Research Projects

2008-2009

Grain Size Stability and Consolidation of Nanostructured Particulates

C. C. Koch, PI; R. O. Scattergood, Co-PI

National Science Foundation

$461,101

9/15/2005 - 5/31/2009

The objective of this research is to develop strategies to stabilize nanoscale microstructures during consolidation of powder particulates at elevated temperatures. The experimental approach to be used will emphasize a systematic study of grain growth and kinetic or thermodynamic factors that influence it in selected metals and alloys prepared by mechanical attrition. The model systems based on bcc Fe and fcc Ni are selected since they exhibit different behavior for nanocrystalline grain growth. Alloy additions will be used to reduce the grain boundary energy (thermodynamic approach) or limit the grain boundary mobility (kinetic approach).

Creep Behavior as a Function of Grain Size in High Fluidity Zn-Al Die Casting Alloys.

C. C. Koch, PI; K. L. Murty, Co-PI

International Lead and Zinc Research Organization

$154,698

1/1/2007 - 12/31/2009

The creep of selected Zn-Al die casting alloys are studied as a function of grain size. The alloys emphasized are those with high fluidity that are readily cast into thin sections. The alloys will have compositions of about 4.5 wt. % Mg and small quantities of Mg and Cu. Grain sizes of these alloys will be varied from about 10 microns down to the nanoscale. Die castings as well as ball milled and consolidated powders will be used to cover the grain size range of interest. Creep measurements will be conducted by impression creep tests from room temperature up to 150?C. The creep data will be analyzed to determine the mechanism responsible so that methods can be explored to minimize the creep rate.

Novel Bulk Nanocrystalline Thermoelectric Materials by Mechanical Milling or Mechanical Alloying.

C. C. Koch

DARPA/DOS, subcontract from RTI International

$193,757

5/15/2008 - 5/14/2013

The objective of the proposed research is to develop strategies for preparation of bulk nanocrystalline thermoelectric materials with increased figures of merit. The methods of preparation will involve high energy ball milling followed by powder compaction. These methods have been found to be very effective in synthesis of nanostructured materials.

The important research issues will comprise materials selection, preparation methods, and powder consolidation methods. Since all materials of interest can be processed in nanocrystalline form by the several ball milling methods, the materials to be selected will be guided by the interests of the team. The preparation methods will include a] “mechanical milling” wherein the single phase component is milled to create a nanocrystalline microstructure; b] “mechanical alloying” whereby a second element can be incorporated into the material, also at the nanoscale; and c] “mechanochemical synthesis” in which a chemical reaction initiated by ball milling results in different chemistry such as formation of sulfides.

Breaking the Strength Ceiling: A Fundamental Study to Make Ultra Strong Bulk Mg Alloys

C. C. Koch, PI; Y. T. Zhu, Co-PI

Army Research Laboratory

$70.001

6/15/2008 - 6/14/2009

There has been increasing interest in Mg and its alloys as light-weight structural materials. Mg has the lowest density of any of the structural metals (1.74 g/cc). However, it is hcp and suffers from relatively low strength, ductility, and formability. A number of studies in recent years have attempted to increase the strength and ductility of Mg-based alloys by grain refinement and/or addition of dispersoids, or changes in texture. With a nanocrystalline matrix, the yield strength of Mg can be increased from about 120 MPa for commercial extruded Mg to about 180 MPa for nanocrystalline Mg with a 45 nm grain size. The yield strength of the strongest Mg alloy is only 340 MPa.

We will attempt to produce bulk nanostructured Mg alloys with ultrahigh strength and good ductility, and to study the mechanisms that gave them the superior properties. We will focus on two Mg alloys: Mg97Zn1Y2 alloy and ZH60A alloy. This will involve the processing and fundamental study of the strength and ductility of three types of nanostructured Mg alloys: 1) Mg97Zn1Y2 alloy via ball milling + consolidation, 2) Mg-Zn-Zr Alloy (ZK60A) via cryogenic rolling + age hardening, 3) High-pressure torsion (HPT) + age hardening of ZK60A.

Materials World Network: Processing-Structure-Property Relationships in Ultra-fine Grained and Nanocrystalline Bulk Cu and Cu-Zn Alloys

C. C. Koch, R. O. Scattergood, and D. Brenner

MWN-NSF

$482,000

8/2009 to 8/2011

This research is a collaboration among North Carolina State University (NCSU), the University of Vienna, Austria (UV), and IFW-Dresden, Germany (IFW-D). The scientific objectives of the research are to exploit a range of processing and characterization techniques and expertise to investigate the mechanical behavior of metals and alloys as a function of their grain size and stacking fault energy. In particular, Cu and Cu-Zn alloys will be prepared by processing techniques at the various institutions that, when combined, will produce artifact-free bulk samples with a grain size range from ultra-fine (< 1000 nm) down to the finest achievable at the nanoscale (~ 10 nm). The combined effort and facilities at NCSU, UV, and IFW-D are needed to achieve the processing objectives. The critical mechanical properties will be determined by tensile, shear and nanoindentation tests at NCSU, compression, tensile and torsion tests at IFW-D, and fatigue and fracture toughness at UV. The deformation mechanisms that are operative at the various grain sizes and stacking fault energies will be probed by in situ x-ray diffraction line broadening studies under stress at UV and NCSU with collaborators. In situ TEM straining studies will be done by IFW-D and NCSU with collaborators, and in situ SEM will be done at IFW-D. An extensive set of density functional theory (DFT) calculations and large-scale MD simulations will be done at NCSU. Mechanism-based and related modeling will be done at NCSU, UV, and IFW-D. The exchange of graduate students and the faculty interactions among NCSU, UV and IFW-D will constitute a major part of the project.