TEM investigation of nanostructures in steels and
in GaN-based violet laser diodes
Professor J. R. Yang
Dept. of Materials Science and Engineering, National Taiwan University
Four case studies on TEM investigation of nanostructures will be presented in the
colloquial at the department.
First case study deals with nanometer-scaled modulated spinodal structures
in a 2205 duplex stainless steel [1-4]. 2205 duplex stainless steel is susceptible to
severe embrittlement when exposed at 475℃; this phenomenon is ascribed to the
degenerated δ-ferrite phase. High-resolution transmission electron microscopy reveals
that an isotropic spinodal decomposition occurred during aging at 475 ℃ in the steel
studied; the original δ-ferrite decomposed into a nanometer-scaled modulated
structure with a complex interconnected network, which contained an iron-rich BCC
phase (α) and a chromium-enriched BCC phase (α΄). It is suggested that the locking of
dislocations in the modulated structure leads to the severe embrittlement.
The second case study is concerned with nanostructures in a heavily
deformed ultra-fine wire of AISI316L stainless steel [5-7]. A heavily deformed
ultra-thin wire with a diameter of 8μm has been made and characterised. A specimen
preparation method for the cross-sectional TEM of ultra-fine wires has been
developed. The TEM images reveal that the extremely fine elongated austenite grains
have become rows of debris; the irregularly deformed structure of austenite
presumably consists of nano-sized dislocation cell structures. The TEM images also
show that nano-sized patches of newly formed martensite are distributed among the
debris of austenite. It is suggested that the effect of mechanical stabilization of
austenite against martensite growth leads to a large amount of nanometer-scaled
martensite in the ultra-fine wire. The strain-induced martensite phase has a smaller
size of dislocation cell than the heavily deformed austenite phase and makes a much
greater contribution to the total strength of the ultrafine wire.
The third case study deals with nanostructural analyses in GaN-based violet
laser diodes [8-14]. GaN-based violet laser diodes (LEDs) have been widely
manufactured for commercial use. Such structures can be produced by epitaxial lateral
over-growth (ELOG) of the GaN layer on a sapphire substrate, followed by deposition of
AlGaN/GaN strained-layer superlattice (SLS) claddings. The ELOG and SLS prevent
dislocations from forming due to mismatch between the Al2O3 and GaN lattices, and
between the GaN and AlGaN lattices, respectively, thereby reducing the density of
threading dislocations (TDs) that propagate to the multiple quantum well (MQW)
InGaN/GaN active layer through the substructures. In spite of these advances, defect-free
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multiple InGaN/GaN QW layers have not yet been obtained. The nanostructures of the
MQW active layers and SLS cladding layers greatly influence the final properties of LDs.
However, there have been very few structural investigations of heterostructures such as
InGaN/GaN layers and AlGaN/GaN layers, because any conventional microanalysis
technique including high-resolution TEM is incompetent for assessing these structure. In
this work, Multiple InGaN/GaN quantum well (MQW) layers in a laser diode were
investigated by high-angle annular dark-field (HAADF) scanning-transmission electron
microscopy (STEM). The detailed structure of the V-defect has been revealed.
Furthermore, the nanostructure of p-type AlGaN/GaN strained-layer superlattice (SLS)
cladding in a GaN-based violet laser diode (LD) has been observed using HAADF-STEM.
The evidence discloses the role of SLS in suppressing threading dislocation propagation.
The fourth case study is about the role of nanometer-sized carbides in newly
advanced high-strength low-alloy steels for automobile application [15-18]. The
nanotechnology has been exploited into the steel metallurgy in order to develop newly
advanced high-strength low-alloy steels for automobile application. A successful
applied case is the fantastic ferritic steel strips invented by National Taiwan
University (NTU) and China Steel Corporation (CSC). Tensile strength of over 980
MPa with total elongation of over 18% has been achieved in Ti and Mo bearing
hot-rolled low-carbon steel strips by producing microstructures that consist of a fully
ferritic matrix with nanometer sized carbides. Because the nanometer-sized carbides is
highly densely-distributed in the ferritic matrix, the steel strips can possess such an
ultrahigh strength with an excellent ductility. The breakthrough in alloy design and the
advanced technique of transmission electron microscopy (TEM) have made the
nanostructure-control and nanostructure-characterisation possible, visible and reliable.
The contribution of the dispersion of interphase-precipitated carbides to the yield
strength of the steel studied has been estimated. It is revealed that an optimum
component about 400 MPa contributed by interphase-precipitated carbides can be
achieved. The development of the ultrahigh-strength nanometer-sized carbide
strengthened hot-rolled steel strips has been extremely competitive in the worldwide
steel markets. Advanced Steel Microstructure Control – Engineering Research Center
(ASMC-ERC) will further carry on this subject for developing newly advanced steels
for automobile application in the five-year term.
