Mechanical Properties of Complex Intermetallics

The following sections are included:

• FOREWORD

• CONTENTS

An introduction to the mechanisms of plastic deformation in metals and alloys is presented. After a brief overview over the basic terms and observables, the microstructural aspects of deformation are addressed. The concept of dislocations, a class of one-dimensional crystal defects, which play a pivotal role in the plasticity of most metals, is introduced. We describe their characteristic parameters and the mechanisms of dislocation construction and movement. Macroscopic experimental approaches for the investigation and characterization of the deformation behaviors are described. For the interpretation of deformation experiments we will introduce a thermodynamic approach including the concept of thermal activation. On this basis, we describe the relations between the thermodynamic activation parameters and the experimentally accessible observables.

Metallic bonding promote a good compromise between metals' stiffness, resistance and deformability. They have a linear elastic behavior below a critical stress and deform plastically by dislocation glide above. Dislocations produce glide along the denser planes of the crystal in the dense directions, constituting slip systems. The critical shear stress depends on the lattice friction, the presence of solute atoms and strain hardening due to dislocation interactions. The three stages of the tensile curve of a single crystal are characteristic of these three contributions. Plasticity can also involve grain boundary sliding, mechanical twinning or martenstic transformations, which lead to outstanding mechanical properties. At the macroscopic scale, tensile tests reveal that the yield stress inversely scales like the square root of the grain size in polycrystals. Their homogeneous elongation is limited by necking, favored by low strain hardening rates. Plasticity is thermally activated and generally enhanced at high temperature and low strain rates. Superplasticity can be observed when the strain rate sensitivity is high, while a negative one produces plastic instabilities. Dynamic recovery and recrystallisation mechanisms intervene in plasticity and reduce strain hardening. The numerous thermally activated mechanisms involved in plastic deformation lead to creep, even below the yield stress.

This course aims at summarizing the relationship that exists between the microstructure of a metallic alloy and its properties related to plastic strain. In a first part, a few techniques that can give access quantitatively to the parameters of the microstructure (phase proportion & size, dislocation density, etc.) will be presented. A second part will be devoted to the relationship between microstructure and yield strength. Next, the material's behavior during plastic strain will be presented. The final part will briefly introduce the relationship between microstructusre and fracture.

An overview is given on the complex high-temperature deformation behavior of intermetallic alloys which results from a tutorial for students of the 4th European School in Materials Science of the EU Network of Excellence “Complex Metallic Alloys” CMA. After a discussion of the basic effects of atomic ordering in intermetallic alloys on high temperature deformation various case studies are presented which are of interest for structural applications at high temperatures. The aim is to point out the special features of the deformation of intermetallic alloys at high temperatures which have to be considered when developing new intermetallic materials for structural applications at high temperatures. Intermetallics have been subject of a long series of excellent reviews which is not to be continued by the present overview. This overview is based on a recent compilation of the knowledge on intermetallics¹ and shorter reviews.^2,3 Data of intermetallic materials were collected in reference 4.

Metadislocations are novel and highly complex structural defects in complex metallic alloys. This Chapter covers the basic geometric concepts of metadislocations involving different metadislocation types and the relation between the number of parson planes, Burgers vector and the elastic energy. Reactions between metadislocations like splitting, the formation of loops and networks are presented. As metadislocations form phase boundaries between complex phases, they are furthermore pivotal defects mediating phase transformations. We will mainly focus on metadislocations in ε-type CMA phases, however also give an example of a metadislocation in the complex phase Al₁₃Co₄.

A common failure mode seen during the testing and operation of spacecraft is termed “cold welding”. European laboratories also refer to this as either “adhesion”, “sticking” or “stiction”. This chapter is intended to give an introduction to the effect of cold welding in general. It surveys basic theory, continues to a test method and equipment developed by AIT. Finally, findings and results on “cold welding” between two contacting surfaces under condition of both impact and fretting will be given. Results will be drawn from “engineering surfaces” which may be considered as bare metallic, inorganically coated, or organically coated metals and their alloys.

