Modern Engineering Materials

Introduction Science and technology have made amazing developments in the design of electronic and machinery using standard materials. Traditionally, standard materials are divided into three basic groups, namely metals, ceramics and polymers. Later three other groups of engineering materials are added. They are composites, semiconductors and biomaterials. Still there is a continuous search for materials with enhanced properties such as strength, high stability, large electrical conductivity etc. A number of materials such as amorphous metals, liquid crystals, smart materials, biomaterials etc have been discovered for high-tech applications.

AMORPHOUS METAL An amorphous metal is a metallic material with a disordered atomic-scale structure. In contrast to most metals, which are crystalline and therefore have a highly ordered arrangement of atoms, amorphous alloys are non-crystalline. amorphous crystalline Materials in which such a disordered structure is produced directly from the liquid state during cooling are called "glasses", and so amorphous metals are commonly referred to as "metallic glasses" or "glassy metals".

In addition to direct cooling, there are several other ways in which amorphous metals can be produced, including: physical vapor deposition solid-state reaction, melt spinning mechanical alloying . Amorphous metals produced by these techniques are, strictly speaking, not glasses; however, materials scientists commonly consider amorphous alloys to be a single class of materials, regardless of how they are prepared.

In the past, small batches of amorphous metals have been produced through a variety of quick-cooling methods. For instance, amorphous metal wires have been produced by sputtering molten metal onto a spinning metal disk. The rapid cooling, on the order of millions of degrees a second, is too fast for crystals to form and the material is "locked in" a glassy state. More recently a number of alloys with critical cooling rates low enough to allow formation of amorphous structure in thick layers (over 1 millimeter) had been produced, these are known as bulk metallic glasses (BMG). Liquid metal sells a number of titanium- based BMGs, developed in studies originally carried out at Caltech. More recently, batches of amorphous steel have been produced that demonstrate strengths much greater than conventional steel alloys.

PROPERTIES of Metallic Glasses Amorphous metal is usually an alloy rather than a pure metal. The alloys contain atoms of significantly different sizes, leading to low free volume (and therefore up to orders of magnitude higher viscosity than other metals and alloys) in molten state. The viscosity prevents the atoms moving enough to form an ordered lattice. The material structure also results in low shrinkage during cooling, and resistance to plastic deformation. The absence of grain boundaries, the weak spots of crystalline materials, leads to better resistance to wear and corrosion. Amorphous metals, while technically glasses, are also much tougher and less brittle than oxide glasses and ceramics.

Thermal conductivity of amorphous materials is lower than of crystals Thermal conductivity of amorphous materials is lower than of crystals. As formation of amorphous structure relies on fast cooling, this limits the maximum achievable thickness of amorphous structures. To achieve formation of amorphous structure even during slower cooling, the alloy has to be made of three or more components, leading to complex crystal units with higher potential energy and lower chance of formation. The atomic radius of the components has to be significantly different (over 12%), to achieve high packing density and low free volume. The combination of components should have negative heat of mixing, inhibiting crystal nucleation and prolongs the time the molten metal stays in super cooled state.

The alloys of boron, silicon, phosphorus, and other glass formers with magnetic metals (iron, cobalt, nickel) are magnetic, with low coercivity and high electrical resistance. The high resistance leads to low losses by eddy currents when subjected to alternating magnetic fields, a property useful for e.g. transformer magnetic cores. Amorphous alloys have a variety of potentially useful properties. In particular, they tend to be stronger than crystalline alloys of similar chemical composition, and they can sustain larger reversible ("elastic") deformations than crystalline alloys. Amorphous metals derive their strength directly from their non-crystalline structure, which does not have any of the defects (such as dislocations) that limit the strength of crystalline alloys.

Amorphous metallic alloys combine higher strength than crystalline metal alloys with the elasticity of polymers.

Mechanical properties and performances of BMGs Fracture toughness vs Young modulus for different materials (reproduced with permission from [25] ©2008, Nature Publishing Group). Comparative chart of fracture toughness vs yield strength for BMGs and other metallic materials (adapted from [26] © 2010 JOM). Fatigue endurance limit (stress-range based) vs yield strength data for BMGs and other metallic alloys (adapted from [26] © 2010 JOM).

PROCESSING OF METALLIC GLASSES Virtually any liquid can be turned into a glass if it is cooled quickly enough to avoid crystallization. The question is, how fast does the cooling need to be? Common oxide glasses (such as ordinary window glass) are quite resistant to crystallization, so they can be formed even if the liquid is cooled very slowly. For instance, the mirror for the 200" telescope at the Palomar Observatory weighed 20 tons and was cooled over a period of eight months, but did not crystallize. Many polymer liquids can also be turned into glasses; in fact, many polymers cannot be crystallized at all. For both oxides and polymers, the key to glass formation is that the liquid structure cannot be rearranged to the more ordered crystalline structure in the time available.

