; quenching and tempering.
Cold work is the severe deforming of a metal at ambient temperature, often by rolling or drawing. This causes a dense array of dislocations and disorders the crystal structure, resulting in an increase in yield strength and a decrease in ductility. Strengthening occurs because the large number of dislocations form dense tangles that act as obstacles to further deformation.
The effects of cold work can be partially or completely reversed by heating the metal to such a high temperature that new crystals form within the solid material, a process called annealing. If this is done following severe cold work, the recrystallized grains are at first quite small. Cooling the material at this state creates a situation where strengthening is said to be due to grain refinement, because the grain boundaries impede dislocation motion.
Solid solution strengthening occurs as a result of impurity atoms distorting the crystal lattice and thus making dislocation motion more difficult.
The effect of a substitutional impurity is greater if the atomic size differs more from that of the major constituent.
Cast irons contain large amounts of carbon,typically 2 to 4% by weight and also 1 to 3% silicon.
Gray iron contains graphite in the form of flakes.These flakes easily develop into cracks under tensile stress, so that gray iron is relatively weak and brittle in tension.
White iron is formed by rapid cooling of a melt that would otherwise form gray iron. Very hard and brittle phase(Fe3C) results in the bulk material also being hard and brittle.
Key process for carbon steels: quenching and tempering.
Stainless steels contain at least 10% chromium and they have good corrosion resistance.
Quenching and temping to produce a martensitic structure is the most effective means of strengthening steels. The higher strength nonferrous metals often employ precipitation hardening.
Titanium alloys: The density is considerably greater than that of aluminum, but still only about 60% of that of steel. In addition, the melting temperature is somewhat greater than for steel and far greater than for aluminum.
Magnesium is the lightest engineering metal.
three groups: thermoplastics, thermosetting plastics and elastomers.
When heated, a thermoplastic softens and usually melts;then, if cooled, it returns to its original solid condition.
Elastomers are distinguished from plastics by being capable of rubbery behavior. In particular, they can be deformed by large amounts,say 100% to 200% strain or more.
An important characteristic of polymers is their light weight.
(a) Thermoplastics: ethylene structure
Aramids (Kevlar, Nomex)
(c) Thermosetting plastics:
crystalline thermoplastics: PE,PP,PTFE,nylon, Kevlar, POM and PEEK.
amorphous thermoplastics:PVC, PMMA, PC
Amorphous polymers are generally used around and below their respective glass transition temperature Tg. Above Tg, the elastic modulus decrease rapidly, and time-dependent deformation (creep) effects become pronounced, limiting the usefulness of these materials in load-resisting applications.
Amorphous polymers chain structure that are linear, branched and cross-linked.
Crystalline polymers tend to be opaque to light, whereas amorphous polymers are transparent.
There is no distinct Tg effect in highly cross-linked thermosetting plastics.
3.6(ceramics and glass)
Ceramics and glasses are solids that are neither metallic nor organic(carbon-chain based) materials.
Ceramics are predominantly crystalline, whereas glasses are amorphous.
Slip of crystal planes does not occur readily in ceramics, due to the strength and directional nature of even partially covalent bonding and teh relatively complex crystal structures. This results in ceramics being inherently brittle, and glasses are similarly affected by covalent bonding. In addition, there is often an appreciable degree of porosity in ceramics, and both ceramics and glasses usually contain microscopic cracks.
3.6.2 Engineering ceramics
The final step in processing is sintering.
3.8.1 Selection Procedure
Express the quantity Q to be minimized or maximized as a mathematical separate function of the requirements and the material properties, in which the gemoetry variable does not appear.