Structural and Microstructural Studies of Ti/Zr and Al-Based Quasicrystals, Approximants, and Metallic Glasses

Kenneth F. Kelton, Washington University, St. Louis, DMR - 0307410Research Directions

• Studies of select metallic glasses by structural and kinetic studies to assess role of local order and diffusion-limited nucleation processes on glass formation and crystallization• Measurements of hydrogenation properties of Ti/Zr-based quasicrystals and related phases to evaluate potential technological usefulness and to probe local structure

Does Ti induce icosahedral short range order (ISRO) in Zr62-xCu20Ni8Al20Tix liquids?

It is widely reported that:• Small additions of Ti reportedly improve

glass formation• Glasses with Ti crystallize to an icosahedral

quasicrystal ISRO in liquid/glass

But our high-q, high energy x-raysynchrotron diffraction studies onelectrostatic-levitated liquids show:• ISRO is present in all glasses and develops in

all liquids, but at higher temperatures in alloys containing Ti

• ISRO is present in all liquids below Tm no simple influence on nucleation barrier

In conflict with previous results and speculations• Glass formation not due to enhanced ISRO• Supercooling studies on electrostatically

levitated liquids show better glass formation in alloys with no Ti

Ti inhibits surface crystallization (SEM below)

DMR METALS RESEARCH PROGRAM

Structural and Microstructural Studies of Ti/Zr and Al-Based Quasicrystals, Approximants, and Metallic Glasses

Kenneth F. Kelton, Washington University, St. Louis, DMR - 0307410

Ti-Zr-Ni Quasicrystal – novel hydrogen storage material?• H/M = 3 or 4 wt.% • DOE Target – 6.5 wt.%• 2 wt.% cycling between 100 – 200oC

Future Studies • Long-term cycling

• Measurements during desorption• Comparison with related phases

Education:Undergraduates Drew Newman, Michael George and Josiah Hartzell, graduate students Van Huett, Youtao Shen, Yeonha Sim, Debujit Saha, Victor Wessels and one-half of a postdoc (Li Qian Xing) were supported by this grant.

Youtao Shen demonstrates techniques for arc-melting of samples for quenching to Karen Spence and Victor WesselsOutreach:PI is President of UCSAC, a science advisory council dedicated to improving science education in University City, MO, a racially and socio-economically diverse school system adjacent to Washington University. A high school student from University City, Mark Bergers, worked in the PI’s lab during summer 2005

DMR METALS RESEARCH PROGRAM

Notes for First Transparency In a past project jointly funded by the NSF and NASA, we demonstrated growing icosahedral short range order (ISRO) in undercooled TiZrNi liquids that acts as a template for the nucleation of the icosahedral quasicrystal ( K. F. Kelton, G.W. Lee, A. K. Gangopadhyay, R. W. Hyers, T. J. Rathz, J. R. Rogers, M. B. Robinson, D. S. Robinson, Phys. Rev. Lett., 90, 195504-1 – 195504-4 (2003)). In that case the ISRO significantly increased the nucleation barrier for the stable C14 Laves phase, enabling the preferential formation of the i-phase. Such ISRO is common, even occurring in some elemental metallic liquids (G. W. Lee, A. K. Gangopadhyay, K. F. Kelton, R. W. Hyers, T. J. Rathz, J. R. Rogers, and D. S. Robinson, Phys. Rev. Lett., 93, 037802 (2004)).

ISRO has been widely invoked to explain metallic glass formation and nanocrystallization, particularly to quasicrystals. It is well known that glass formation and stability can be greatly influenced by the addition of small amounts of particular elements (microalloying). As one example, it has been widely reported that glass formation improves in Zr62Cu20Ni8Al10 alloys when small amounts of Ti are substituted for Zr, with Zr57Ti5Cu20Ni8Al10 forming

a bulk metallic glass. Glasses containing greater than 3 at.% Ti crystallize to a metastable icosahedral phase, suggesting that the Ti enhances ISRO in the liquid/glass, making crystallization more difficult during cooling.

However, we have recently shown that this is incorrect. By combining electrostatic levitation techniques with x-ray synchrotron methods, we measured directly the structures of the liquids as a function of supercooling. All of the supercooled liquid Zr-Ti-Cu-Ni-Al alloys show a distinct shoulder on the high-q side of the second peak in S(q) that is independent of the Ti concentration. The temperature at which the shoulder emerges increases with increasing Ti concentration, but it is present in all samples at and below the liquidus temperatures. These data demonstrate that there is no significant difference in the ISRO in the supercooled alloys as a function of Ti and hence no impact on glass formation, in conflict with the earlier speculations. Our solidification studies of electrostatically levitated liquids also showed that in conflict with earlier claims, Ti actually hinders rather than helps glass formation in these alloys. The Ti did, however, suppress surface crystallization, as can be seen in the SEM images of arc-melted samples shown in the transparency. The surface of the Ti-free sample not in contact with the Cu hearth clearly shows a thick surface crystallized layer, which is absent in the sample containing 5 at.% Ti. The interiors of both samples are amorphous. The mechanism by which surface crystallization is suppressed is unknown at present.