Design and fabrication of BMGs A promising material for micro-devices and micro-manufacture -courtesy by prof. Wei Hua Wang Zr-based BMG pieces obtained by TPF based blow molding (adapted with permission from [43] © 2011 Elsevier). BMG Medalof Institute of Physics (CAS) Beijing -courtesy by prof. Wei Hua Wang

Kinetic Energy Penetrators ‘‘KEPs’’ (reproduced from [64]). Engineering parts at micro-scale fabricated from BMGs (adapted with permission from [38] © 2011 Elsevier Images with spacecraft (artist rendering during collection phase) and BMG components for Genesis Mission (image credit NASA/JPL – Caltech/USC [66,67]).

Shape Memory Alloys

Introduction Shape memory alloy (SMA) is a metallic alloy that remembers its shape and can be returned to its initial shape after being deformed, by applying heat to the alloy. A material that can remember its shape SMA also exhibits superelastic (pseudoelastic) behavior When shape memory effect is correctly used, the material becomes a light weight, solid-state alternative to conventional actuators such as hydraulic, pneumatic and motor-based systems. SMAs have several applications in the medical and aerospace industries. Some examples for SMAs are Ni-Ti alloy; Cu-Al-Ni alloy; Cu-Zn-Al alloy; Au-Cd alloy; Ni-Mn-Ga alloy and Fe-based alloys.

Two Phases Austenite Martensite Hard, firm Inelastic Resembles titanium Simple FCC structure Martensite Soft Elastic Complex structure

Austenite High temperature phase Cubic Crystal Structure Martensite Low temperature phase Monoclinic Crystal Structure Twinned Martensite Detwinned Martensite

Principles of Shape Memory Alloys Shape Memory Alloys (SMA) are alloys that exhibit the shape memory effect. The shape memory effect is the process of restoring a deformed material back to an initial shape through a thermally induced crystalline transformation The crystalline transformation occurs between a low temperature ductile martensitic phase and a high temperature high strength austenitic phase.

Advantages of SMA’s The main advantages of SMA’s for micro-actuation are: SMA’s are capable of producing a large actuation force SMA’s are capable of producing large displacements SMA’s are activated through thermal heating

Disadvantages of SMA’s The main disadvantages of SMA’s are: Sensitivity of material properties in fabrication Residual Stress’s developed in thin films Nonlinearity of actuation force Lower maximum frequency compared to other microactuator devices

Characteristic temperatures of SMAs There are four characteristic temperatures describing the phase transformation. They are (i) Martensite Start temperature, Ms It is the temperature at which material starts transforming from austenite to martensite. The transformation proceeds with further cooling and is complete at the martensite finish temperature, Mf. Below Mf, the entire body is in the martensite phase and a specimen typically consists of many regions each containing a different variant of martensite. The boundaries between the variants are mobile under small applied loads.

(ii) Martensite Finish temperature, Mf It is the temperature at which the transformation is complete and the material is fully in the martensite phase. (iii) Austenite Start temperature, As It is the temperature at which austenite first appear in the martensite, with heating. With further heating, more and more of the body transforms back into austenite (iv) Austenite Finish temperature, Af It is the temperature at which the reverse phase transformation is completed and the material is in the austenite phase. Above Af , the material is in the original undistorted state.

Shape Memory Alloy Effect “When an SMA is cooled below its transformation temperature, it can deform into any new shape which it will retain, and when the material in new phase is heated above its transformation temperature it recovers its original shape. This effect is known as shape memory effect”. Let us consider a shape memory alloy in cubic austenite phase. When it is cooled below a temperature, called the martensite finish temperature Mf and the material is applied a constant load, it gets deformed. On cooling, the SMA goes into twinned martensite phase and when it is loaded in this state, the SMA deformed in to the detwinned martensite.

Suppose the material in detwinned phase is heated above the temperature called the austenite finish temperature Af, the SMA recovers its original shape i.e., cubic austenite phase. Following figure illustrates the graphical representation of shape memory alloy effect.

Heat supplied to the material is used to drive the molecular rearrangement of the alloy. It is clear that an SMA undergoes phase changes while remaining a solid. The phase changes occur below its melting point. The phase changes involve the rearrangement of the position of particles within the crystal structure of solid. Thus, the alloy can retain its shape without melting. Under the transition temperature, the alloy is in the martensite phase and can be bent in any shapes. To get the initial shape, the allo must be in position and heated to about 500oC. This high temperature causes the atoms to arrange themselves into a high symmetry cubic arrangement called the austenite phase.

