Oxygen and Nitrogen Solid Solution
Strengthened Ti Materials by SLM Process1

Background－ Oxygen & Nitrogen Solid Solution Strengtheningof Powder Metallurgy Titanium Materials －2

3

 Lightweight (~4 g/cc)
 High corrosion resistance Biocompatibility (no toxic) α-Ti (hcp) Not enough strength

Aircraft engine components
Micro-surgerydevices

 Strengthened by rare metals
 High materials cost
 Some allergenic properties Poor ductility & machinability

Artificial bone

Dental implant

4Ubiquitous elements* addition significantly effective to improvemechanical properties of powder metallurgy (PM) Ti materials*O, N, Fe, Si, Sn, Cu, Hydrogen, Carbon etc.

Nominal strain, εNominal stress, σ/MPa

Nominal stress, σ/MPa(a) Ti-O(b) Ti-N

Nominal strain, ε5Reaction behavior between Ti powderand TiO2 particles in vacuum sintering@800℃ for 30 min.
6

Ti [s] + O2 [g] → TiO2 [s] + ΔGf
ΔGf ;Formation-Gibbs free energy
ΔGf = -940.4+0.181T (kJ/mol)*at 800℃; ΔGf = -746 kJ/mol*Charlu T.V et al., J Chem Thermodynamics 6 (1974) 1065.

TiO2 decomposition occurs?

7①Ti+O2 ：O2 gas and Metal Ti [Ef =0, Standard］②Ti+TiO2 ： TiO2 existing with α-Ti
③Ti(O) ：Oxygen solution α-Ti
State ➂ : most thermally stable➡TiO2 decomposition andOxygen atoms solution into α-Ti

O atomDifference electron densities of α-Ti[hcp] (a) and Ti-O (b) analyzed byFirst Principles Calculation method
Yellow & Blue colors; Increase anddecrease of D.E.D in α-Ti crystal89Cast Ti-0.21～0.96 mass% OCast Ti-0.59%OO

Ti-0.21, 0.42, 0.59, 0.82, 0.96 wt.% O

100 μm(prepared by TOHO Titanium Co.)

Pure Titanium (JIS)(mass%)
Grade 1
Grade 2
Grade 3
Grade 4

ON

HCFe

≦0.15
≦0.20
≦0.30
≦0.40

≦0.03
≦0.03
≦0.05
≦0.05

≦0.013≦0.013≦0.013≦0.013

≦0.08
≦0.08
≦0.08
≦0.08

≦0.20
≦0.25
≦0.30
≦0.50

Ti-6Al-4V/Ti64 (JIS)ONHCFe

≦0.20

≦0.05

≦0.015

≦0.08

≦0.40

AlV
5.5-6.75 3.5-4.510

Cast Ti-0.21～0.96 mass% O
Cast Ti-0.59%O

Ti-0.21, 0.42, 0.59, 0.82, 0.96 wt.% O(prepared by TOHO Titanium Co.)

O100 μm

Cast Ti-O materials

11

Objectives－ O & N Solid Solution Strengthening
of SLMed Titanium Materials －12

 Molten pool rapidly solidified after laser irradiation in SLM
process (Solidification rate;105~106 K/s*)➠ TiO2 /TiN completely decomposed and O/N atoms solution
into α-Ti proceeds during SLM as well as PM process?

Or➠ O/N agglomeration occurs at some grain boundaries aftersolidification of molten Ti material as well as cast process?

*J. Yang, et al., Formation and control of martensite in Ti-6Al-4V alloy produced

by selective laser melting, Materials and Design, 108 (2016) 308-318.

13❶Pre-mixed Ti powder with TiO2/TiN particles by milling

Mechanical milling❷Ti2N/TiN layer covered Core-Shell Ti powder

Heat treatment in N2 gas

14

(a-1) TiO2 particle (~1.8µm)(a-2) Ti/TiO2 powder

TiO2
Pure Ti powder (~28µm)(b-1) TiN particle (~1.2µm)

Milling conditions
Ball-milling@3.6 ks
Ball to powder ratio; 5(b-2) Ti/TiN powder

TiN15 Increasing in nitrogen content of Ti powder
after H.T @1100℃ for 10 min., Micro-hardness of matrix (core part) drasticallyincreased（173Hv ➡ 315Hv ➡ 973Hv）.
 Not only Ti2N but TiN compounds formed.

d>100 µm (1100℃)

d=40~100 µm (1100℃)

16(a) H.T @900℃-10 min.
O; 0.15%, N; 4.34%

20 μm(b) H.T @1100℃-10 min.
O; 0.12%, N; 7.59%

50 μm
17

Formation energy (eV / N atom)

 TiN & Ti2N surface layers are able to be
decomposed during solidification in SLM

Ti2N/TiN

10①Ti+N2 [Standard］②Ti+TiN-1-2③Ti+Ti2N①Ti+N2-3④Ti(N)-4-50[Standard］： Metal Ti and N2 gas②Ti+TiN ： TiN existing with α-Ti
③Ti+Ti2N ： Ti2N existing with α-Ti

500

1000

1500

2000 ④Ti(N)： Nitrogen solution α-Ti

Temperature, T / K18

① Pre-mixed Ti+TiO2 powder
TiO2 content; 0.5~2 wt%② SLMed Ti-O materials- Laser power ; 140 W- Scan speed ; 1400 mm/s
- Hatch distance ; 60 µm- Layer thickness ; 20 µm- Laser spot ; 30 µm- Scan pattern; chessboard③ Microstructures & mechanicalproperties analysis of SLMed
Ti-O materials- XRD analysis (O solid solution)- SEM-EBSD (grains orientation)- EMPA (O atoms distribution)- Tensile test at room temperature
19