The motivation for the topic of this chapter is materials issues in microelectronics and microelectromechanical systems (MEMS). The core of the course is the materials behavior in small dimensions. Focus is laid on scaling of electronic and mechanical properties, thin film mechanics, device reliability and integration issues when dissimilar materials are joined. The chapter is divided into four parts focusing on an introduction to scaling laws and size effects, mechanical properties of thin films, properties of interconnects in microelectronics and finally complex nanostructures geometries. As far as possible, scaling laws are introduced that describe the change in a property as a function of the external dimension. As many of these laws would diverge at extremely small length scales, the limits of scaling laws are discussed. The mechanical properties range from elasticity to strength and fracture toughness, whereas the electrical properties are focused on resistively and the phenomenon of electromigration. A section on measurement techniques is omitted as these will be the focus of a review article to be published in May 2010. More complex geometries than they are currently found in microelectronics require more sophisticated theories, such as strain gradient plasticity for mechanical aspects, provided they coincide with nonsocials dimensions

Mechanical properties of nanocrystalline alloys with grain size ranging from few to several tens nanometers have been examined. Sufficiently dense and structurally well characterized nanocrystalline materials have been prepared by dynamic compaction of mechanically alloyed nanocrystalline powders and also by electrodeposition techniques. Both dynamically consolidated (Fe, Ni)-TiN materials and electrodeposited Ni-W materials exhibited Hall-Petch hardening to grain sizes near about 10 nm whereas at grain size finer than about 10 nm, the hardness decreases in an apparent breakdown of the Hall-Petch relationship. High temperature hardness tests and indentation creep tests were performed between room temperature and up to 1000 K on (Fe, Ni)-TiN materials. With increasing temperature, softening occurs by two stages: hardness decreases slightly at first stage and decreases significantly at second stage from about 500 K. The grain size softening dependence suggests that softening in second stage occurs via grain boundary diffusion creep. Conversely, the tensile strength and ductility of nanocrystalline Ni-W electrodeposits have been strongly affected by the inclusion of impurities such as hydrogen and oxygen during the deposition process. Under optimal electrode position conditions, high-strength nanocrystalline Ni-W alloys with tensile strength of about 2300 MPa combined with good ductility were obtained.

Minerals raise the question of the core structure and mobility of dislocations in materials with large unit cells and complex crystal chemistry. Completely unknown for a long time, the structure of dislocation cores in minerals has recently been studied by numerical modeling. The Pearls model and its recent numerical developments are first recalled since most of the studies have been performed within this framework. We discuss then some examples on perovskites, olivine, wadsleyite and garnets which illustrate some cases corresponding to Burgers vectors lengths ranging from 0.4 to 1.15 nm.

Inorganic and carbon nanotubes distinguish by important peculiarities, from growth mechanisms to various physical and chemical properties and possible applications. A list of inorganic compounds synthesized in a cylindrical shape using a variety of preparation techniques is presented and the most promising directions of applications are indicated. In particular, structural and morphological properties of MoS₂ and WS₂ nanotubes determining their chemical stability and mechanical properties are explained in details. Recently discovered hybrid inorganic nanotube structures, i.e. WS₂ nanobuds and MoS₂ “mama” tubes are shown in a view of their morphological hybrid structure.

Spin electronics exploits the spin angular momentum and magnetic moment of the electron to add new functionality to electronic devices. A first generation of applications was based on magnetoresistive sensors and magnetic memory. The sensors have numerous applications, not least in digital recording. Read heads are usually spin-valves exhibiting giant magnetoresistance (GMR) or tunneling magnetoresistance (TMR). Magnetic random-access memory (MRAM) is based on switch able TMR spin valve cells, similar in structure to the read head. New devices are under development where the angular momentum of a spin-polarized current is used to exert spin transfer torque, or the flow of spin-polarized electrons is controlled via a third-electrode in a transistor-like structure.

Spintronics is the spin electronics in which both charge and spin degrees of freedom play the determining role in solid state environment. The goal of the spintronic application is to realize functions controlling electronic properties by spin or magnetic field as well as controlling magnetic properties by current or electric field. In practical sense, metal-based spintronics (i.e. giant magnetoresistance; GMR) is used as hard disk read heads and semiconductor-based spintronics (i.e. tunnel magnetoresistance; TMR) is used as magnetic random access memory (MRAM) devices. Spin transfer torque (STT) technology is applied to the existing MRAM devices of magnetic tunnel junctions with MgO barriers. STT-MRAM has the advantages of better selectivity and better scalability over conventional MRAM.

Thermoelectric materials can be used for refrigeration and power generation. To date efficient materials for applications in the vicinity of room temperature exist and thermoelectric modules fabricated from these are commercially available. However, both at low and high temperatures, efficiencies are still too low and much effort is put worldwide in optimizing known and finding new materials. After a brief introduction to thermoelectric phenomena and a presentation of the state-of-the-art in traditional Bi-Te based thermoelectrics, focus will be on material classes considered more recently for thermoelectric applications. Special emphasis will be on “cage compounds” as one important representative of the class of “complex metallic alloys”.