The primary point of these results is that while ISRO may be responsible for glass formation in some cases, it is probably not as widespread as has been argued in the literature.

Notes for Second Transparency The depletion of the world's petroleum reserves and the increased environmental impact of conventional combustion-engine-powered automobiles are leading to renewed interest in hydrogen powered vehicles. Hydrogen storage is a key issue, with the figure of merit being the weight of the materials needed to store the hydrogen. Recently, the US Department of Energy set a goal of 6.5 wt.% for transportation needs.

In the mid 1990’s, Ti/Zr/Hf-based quasicrystals were demonstrated to store more hydrogen than competing crystal intermetallic phases. Unfortunately, recovery of the hydrogen required temperatures in excess of 400oC, severely limiting the technological application of these materials. Recently, we have demonstrated the existence of a relatively flat plateau at high pressures (100 – 300 psi) for hydrogen concentrations exceeding 3 hydrogen atoms per metal atom (H/M = 3) in the Ti41.5Zr41.5Ni17 quasicrystal, more than 4 wt.% (see figure). Hydrogen desorption from

H/M = 3 to 1.5 was demonstrated at temperatures as low as 200 °C. These advances have improved the prospects of the Ti/Zr-based quasicrystals for hydrogen storage applications. Future studies will focus on exploring the behavior to higher pressures, up to 500 psi, establishing their cycling behavior, investigating the hydrogenation properties of related phases (metallic glasses and crystal approximants) and investigating the hydrogenation characteristics of nanostructured materials, prepared by our collaborator in Japan, Akito Takasaki.

Figure Photograph of senior graduate student, Youtao Shen, demonstrating to graduate students Karen Spence and Victor Wessels the techniques of arc-melting for samples to be rapidly quenched.

Outreach – The PI is the President of UCSAC, a a science advisory council dedicated to improving science education in University City, MO, a racially and socio-economically diverse school system adjacent to Washington University. This past year UCSAC focused on science in the High School. The High School teachers requested that scientists give regular seminars on topics of current scientific interest; a schedule was drawn up and scientists were recruited. In addition to working on the organization, the PI participated in the program, giving a seminar in February, 2005. The PI and secretary of UCSAC also met with the science teachers to connect students interested in working in a research laboratory with interested PI’s. Three students were placed for the summer of 2005. One student, Mark Bergers, worked in Kelton’s laboratory, assisting two graduate students in measuring particle size distributions of nanocrystals in partially crystallized metallic glasses. These data will be used in testing predictions from the coupled-flux model for nucleation, a model developed by the PI that couples the long-range diffusion and interfacial fluxes for partitioning transformations.

Papers and presentations over the past year:

Most notable publications:1. “In-situ High-Energy X-Ray Diffraction Study of the Local Structure of Supercooled Liquid Si,” T. H.

Kim, G. W. Lee, B. Sieve, A. K. Gangopadhyay, R. W. Hyers, T. J. Rathz, J. R. Rogers, D. S. Robinson, K. F. Kelton and A. I. Goldman, Phys. Rev. Lett., 95, 085501 (2005).

2. “Influence of Order on the Nucleation Barrier,” K. F. Kelton, A. L. Greer, D. M. Herlach, and D. Holland-Moritz, MRS Bulletin, 29, 940 (2004).

3. “Unterkühlt und dicht gepackt,” Physik Journal, 4, 37 (2005). 4. “Formation and crystallization of ZrCuTi metallic glasses,” Y. T. Shen, L. Q. Xing and K. F. Kelton,

Phil. Mag. (in press).Most notable invited presentation of PI:1. “Influence of local structure and alloy composition on glass formation and stability, Hume-Rothery

Symp., TMS2005, Feb. 13-17, 2005, San Francisco, CA2. “Coupled processes in nucleation,” Frontiers in Solidification Science, TMS2005, Feb. 13-17, 2005,

San Francisco, CA.3. “Icosahedral order in undercooled metallic liquids and the influence on the nucleation barrier,” Neutron

Diff.Charac. of Mech. Behav.: Deformation II, TMS2005, Feb. 13-17, 2005, San Francisco, CA4. “Icosahedral order in undercooled metallic liquids – Impact on the crystal nucleation barrier and thermo

physical properties,” Am. Phys. Soc., March 21-25, 2005, Los Angeles, CA.5. “Liquid structure and long-range diffusion – Their impact on glass formation and nanoscale

devitrification,” Bulk Metallic Glasses IV, May 1-5, 2005, Gatlinburg, TN.6. “Hydrogen storage in Ti-Zr and Ti-Hf- based quasicrystals,” 9 th Int. Conf. On Quasicrystals, May 22-26,

2005, Ames, IA.7. “The importance of local structure and chemistry (diffusion) on glass formation and crystallization,” K.

F. Kelton, Invited Presentation, PTM05, May 29 – June 3, 2005, Phoenix, AZ. 8. “Influence of microalloying on glass formation and crystallization,” K. F. Kelton, Invited Presentation,

ISMANAM 2005, July 4-7, 2004, Paris, France.