Equilibrium and metastable solidification in Ti-Al-Nb and Al-Ni systems [Elektronische Ressource] / vorgelegt von Olga Shuleshova

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• Physik

Informations

• Publié par: technische_universitat_dresden
• Publié le: 01 janvier 2009
• Nombre de lectures: 23
• Langue: English
• Poids de l'ouvrage: 68 Mo

Extrait

Equilibrium and metastable solidi cation
in Ti-Al-Nb and Al-Ni systems
Dissertation
zur Erlangung des akademischen Grades
Doktor der Naturwissenschaften
vorgelegt von
Olga Shuleshova
geboren am 21. April 1980 in Dneprodzerzhinsk
Technische Universit at Dresden
2009Gutachter: Prof. Dr. Bernd Buchner
Prof. Dr. Dieter Herlach
Tag der mundlic hen Prufu ng: June 1, 2010Contents
Introduction 1
1 Principles of solidi cation 5
1.1 Thermodynamics . . . . . . . . . . . . . . . . . . . . . . . . . 5
1.1.1 Phase diagrams . . . . . . . . . . . . . . . . . . . . . . 7
1.2 The nucleation threshold . . . . . . . . . . . . . . . . . . . . . 8
1.2.1 Surface energy of crystalline solid . . . . . . . . . . . . 11
1.3 Solidi cation . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
1.3.1 Free dendritic growth . . . . . . . . . . . . . . . . . . 13
1.3.2 Attachment kinetics . . . . . . . . . . . . . . . . . . . 16
1.3.3 Solute trapping . . . . . . . . . . . . . . . . . . . . . . 17
2 Experimental 19
2.1 Levitation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
2.1.1 The electromagnetic levitation method . . . . . . . . . 20
2.1.2 Non-contact observations of solidi cation processes . . 22
2.1.3 In situ structural analysis with synchrotron
radiation . . . . . . . . . . . . . . . . . . . . . . . . . 27
2.2 Alloy preparation . . . . . . . . . . . . . . . . . . . . . . . . . 31
2.3 Sample characterization . . . . . . . . . . . . . . . . . . . . . 32
2.3.1 Metallographic analysis . . . . . . . . . . . . . . . . . 32
2.3.2 Composition control . . . . . . . . . . . . . . . . . . . 33
3 Phase diagram studies of the ternary system Ti-Al-Nb 35
3.1 Motivation . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35
3.2 In situ X-ray di raction data . . . . . . . . . . . . . . . . . . 39ii Contents
3.2.1 Non-peritectic alloys on the 8 at.% Nb isopleth . . . . 39
3.2.2 Alloys on the 5 at.% Nb isopleth . . . . . . . . . . . . 44
3.2.3 Alloys on the 10 at.% Nb isopleth . . . . . . . . . . . 47
3.2.4 Alloys on the 38 at.% Ti isopleth . . . . . . . . . . . . 49
3.3 Comparison with thermodynamic calculations . . . . . . . . . 52
3.4 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55
4 Solidi cation of undercooled Ti-Al-Nb melts 57
4.1 Motivation . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57
4.2 Primary phase selection . . . . . . . . . . . . . . . . . . . . . 60
4.2.1 solidi cation domain . . . . . . . . . . . . . . . . . . 60
4.2.2 and solidi cation domains . . . . . . . . . . . . . 65
4.3 Microstructure of undercooled samples . . . . . . . . . . . . . 67
4.3.1 solidi cation domain . . . . . . . . . . . . . . . . . . 67
4.4 Growth kinetics of the primary phase . . . . . . . . . . . . 68
4.5 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . 72
5 Solidi cation of undercooled Al-Ni melts 75
5.1 Motivation . . . . . . . . . . . . . . . . . . . . . . . . . . . . 75
5.2 Al-Ni phase diagram . . . . . . . . . . . . . . . . . . . . . . . 78
5.3 In situ X-ray diraction data . . . . . . . . . . . . . . . . . . 82
5.3.1 The Al Ni alloy . . . . . . . . . . . . . . . . . . 8268:5 31:5
5.3.2 Alloys with 18{25 at.% Ni . . . . . . . . . . . . . . . . 89
5.3.3 Identi cation of the metastable phase . . . . . . . . . 91
5.4 Microstructure of undercooled samples . . . . . . . . . . . . . 92
5.5 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . 94
Summary 97
Appendix 101
Bibliography 110Introduction
For a long period of time the human development was determined by an metal-
lic alloy that they were able to melt and cast [1]. Despite the big variety of
the production routes invented nowadays, the solidi cation is still involved
in the majority of man-made products. On industrial demand the design
concepts for the novel materials need to ful l quite high requirements. Very
promising candidates for a wide range of potential applications are inter-
metallics { ordered chemical compounds of two or more metallic elements.
Among the others of current importance are the intermetallic Ti-Al and Al-
Ni alloy systems investigated within the European integrated project IM-
PRESS (Intermetallic Materials Processing in Relation to Earth and Space
Solidi cation) [2].
After several decades of research and development of titanium aluminides,
the intermetallic -TiAl based alloys are considered as the most promis-
ing candidates for aero-engine and automotive components [3], mainly due
3to their potential of signi cant weight savings ( = 3:9 { 4.2 g cm ).
Cast alloys with 42{46 at.% Al, which solidify via the disordered body-
centered cubic (Ti) phase, exhibit an isotropic, equiaxed and texture-free
microstructure with modest microsegregation, bene cial over the peritectic
alloys, where solidi cation involves the hexagonal (Ti) phase. By stabil-
ising the (Ti) phase through alloying with Nb, Ta, Mo or other elements,
an improvement of the thermal and mechanical properties can be achieved.
In particular addition of 5{10 at.% Nb reduces the stacking fault energy,
retards di usion processes and modi es the structure of the oxidation layer.
Despite remarkable progress, additional e orts are required for the in-
dustrial manufacture of these materials. Further development of concepts