B

Note in Fig. B.13 that since the eutectic is the lowest melting composition in the alloy, it can often cause problems during hot working and heat treating operations. During casting of an ingot, due to non-uniform cooling rates, there is often quite a bit of segregation of the alloying elements. The low melting eutectic composition is normally the last portion of the metal to freeze, normally at the grain boundaries. On reheating the metal for hot working operations, or for heat treatment, if the lowest melting eutectic temperature is exceeded, melting can occur along the grain boundaries and the part is frequently ruined. It should also be noted that alloying systems with a number of different alloying elements will often form lower melting point eutectics than for the simple binary system shown in the figure. To help prevent some of these problems, as-cast ingots are often reheated to temperatures just below the melting point and soaked for long times (called homogenization) to create more uniform structures prior to hot working or heat treating.

A note of caution: it should be remembered that phase diagrams represent equilibrium conditions, which are often not obtained in industrial processes; nevertheless, phase diagrams are very useful tools in processing.

B.3 Ceramics

Ceramics are inorganic non-metallic materials which consist of metallic and non-metallic elements bonded together with either ionic or covalent bonds. Although ceramics can be crystalline or non-crystalline, the important engineering ceramics are all crystalline. Due to the absence of conduction electrons, ceramics are usually good electrical and thermal insulators. In addition, due to the stability of their strong bonds, they normally have high melting temperatures and high chemical stability to many hostile environments. However, ceramics are inherently hard and brittle materials that, when loaded in tension, have almost no tolerance for flaws. As a material class, few ceramics have tensile strengths above 25 ksi, while the compressive strengths may be 5-10 times higher than the tensile strengths.6

Under an applied tensile load at room temperature, both crystalline and non-crystalline ceramics almost always fracture before any plastic deformation can occur. Stress concentrations leading to brittle failure can be minute surface or interior cracks (microcracks), or internal pores, which are virtually impossible to eliminate or control. Plane strain fracture toughness (KIc) values for ceramic materials are much lower than for metals; typically below 9ksi^/in., while for metals they can exceed 100 ksi ^/in.). There is also considerable scatter in the fracture strength for ceramics, which can be explained by the dependence of fracture strength on the probability of the existence of a flaw that is capable of initiating a crack. Therefore, size or volume also influences fracture strength; the larger the size, the greater the probability for a flaw and the lower the fracture strength.

In metals, plastic flow takes place mainly by slip. In metals, due to the non-directional nature of the metallic bond, dislocations move under relatively low stresses, and because all atoms involved in the bonding have an equally distributed negative charge at their surfaces. In other words, there are no positive or negatively charged ions involved in the metallic bonding process. However, ceramics form either ionic or covalent bonds, both of which restrict dislocation motion and slip. One reason for the hardness and brittleness of ceramics is the difficulty of slip or dislocation motion. While ceramics are inherently strong, they cannot slip or plastically deform to accommodate even small cracks or imperfections, i.e. their strength is never realized in practice. They fracture in a premature brittle manner long before their inherent strength is approached.

The nature of the ionic and covalent bonds is shown in Fig. B.3. In the ionic bond, the electrons are shared by an electropositive ion (cation) and an electronegative ion (anion). The electropositive ion gives up its valence electrons, and the electronegative ion captures them to produce ions having full electron orbitals or suborbitals. As a consequence, there are no free electrons available to conduct electricity. In ionically bonded ceramics, there are very few slip systems along which dislocations may move. This is a consequence of the electrically charged nature of the ions. For slip in some directions, ions of like charge must be brought into close proximity to each other, and because of electrostatic repulsion, this mode of slip is very restricted. This is not a problem in metals, since all atoms are electrically neutral. In covalently bonded ceramics, the bonding between atoms is specific and directional, involving the exchange of electron charge between pairs of atoms. Thus, when covalent crystals are stressed to a sufficient extent, they exhibit brittle fracture due to a separation of electron pair bonds, without subsequent reformation. It should also be noted that ceramics are rarely either all ionically or covalently bonded; they usually consist of a mix of the two types of bonds. For example, silicon nitride (Si3N4) consists of about 70% covalent bonds and 30% ionic bonds.

Due to their extremely high melting temperatures, ceramics are often processed by powder processing methods in which powders are mixed, consolidated under pressure, and then sintered at high temperatures. During the high temperature sintering operation, the particles fuse together through diffusion processes, much in the manner as shown in Fig. B.14. Although many industrial ceramic products are sintered without external pressure, high performance ceramics, since they are extremely sensitive to all flaws, even very small voids, are usually sintered with high external pressures to minimize the amount of porosity.

B.4 Polymers

While polymers are light and some exhibit high levels of ductility, they are simply not strong enough by themselves to classify as high strength structural materials. However, when combined with high strength fibers, they become

structural materials (i.e., composites) with high strength and stiffness-to-weight ratios and are becoming increasingly important in aerospace.

Polymers can be classified as either thermosets or thermoplastics. As shown in Fig. B.15, a thermoset crosslinks during cure to form a rigid intractable solid. Prior to cure, the resin is a relatively low molecular weight semi-solid that melts and flows during the initial part of the cure process. As the molecular weight builds during cure, the viscosity increases until the resin gels, and then strong covalent bond crosslinks form during cure. Due to the high crosslink densities obtained for high performance thermoset systems, they are inherently brittle, unless steps are taken to enhance toughness. On the other hand, thermoplastics are high molecular weight resins that are fully reacted prior to processing. They melt and flow during processing but do not form crosslinking reactions. Their covalently bonded main chains are held together by relatively weak secondary bonds. However, being high molecular weight resins, the viscosities of thermoplastics during processing are orders of magnitude higher than that of thermosets. Since thermoplastics do not crosslink during processing, they can be

Polymer Before Processing Polymer After Processing

Polymer Chains

Polymer Chains

Thermoset

-Crosslink

-Crosslink

No Branches '

Linear Thermoplastic

No Crosslinks

No Crosslinks

Branches

Branches

No Crosslinks

Branched Thermoplastic

No Crosslinks

Fig. B-15. Comparison of Thermoset and Thermoplastic Polymer Structures8

reprocessed; for example, they can be thermoformed into structural shapes by simply reheating to the processing temperature. Thermosets, due to their highly crosslinked structures, cannot be reprocessed and, if reheated to high enough temperatures, will thermally degrade and eventually char. However, there is a limit to the number of times a thermoplastic can be reprocessed. Since the processing temperatures are often close to the polymer degradation temperatures, multiple reprocessing will eventually degrade the resin and in some cases it may crosslink.

Many thermoplastics are polymerized by what is called addition polymerization, as shown in Fig. B.16 for the simple thermoplastic polyethylene. Addition polymerization consists of three steps: initiation, propagation, and termination. During initiation, an active polymer capable of propagation is formed by the reaction between an initiator species and a monomer unit. In the figure, R represents the active initiator that contains an unpaired electron (■). Propagation involves the linear growth of the molecule as monomer units become attached to each other in succession, to produce a long chain molecule. Chain growth, or propagation, is very rapid, with the period required to grow a molecule consisting of 1000 repeat units on the order of 100-1000 s.7 Propagation can terminate in one of two ways. In the first, the active ends of two propagating chains react together to form a non-reactive molecule. The second method of termination occurs when an active chain end reacts with an initiator. Polyethylene can normally have anywhere from 3500 to 25 000 of these repeat units.9

Unpaired Electron

Initiator or Catalyst

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