계층적 코어-쉘 신 합금 · 김래언 발표실

 김래언

포항공과대학교 박사과정

계층적 코어-쉘 구조를 통한 우수한
강도-경량화-가격경쟁성 달성 방안
 
:우주 발사체를 위한 신 합금

대한금속재료학회

지도교수: 김형섭 교수님

2026.04.28

1 장

Profile

김래언 (Rae Eon Kim)

Present: POSTECH, GIFT Ph.D candidate, 지도교수: 김형섭 교수님
M.S: POSTECH, GIFT
B.S: POSTECH, Mechanical Engineering

▪Research field

Additive manufacturing

Alloy Design

Formability

Hydrogen embrittlement

Heterostructure

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고온 구조 재료

Space rocket engine, nozzle extension

Gas turbine

Electric vehicle battery pack

High-tech industry requires high T structrual materials 

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고온 구조 재료

Inconel

A286

[1]

[3]

[2]

Ni-20Cr-6Mo-5Fe-

Fe-27Ni-16Cr-1Mo-

Co-9Al-7.5W

Ni based Superalloy

Co based Superalloy

Fe/Ni based Superalloy

[1] M.M. Oliveira et al. metals 9 (2019) 301

[2] J. Sato. et al. Science 5770 (2006) 90-91

[3] https://www.extreme-bolt.com/

Superalloys show high T stability and strength

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고온 구조 재료

[1] Lei H. et al. Fire Technol. 54 (2018) 1609-1628

[2] J. Chen et al. J. Struct. Eng. 132 (2006) 12

High content of 
Ni and Co

Inconel 718 TTT

γ’ (L1₂) Hardening

Conventional superalloys relie on L1₂ precipitate and

solid solution hardenings

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고온 구조 재료

Price

Ni : 8.90 g/cm3
Co : 8.83 g/cm3
Fe : 7.87 g/cm3

Ni : 1.57 $/kg
Co : 2.43 $/kg

Weight

High T
properties

Cost

Effetiveness

Low alloy density

Excellent mechanical properties at high T

The materials with high T properties, price, and low alloy density are required

Trade-off

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합금 설계

60FeHR

60FeAG

60Fe-23.2Ni-10Cr-3Ti-3Al-0.5Nb-0.3C (at%)

Fe	To ensure cost competitiveness, it is added at 60 at%	Cr	Oxidation resistance Fraction of γ' is increased	C	High-temperature stability (Nb,Ti)C increased strength
Ni	stability is enhanced Fraction of γ' is increased	Al, Ti	Essential elements for γ' precipitates weight reduction	Nb	Solid solution strengthening Fraction of γ' is increased

Low precipitate fraction → Low strength 
Heterostructures to compensate for the low strenth from the precipitates

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고온 구조 재료

RE Kim et al, Acta Mater. 281 (2024) 120397 

Both materials exhibit FCC matrix+carbide
60FeAG sample shows core-shell structure

■ Microstructure

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고온 구조 재료

RE Kim et al, Acta Mater. 281 (2024) 120397 

Local discontinuous precipitation
 in core-shell grain structures

Sample		Grain size	Precipitates
60FeHR		35.2 μm	X
60FeAG	Shell	40.3 μm	Rod shape Discontinuous precipitates (DP)
60FeAG	Core	3.2 μm	Spherical shape Continuous precipitates (CP)

60FeHR

60FeAR

Table. Microstructrual features of each samples

BSE images 

Carbide

■ Dual-graded Core-Shell structure

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고온 구조 재료

RE Kim et al, Acta Mater. 281 (2024) 120397 

Heterogeneous precipitates embedded 
in core-shell grain structure

TEM Analysis

■ Dual-graded Core-Shell structure

Schematic of Dual-graded Core-shell

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Dual-graded core-shell 생성 원리

Step 1. 

Step 2. 

Unlike cold rolling, dislocations accumulate primarily at interfaces (unsaturated states), creating a significant gap in recrystallization driving force between the grain boundaries and the interiors.

The excessive free energy at grain boundaries triggers boundaries to bulge and serrate (Strain induced boundary migration). During this process, L1₂-phase precipitates form preferentially at these boundaries. These newly bulged regions contain annealing twins boundaries through growth accidents.

LAGB and twin boundaires turned into HAGB. (shell grains). The newly created grain boundaries interact with DP formation, resulting in the emergence of a hierarchical harmonic structure

Step 1. 

Step 2. 

Step 3. 

Step 3. 

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기계적 물성

Sample	Mechanical Properties					Materials Properties
Sample	YS	UTS	U. El	T. El	YS at 600 C	Desity	Cost
60FeHR	1020 MPa	1242 MPa	10.9%	27.8%	-	7.73 g/cm³	6.5 $/Kg
60FeAG	745 MPa	845 MPa	4.3%	26.2%	798 MPa	7.73 g/cm³	6.5 $/Kg

*YS: Yields trength, UTS: Ultimate tensile strength, U. El: Uniform Elongation, T. El: Total Elongation

RT

High T

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HDI hardening 원리

RE Kim et al, Acta Mater. 281 (2024) 120397 

60FeAG shows the significant HDI hardening

■ *HDI hardening analysis

*Hetero-deformation induced (HDI) hardening

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고온 변형 후 미세구조

RE Kim et al, Acta Mater. 281 (2024) 120397 

Dual-graded core-shell grain structure 
is maintained at high T

<Side view>

<Fractography>

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연구 요약

The alloy demonstrates excellent mechanical performance across a wide range of temperatures while simultaneously offering low density and cost-effectiveness

Conventional superalloys contained generally more than >50 Ni at%, leading to high cost and density. The high Fe in L1₂-precipitated alloy generally reduces the precipitation fraction, potentially leading to decreased mechanical properties.

Limitation of the conventional superalloys

The hierarchical structure was designed by hot rolling and direct aging treatment without an additional process in L12-precipitated ferrous MEA. The present alloy features multi-scaled heterogeneity.

Dual-graded heterostructure

Superior HDI hardening

This structure can achieve superior HDI hardening across a wide range of temperatures. Further the structure can be maintained in high temperatured deforamtion. This structure induce local TRIP behavior in cryogenic temperature.

 김래언

포항공과대학교 박사과정

감사합니다 !

박사학위논문 심사

지도교수: 김형섭 교수님

2026.04.21

1 장

Appendix

Figure. Calculated and experimental yield strengths of the 60FeAG sample.

Figure. Composition maps of (a) the 60FeHR and (b) 60FeAG samples.

Figure. (a) STEM image, (b) bright-field TEM image and SAED pattern, and (c) dark-field TEM image of the 60FeAG samples deformed to a true strain of ∼10%.

Figure. Pole figure and ODF maps of (a) 60FeHR and (b) 60FeAG samples.

Figure. True stress-strain curves along the rolling direction (RD) and transverse direction (TD).

Figure. One-dimensional concentration profile across the two Ni-Ti-rich particles. The iso-surface of Ni is 25 at%.

Figure. 3D reconstruction map and line profile of APT for (a) core region and (b) shell region in 60FeAG. The line profile is presented along the white line in the 3D APT maps.

Figure. (a) Engineering stress-strain curves and (b) strain hardening rate curves of the present samples. (c) Engineering tensile stress-strain curves of 60FeAG at elevated temperatures of 600°C and 700°C. (d) Comparison of the tensile strengths as a function of performed temperature with those of conventional superalloys.