Electronic structure and mechanism for martensitic transformation in in Co2NiGa Shape Memory Alloys.

Computational and Experimental Design of Novel CoNiGa High Temperature Shape Memory Alloys (HTSMAs)

Raymundo Arroyave, Texas Engineering Experiment Station, DMR 0805293

• The ultimate goal of the current work is to develop CoNiGa HTSMAs through a combined computational/experimental approach.

• The intellectual merits of this transformative research are:o The potential of revolutionizing the field of SMAs, by developing SMAs

possessing high TTs, high thermal stability, low susceptibility to creep, aging and cyclic damage as well as good ductility

o Transformation of the design process for new HTSMAs through the synergistic experimental-computational approach

o The creation of an entirely new family of SMAs, i.e. shape memory superalloys, with high temperature SM and PE properties.

• Research Highlights: Microstructural degradation mechanisms for the superelastic response

of two phase CoNiGa high temperature SMA with the highest reported superelastic temperature were revealed.

A thermally and chemically stable CoNiGa HTSMA composition was discovered with phase transformation temperature of 172 °C and transformation hysteresis of 38 °C.

For the first time, we have identified not only the electronic valence ratio (e/a) but also the magnetic valence Zm as the two major alloy design parameters that determine the transformation temperature of CoNiGa SMAs. Electronic structure calculations also corroborate this observation.

Through first-principles calculations, the mechanism for the transformation in CoNiGa SMAs was shown to be fundamentally different from that of other Heusler systems, such as Ni2MnGa. Also, we have developed MonteCarlo simulations of the magnetic transitions in CoNiGa SMAs. The degree of configurational disorder has been shown to greatly affect transformation temperatures of CoNiGa SMAs.

A full thermodynamic model for the Co-Ni-Ga system over the entire composition and temperature range has been constructed using experimental as well as ab initio data within the CALPHAD approach. Model predictions have been satisfactorily compared with alloys prepared by the experimental components of this collaboration.

TEM image of Co46Ni27Ga27 showing martensite structure and the γ precipitate

Martensite

γ phase

350

300

250

200

150

100

50

0

-50

-100

-150

Ms (

°C)

7.707.657.607.557.507.457.407.357.307.25e/a

Ga30.6Ga30

Ga32 Ga28Ga30 Ga content

Ga26

Ga30

Ga29.27

Ga28.11

Ga29.76

Ga28.31

Ga28.4

Ms temperatures of CoNiGa SMAs with single or

multiphase structures as a function of the e/a ratio and

Ga content.

Predicted and experimental phase diagram in Co-Ni-Ga system at 1000 C

Monte Carlo simulations of magnetic behavior in CoNiGa SMAs

Predicted magnetic properties of austenite and martensite in stoichiometric Co2NiGa SMAs

Typical two-phase microstructure in formable CoNiGa SMAs

Computational and Experimental Design of Novel CoNiGa High Temperature Shape Memory Alloys

Raymundo Arroyave, Texas Engineering Experiment Station, DMR 0805293

• The broader impacts of the proposed activity are reflected in the following areas:o Maintaining US leading role in active materials research despite the vast amount of recent works at overseas on SMAso Development of teaching modules for incorporation into undergraduate courseso Helping K-12 students in developing science projects with SMAs o Enriched graduate and undergraduate research experiences in coordination with the new IGERT and Nanomaterials certificate programso Development of a graduate course in computational materials scienceo Disseminating the knowledge generated to both academia and industry through the workshops and symposiums on SMAs, presentations,

publications, and a website, and close collaboration with industry.

Broader Impact Highlights: Five students (including one female and one Hispanic), two M.

S., two Ph. D., and one undergraduate are employed in the project. Two M. S. students already graduated.

A course on thermodynamics of materials has been developed and offered during Spring 2008 and 2010.

Ongoing interaction with a local high school for new science projects. Several 8-10th grade students advised by Drs. Karaman and Arroyave have participated in the International Sustainable Energy, Engineering and Environment Project Olympiad (I-SWEEEP) 2011, Senior Engineering Category. I-SWEEEP is a groundbreaking science fair open to middle and high school students. It is the largest science fair event of its kind world-wide. Each year, around 500 projects from 70 different countries competes in I-SWEEP (www.isweeep.org)

The PI and Co-PI have been active in the ISWEEP Olympiad as judge and member of the organizing committee, respectively, for the last two years. Also they have been involved in recruitment activities at Minority Institutions, such as the University of Texas-Pan-American and assisted in the organization of a Materials Camp for Teachers

In the Summer of 2010, the PI hosted two high school teachers as part of the NSF-funded Enrichment Experiences in Engineering (E3) Program at Texas A&M University

The Co-PI has organized four international symposiums and three workshops for oil/gas, aerospace, and automotive industry on SMAs, up to date.

Nine research articles have been published, one accepted, and four in review (total of 14).

PhD students giving a lab tour to 36 female students in collaboration with the Society of Women Engineers at TAMU

Two 10th grade female students, advised by Ebubekir Dogan, present their work in the I-SWEEP 2011 Olympiad

Undergraduate students in Dr. Arroyave’s class synthesized, characterized and tested NiTi shape memory alloys in Dr. Karaman’s group