Types of Shape Memory Alloy Effects There are two types of SMA effects, namely : One-way shape memory alloy effect and Two-way shape memory alloy effect. One-way shape memory alloy effect: This SMA exhibits shape memory alloy effect only on heating. On cooling, the SMA retains the shape that it had before heating. A schematic view of the one-way shape memory alloy effect is given fig. Fig. One-way memory effect: (a) material in martensite phase, (b) material deformed, (c) sample heated and (d) sample cooled again

When a SMA is in its cold state (i. e When a SMA is in its cold state (i.e., below As), the metal can be bent or stretched into different new shapes and will hold in that shape until it is heated above the transition temperature. Upon heating, the shape changes back to its original shape. When the metal cools again it will retain in the hot shape, until deformed again. Note that the shape recovery is achieved only during heating. With one way effect, cooling from high temperature does not cause a macroscopic shape change. A deformation is necessary to create the low-temperature shape. On heating, transformation starts at As and is completed at Af (typically2 to 20oC or hotter, depending on the alloy or the load conditions).

Two-way shape memory alloy effect: The mechanism of SME describe above, only the shape of the austenitic phase is remembered. However, it is possible to remember the shape of the martensitic phase under certain condition. This behavior is a common property of SMA, it is called Two Way Shape Memory Effect (TWSME). TWSME is not intrinsic property (as the OWSME) to SMA, but it can be exhibited after specific thermo- mechanical treatments known as training procedures. TWSME refers to the reversible and spontaneous shape change of materials with thermal cycling, in other words, this property permitted to SMA a spontaneous shape change on both heating and cooling.

TWSME refers to the reversible and spontaneous shape change of materials with thermal cycling, in other words, this property permitted to SMA a spontaneous shape change on both heating and cooling. Once that the material has learned the behavior, it is possible to modify the shape of the material, in a reversible way between two different ones and without applied stress or load, only by changing of temperature across Af and Mf. A schematic representation of the macroscopic observed behavior is reported in Fig. Fig. Two-way memory effect: (a) material in martensite phase, (b) material deformed, (c) sample heated and (d) sample cooled again

At microscopic level, the reason for which a specimen remembers the shape is explanation as follows. Upon heavy deformation in martensitic phase, dislocations are introduced so as to stabilized the configuration of martensitic phase. These dislocation exist even in the parent phase after reverse martensite upon heating. In particular, there are three key microstructural forms for SMA. This key microstructure is preferred in which becomes learned by the alloy during training process. Therefore, this structure promotes the TWSME.

Shape Memory Alloy Qualities Ability to “remember” its austenite phase As the metal is cooled to the martensite phase, it can be easily deformed. When the temperature is raised to the austenite phase, it reforms to the original shape of the material. Pseudoelasticity When the metal is changed to the martensite phase simply by strain. The metal becomes pliable and can withstand strains of up to 8%. A mix of roughly 50% nickel and 50% titanium is the most common SMA. Also CuZnAl and CuAlNi are widely used.

Pseudoelasticity Pseudoelasticity (superelasticity) occurs when the alloy is above the martensite temperature, but there is a load strong enough to force the austenite into the martensite phase. The alloy will not return to the austenite phase until the loading is decreased or there is a large enough change in temperature. The figure shows load versus temperature on an SMA.

The figure below shows NiTi’s ability to change its shape along phase planes. Other metals, as we know, slide along slip planes when there is an induced stress. The above figure shows the Martensitic transformation and hysteresis (= H) upon a change of temperature. As = austenite start, Af = austenite finish, Ms = martensite start, Mf = martensite finish and Md = Highest temperature to strain-induced martensite. Gray area = area of optimal superelasticity.

Biological Applications Bone Plates Memory effect pulls bones together to promote healing. Surgical Anchor As healing progresses, muscles grow around the wire. This prevents tissue damage that could be caused by staples or screws. Clot Filter Does not interfere with MRI from non-ferromagnetic properties. Catheters Retainers Eyeglasses

Aircraft Maneuverability Nitinol wires can be used in applications such as the actuators for planes. Many use bulky hydraulic systems which are expensive and need a lot of maintenance. USAF Aircraft Pictures

Typical actuator in the wing of a plane. Picture of wing with SMA wires. The wires in the picture are used to replace the actuator. Electric pulses sent through the wires allow for precise movement of the wings, as would be needed in an aircraft. This reduces the need for maintenance, weighs less, and is less costly.

Problems With SMAs Fatigue from cycling Overstress Causes deformations and grain boundaries Begin to slip along planes/boundaries Overstress A load above 8% strain could cause the SMA to completely lose its original austenite shape Difficulty with computer programming More expensive to manufacture than steel and aluminum Relatively new