FOLLOW THE RECIPE. Sub-topics. Processing for properties Microstructure Microstructure evolution during processing Non-metals processing

Transcription

1 FOLLOW THE RECIPE Sub-topics 1 Processing for properties Microstructure Microstructure evolution during processing Non-metals processing

2 HOW MANY OPTIONS? Often a materials problem is really one of selecting the material that has the right combination of characteristics for a specific application. Intrinsic properties depend on microstructure and microstructure depends on processing 2

3 HISTORICAL REVIEW OF AN ALUMINUM BIKE FRAME Materials processing involves more than one step Each process step has a characteristic history Designers should watch out for unintended side-effects in the joining stage Design focuses on the properties of finished products, but some of these properties are also critical during processing In choosing materials for a component 3 it is important to examine their suitability for processing as well as for performance in service.

4 METALS 4

5 CERAMICS 5

6 POLYMERS 6

7 PHASES Phase: region of a material with a specified atomic arrangement Phase diagram: maps showing the phases expected as a function of composition and temperature, if the material is in its lowest free energy state Phase transformation: occur when the phases present change requires a driving force and a mechanism Phase diagrams show us when changes are possible. Phase transformations are central to microstructural control. 7

8 THERMODYNAMICS OF PHASES Microstructures can only evolve from one state to another if it is energetically favorable to do so The Gibbs free energy G helps to determine what phase an alloy will be in at a given composition held at a fixed temperature U intrinsic energy of the material p pressure V volume U + pv enthalpy T temperature S entropy The state of lowest free energy is the state of thermodynamic equilibrium at equilibrium no phase change is physically possible 8

9 PHASE DIAGRAM: LEAD-TIN SYSTEM The diagram divides up into single-and two-phase regions, separated by grain boundaries At any point in the two-phase region, the present phases are those found at the phase boundaries at either end of the of a horizontal tie-line through the point defining the composition and temperature concerned Both Pband Snwill dissolve in one another to some extent, with the maximum solubility in both cases being at the same temperature The pure elements have a unique melting temperature Alloys show a freezing range between the boundaries known as the liquidus and solidus, so there is no longer a single melting point At the eutectic point, the alloy can change from 100% liquid to 100% two-phase solid 9

10 COMMERCIAL CAST IRON Pearlite-the lamellar structure in carbon steels and cast irons that consists of plates of pure iron and iron carbide 10

11 OPTICAL PHOTOMICROGRAPHS OF VARIOUS CAST IRONS Gray iron: the dark graphite flakes are embedded in an ferrite matrix Nodular (ductile) iron: the dark graphite nodules are surrounded by an ferrite matrix. Malleable iron: dark graphite rosettes (temper carbon) in an ferrite matrix White iron: the light cementiteregions are surrounded by pearlite, which has the ferrite cementite layered structure 11

12 KINETICS OF PHASE TRANSFORMATIONS The boundaries on phase diagrams show where two different states can both be in equilibrium. At the melting point, both phases have the same free energy and can co-exist Above the melting point, liquid is the state of lower free energy if a liquid is cooled beyond its melting point, its free energy as a liquid is greater than that of a solid the system can release energy if it solidifies this is the driving force for phase transformation 12

13 RATE OF PHASE TRANSFORMATION The rate of phase transformation depends on both the driving force and the diffusion rate both are dependant on but have opposite reaction to temperature 13

14 NUCLEATION AND GROWTH When a liquid solidifies, solid first has to appear from somewhere, after which the interface between solid and liquid can migrate to enable atoms to switch from one phase to the other at the boundary the two stages are nucleation and growth 14

15 TIME-TEMPERATURE-TRANSFORMATION (TTT) DIAGRAMS TTT diagrams are used to study diffusion-controlled transformation: C-curves reflect the transformationrateas a function of temperature; TTT curves assume rapid cooling from above the transformation temperature 15

16 METALS PROCESSING Most alloys offer both cast and wrought variants Metals with an HCP structure are primarily castat high temperatures because of their inherent lack of ductility Casting and wrought alloys in a given system tend to have different compositions Casting leads to coarser microstructures and poorer strength and toughness than a wrought alloy Good castabilityrequires higher levels of alloying additions than wrought alloys to lower the melting temperature 16

17 SOLIDIFICATION: METAL CASTING In casting, a liquid above its melting point is poured into a mold where it cools by thermal conduction it is relatively cheap and well suited for complex 3-d shapes New solid forms by nucleation new crystals form in the melt, on the walls of the mold, or on foreign particles Crystals grow in opposing directions and impinge on one another to form grain boundaries 17

18 DEFORMATION PROCESSING OF METALS Deformation processes exploit the plastic response of metals, that is, their ability to remain intact without damage when subjected to large strains and shape changes Deformation processes influence Microstructure by various means: Temperature determines the phases present and is relevant during forming and during cool-down Grain size forming process can strengthen the metal by changing both the size and the shape of the grains 18

19 FORMING PROCESSES increasing the strength of metals often reduce their ductility it is necessary to follow forming with an annealing heat treatment to restore some of the materials ductility Grain structure evolution through deformation and annealing 19

20 MATERIAL ALLOYS USED FOR EURO COINS On January 1st, 2002 the euro became the single legal currency in some European countries. Euro coins are minted in eight different denominations: 2 and 1 euros, as well as 50, 20, 10, 5, 2, and 1 cent euros. Outer part: copper-nickel Inner part: Layers of nickel brass, nickel, nickel brass Nordic gold - 89% copper, 5% aluminium, 5% zinc, and 1% tin 20

21 HEAT TREATMENT OF METALS In a typical heat treatment, a component is heated to high temperature, cooled at a controlled rate, and usually reheated to an intermediate temperature The thermal profile and resulting microstructure for a heat treatment process A solid solution is formed at high temperature followed by precipitation hardening at an intermediate temperature after being cooled 21

22 EFFECTS OF HEAT TREATMENT ON MECHANICAL PROPERTIES Normalizing slow cooling from high temperature producing a microstructure of lower strength but high toughness no follow-up heat treatment Quench and temper cooled faster than the critical cooling rate producing a martensitic microstructure with high strength and low toughness then reheated at an intermediate temperature to restore toughness 22

23 JOINING PROCESSES Thermal welding of metals involves heating and cooling which may cause phase transformations in the weld metal and in the heated regions of the weld A weld cross-section with corresponding thermal histories in the weld metal and heat-affected zone. Typical hardness profiles induced across welds in aluminum alloys, low carbon steel, and low alloy steel 23

24 SURFACE ENGINEERING Surface treatments exploit many different mechanisms and processes to change the surface microstructure and properties Laser hardening is a surface treatment process that modifies microstructure the traversing laser beam induces a rapid thermal cycle, causing phase changes on both heating and cooling the track below the path of the laser has a 24 different microstructure of high hardness

25 FERROUS METALS Strength and toughness tend to dominate the property profile desired for applications that use ferrous metals The range of mechanical properties able to be achieved through alloying and processing makes ferrous metals the most versatile group of engineering material 25