(a) Pure Ti(b) Ti-0.5%TiO250 µm

50 µm(c) Ti-1.5%TiO2
Building direction
50 µm20

21

(a) SLM Ti-2.0%TiO2 (0.89%O)O1µm(b) Cast Ti-0.62%OEBSD analysis results of SLM Ti-2.0%TiO2 material

50µm

EPMA results of SLM Ti-0.89%O (a)and cast Ti-0.62%O materials (b)2223① Ti(0.99%N) powder preparation(H.T @650℃-7.2 ks)② Pre-mixed powder of pure Ti
and Ti(0.99%N) to control
N content; 0.1 ~ 0.7 wt%

O; 0.16 wt.%, N; 0.99 wt.%

D50=24.5 μm③ SLMed Ti-N materials- Laser power ; 160 W- Scanning speed ; 535 mm/s
- Hatch distance ; 110 µm- Layer thickness ; 20 µm- Laser spot ; 30 µm- Scan pattern; Chessboard

Ti2N

24Intensity (a.u.)

SLMed Ti-N materials
α-Ti(0002)

α-Ti(10-10)

Ti2N
Ti-0.5%N

α-Ti(0002)

Ti-0.5%N
Ti-0.3%N

Ti-0.3%N
Pure Ti

Diffraction angle, 2θ/ °Pure Ti

25

α-Ti(10-10)

Ti2N
Ti-0.5%N
Ti-0.3%N
Pure Ti

Diffraction angle, 2θ/ °Lattice constant in a-axis

Lattice constant in a-axis (Å)α-Ti(0002)

Lattice constant in c-axis (Å)

Intensity (a.u.)

SLMed Ti-N materials

Nitrogen content (wt%)
26

Laser power; 83.3 J/mm327

α-Ti grains refinement withincrease in nitrogen content,and by laser power decrease

Laser power; 83.3 J/mm328

 Ti-0.01%N：L➠ β→α phase transformation
 Ti-0.31%N：L➠ β→α+β→α phase trans.
Primary α prevents 2nd α grains growth Large laser power causes decrease ofcooling rate in SLM➠ Grains growth29

Tensile strength, σ / MPa

Ti-0.5%N
Ti-0.3%N
Ti-0.2%N
Ti-0.1%N
Pure Ti
Ti-0.7%N
Strain rate; 5×10-4 /sStrain, ε30

S. Kariya et. al, Mater. Trans, 60 (2019) 263.

SLMed Ti-O
Pure Ti
Ti-0.5 wt.%TiO2
Ti-1.0 wt.%TiO2
Ti-1.5 wt.%TiO2
Ti-2.0 wt.%TiO2

SLMed Ti-N
Pure Ti (0.01%N)
Ti-0.1 wt.% N
Ti-0.2 wt.% N
Ti-0.3 wt.% N
Ti-0.5 wt.% N

O

0.460
1.050
1.590
1.970
2.610
O

0.470
0.430
0.510
0.510
0.530

SLMed materials (at.%)
NH

0.030
0.040
0.040
0.040
0.040

0.090
0.050
0.005
0.050
0.050

SLMed materials (at.%)
NH

0.030
0.290
0.730
1.060
1.7600.090
0.090
0.002
0.000
0.000

Yield stress by solid solution (MPa)Ti99.42098.86098.36597.94097.300σss[O]

σss[N]

Total

Schimdfactor,Sf

333.3
547.8
701.3
802.6
973.2

42.6
48.9
47.5
47.1
47.4

376.0
596.8
748.8
849.7
1020.6

0.346
0.365
0.376
0.379
0.377

Yield stress by solid solution (MPa)Ti99.41099.19098.75898.43097.710σss[O]

σss[N]

Total

Schimdfactor,Sf

346.2
295.7
330.0
335.7
337.1

43.6
179.4
330.7
431.4
592.0

389.8
475.1
660.7
767.1
929.2

0.338
0.373
0.374
0.368
0.376
31

Increment of measured σYS, ΔσYS* / MPa*ΔσYS; Difference in yield stress (σYS)
between pure Ti and Ti-O/N materials

Increment of calculated σYS[ss], ΔσYS[ss]** / MPa**σYS[ss] = σSS[Ti-x%O]－σSS[pure Ti]

H-P equation ∆𝜎𝑘 𝑑 ⁄𝑑⁄k ： H-P constant （k = 18 MPa/mm-1/2）∆𝜎185.0 𝑀𝑃𝑎ΔσYS－σYS[ss] at N=0.3% ＝ 176.6 MPa
32

As-SLMed Ti-0.5%N
Fully acicular structure

SLM Ti-0.5%N after HT

Ti-0.5%N after HT33

 HT@1030C（α+β）→W.Q➠Nitrogen discharge from β➠N-poor phase formation

5μm

Ti-0.5%N after HTTi

NDistance, L/μm34

 KAM analysis on tensile test specimen of Ti-0.7%N-HT（1060C→W.Q）

Fractured surface

αp grains

αp grains with highNitrogen content35

 Pre-mixed Ti+TiO2 powder and “Core-shell” structured Ti
powder uniformly covered with Ti2N/TiN thin film weresuccessfully employed to fabricate SLMed Ti materials
with oxygen/nitrogen elements solid solution.
 O/N elements were effective to α-Ti grains refinement
because of β→α phase transformation during rapid
cooling after solidification.
 Solid solution strengthening behavior by O/N atoms was
obtained to significantly improve mechanical properties
of SLM Ti materials as well as PM Ti strengthened bytheir solution.
 Solid solution & grains refinement strengthening factors
were dominant of SLMed Ti-O/N materials.
36