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An Introduction to Materials by Design Including a Dynamic Stress Environment

James W. McCauley

Army Research Laboratory and Johns Hopkins University

Abstract

Materials by Design, conceptually, describes a process of designing materials from the atomic to the macroscopic scale for a particular suite of mechanisms and properties that are required for defined performance/applications. Very simply, it is not about how to design components (systems) with existing materials, but about how to select and design materials for defined applications. Initially, the importance and definition of "materials characterization" are presented. including a materials "unique signature" at the atomic, microstructure, and macrostructure scales. A brief history of the emergence of Materials by Design is summarized, followed by progressively more complex examples, including atomic structure and microstructure design in ceramic matrix composites. Historically, the Materials by Design approach has been primarily used for systems in quasi-static mechanical environments; utilization in extreme dynamic environments presents significant challenges. A simplified example of the approach for solid-on-solid impact on a structural ceramic is described. Finally, the influence of this approach in a strategic materials basic research program and the new transformational Materials in Extreme Dynamic Environments program at the Army Research Laboratory is briefly summarized.

Keywords: Materials by Design; characterization; materials selection; unique signature; ceramic matrix composites; extreme dynamic environments

1.1 Introduction

Materials are ubiquitous in all Army materiel. The performance and function of every Army system are determined by the underlying properties of the materials and the structural/electrical design of the engineering system. In turn, the properties of the materials themselves are products of the hierarchy of structures found within. From atoms, to unit cells, to crystals, to grains, to assembly of grains, and to multiphase materials, the final performance of any system is a "sum of the parts," sometimes synergistic, of the underlying physics down to the smallest (nano/atomic) scale, or simplified as follows.

1.2 Crystal Structure Microstructure Macrostructure Property Relationships

In real estate, the three most important factors are location--location--location. In materials, it is structure-structure-structure.

The potential to gain extraordinary system or component improvements through the effective design and control of the constituent materials remains untapped. Enabling the design of these hierarchical material structures in concert with the overall design and function of the system will allow for transformational gains in the performance of engineering systems. This vision is consistent with the National Materials Genome Initiative to "Discover, develop, manufacture, and deploy advanced materials in a more expeditious and economical way." [1].

1.3 Scope of Manuscript

First, the appropriate context will be established by introducing the importance of the definition of "materials characterization," especially in materials science and engineering research and development. This will lead to the development of the material "unique signature" concept at multiple scales. A brief history of the emergence of a formalized materials selection process and the Materials by Design (MbD) approach in the 1990s is then introduced. This is followed by progressively more complex examples of MbD:

1. RADOME materials: materials selection, combining mechanical and electrical properties;
2. Multilayer ceramic capacitors (MLCCs) combining electrical, geometry, and MbD at the single crystal/grain level;
3. Particulate dispersion ceramic matrix composites demonstrating atomic and microstructural control of mechanical and thermal properties; and
4. MbD in a dynamic mechanical environment.

Finally, a case is made that the use of the MbD approach in the Materials R&D environment significantly improves the probability of success in the utilization of new materials. This is followed by a brief description of the new Army Research Laboratory program, Materials in Extreme Dynamic Environments (MEDE), which utilizes the multiscale MbD approach in a robust modeling and simulation and advanced experimental framework.

1.4 Characterization of Materials and Unique Signatures at Multiple Scales

Professor Rustum Roy, the founder and Director of the Materials Research Laboratory at Penn State for many years, published a catalytic paper in 1965, emphasizing the importance of material characterization in materials science and engineering [2]. It is believed that this paper led to an important National Academy Materials Advisory Board study in 1967, focusing on the characterization of materials [3]. The most important conclusion from the committee, besides underscoring the importance of materials characterization, was a universally agreed-upon definition of "Materials Characterization" as follows:

CHARACTERIZATION DESCRIBES THOSE FEATURES OF THE COMPOSITION AND STRUCTURE (INCLUDING DEFECTS) OF A MATERIAL THAT ARE SIGNIFICANT FOR A PARTICULAR PREPARATION, STUDY OF PROPERTIES, OR USE, AND SUFFICE FOR REPRODUCTION OF THE MATERIAL

Building on these recommendations, the Army Materials and Mechanics Research Center, in cooperation with Syracuse University, convened the twentieth Sagamore Army Materials Research Conference on "Characterization of Materials in Research-Ceramics and Polymers" at the Sagamore Conference Center, Raquette Lake, New York, during September 11-14, 1973. At this conference, McCauley [4] presented a paper titled "Structural and Chemical Characterization of Processed Crystalline Ceramic Materials." The following is a quotation from this paper: "Research on new materials demands systematic characterization, not only to optimize fabrication parameters and to insure future quality control, but also to enable optimum engineering properties to be achieved. Any processed ceramic material should be uniquely defined by a necessary and sufficient set of parameters including composition, grain size, shape orientation, and packing. Quantitative relationships derived among these parameters, engineering properties and utilization functions can be used to control and optimize their properties and use." A simple material defining equation was suggested to uniquely define a material:

These concepts were later reworked [5] into a "unique signature" of a material to represent the critical importance of processing defects:

The unique signatures for boron carbide (nominally B4C) at five scales are illustrated in Figure 1.1.

Figure 1.1 Unique signatures at multiple scales for polycrystalline boron carbide (B4C).

In addition, "utilization functions" were introduced [4], which were defined as a "mathematical combination of critical properties which can quantitatively predict how well a material will perform in a certain use or environment." These two simple ideas form the basis of what is now called "Materials by Design" and material "Figures of Merit." Finally, a flow chart for materials research and development (Figure 1.2) was presented to emphasize the importance of characterization in several stages of an overall materials development program. Using these concepts can help define materials science and materials engineering:

• Materials Science = the creation of new materials and the understanding of the relation of material characteristics [unique signature = chemistry (c), microstructure (M), defects (PD)] to properties.

• Materials Engineering = the fabrication (processing) of materials with controlled properties for certain performance. Materials Figures of Merit (FoM) are a critical link here as they define a quantitative relationship between combinations of properties to desired performance.

Figure 1.2 Flow chart for materials research and development.

1.5 Historical Emergence of Materials by Design

In the 1990s, the stage was being set by a few key publications and activities. Mike Ashby from Cambridge University published a classic work on "Materials Selection in Mechanical Design" in 1992 [6]. Ashby introduced the concept of a "Material Index," which is a combination of material properties that characterizes the performance of a material in a given application. A simple example is the material index (M) for a light, stiff structural beam:

where E is Young's modulus (stiffness) and ? is the density. The objective is to maximize M. This index can also be referred to as a "Figure of Merit (FoM)."

In 1997, Professor Greg Olson [7], from Northwestern University, building on the work by Cyril Stanley Smith [8], published a seminal paper in Science titled "Computational Design of Hierarchically Structured Materials," In this paper, he presented a very simple model (Figure 1.3) that he called...