References
1. T. H. Chen and J. R. Yang, “Effects of Solution Treatment and Continuous
Cooling on σ-phase Precipitation in a 2205 Duplex Stainless Steel”, Materials
Science and Engineering A, A311(2001)28-41.
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2. T. H. Chen, K. L. Weng and J. R. Yang, “The effect of high-temperature exposure
on the microstructural stability and toughness property in a 2205 duplex stainless
steel”, Materials Science and Engineering A, A338(2002)259 –270.
3. T. H. Chen and J. R. Yang, “Microstructural characterization of simulated heat
affected zone in a nitrogen-containing 2205 duplex stainless steel”, Materials
Science and Engineering A, A338(2002) 166-181.
4. K. L. Weng, H. R. Chen and J.R. Yang, “The low-temperature aging embrittlement
in a 2205 duplex stainless steel”, Materials Science and Engineering A,
A379(2004)119-132.
5. H.S.Wang, J.R.Yang and H.K.D.H.Bhadeshia, “Characterisation of Severely
Deformed Austenitic Stainless Steel Wire”, Materials Science and Technology,
21(2005)1323-1328.
6. H.S.Wang, C.Y. Huang and J.R. Yang, “Cross-sectional transmission electron
microscopy of ultra-fine wires of AISI 316L stainless steel”, Philosophical
Magazine, 86(2006)237-251.
7. S. Chatterjee, H.S.Wang, J.R.Yang and H.K.D.H.Bhadeshia, “The Mechanical
Stabilisation of Austenite”, Materials Science and Technology, 22 (2006)
641-644.
8. H. L. Tsai, T. Y. Wang, J. R. Yang, C. C. Chuo, J. T. Hsu, M. Ceh, H. Saijo and M.
Shiojiri, “Structural analysis of strained p-type AlGaN/GaN superlattice, Journal
of Applied Physics, 101, 023521 (2007).
9. M. Shiojiri, C. C. Chuo, J. T. Hsu, J. R. Yang, H. Saijo, “Structure and formation
mechanism of V defects in multiple InGaN/GaN quantum well layers”, Journal of
Applied Physics, 99, 073505 (2006).
10. H. Saijo, J. T. Hsu, R. C. Tu, M. Yamada, M. Nakagawa, J. R. Yang, M. Shiojiri,
“Mapping of multiple-quantum-well layers and structure of V defects in InGaN /
GaN diodes”, Applied Physics Letters, 84(2004)2271-2273.
11. Y. C. Chen, C. M. Wu, M. K. Chen, C. C. Yang, Z. C. Feng, and J.R. Yang,
“Improvements of InGaN/GaN quantum-well interfaces and radiative efficiency
with InN interfacial layers”, Applied Physics Letters, 84(2004)5422-5424.
12. K. Watanabe, J. R. Yang, N. Nakanishi, K. Inoke and M. Shiojiri, " Atomic-scale
strain field and In atom distribution in multiple quantum wells InGaN / GaN" ,
2003, Applied Physics Letters, 82(2003)715-717.
13. K. Watanabe, J. R. Yang, N. Nakanishi, K. Inoke and M. Shiojiri, 2003,
“Formation and structure of inverted hexagonal pyramid defects in multiple
quantum wells InGaN/GaN”, Applied Physics Letters, 82(2003)718-720.
14. K. Watanabe, J. R. Yang, N. Nakanishi, K. Inoke and M. Shiojiri, “Direct
Determination of Atomic Structure in MQW-InGaN / GaN”, Applied Physics
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Letters, 80(2002) 761-762.
15. H. W. Yen, C. Y. Huang and J. R. Yang, “Characterization of
interphase-precipitated nanometer-sized in a Ti-Mo-bearing steel”, Scripta
Materialia, 61 (2009) 616-619.
16. H.W. Yen, C.Y. Chen, T.Y. Wang and J.R. Yang, “Orientation relationship
transition of nanometer-sized interphase-precipitated TiC carbides in a Ti-bearing
steel”, Materials Science and Technology, 26(2010) 421-430.
17. F. G. Caballero, H. Y. Yen, M. K. Miller and J. R. Yang, “Complementary use of
transmission electron microscopy and atom probe tomography for the examination
of plastic accommodation in nanocrystalline bainitic steels”, Acta Materialia, 59
(2011) 6117-6123.
18. H. W. Yen, P. Y. Chen, C. Y. Huang and J. R. Yang, “Interphase precipitation of
nanometer-sized carbides in a titanium-molybdenum-bearing low-carbon steel”,
Acta Materialia, 59 (2011) 6264-6274.
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