Introduction
Materials, Processes, and Experimental Procedures
General Manufacture of Nitinol
Tube Manufacturing Process TM-1
Mill product certification of compliance data | |||||||
---|---|---|---|---|---|---|---|
Element, w/o | Current study | Robertson et al. [1] study | |||||
Standard VIM-VAR | Standard VAR | Standard VIM-VAR | Standard VIM | Process optimized VIM-VAR | High-purity VAR | ||
Ingot 1 | Ingot 2 | Ingot A | Ingot B | Ingot C | Ingot D | Ingot E | |
Nickel | 55.90 | 55.93 | 55.83 | 55.89 | 55.8 | 55.98 | 56.1 |
Titanium | Balance | Balance | Balance | Balance | Balance | Balance | Balance |
C | 0.0272 | 0.0283 | 0.0020 | 0.0268 | 0.0330 | 0.0269 | 0.0020 |
Cr | 0.0004 | 0.0002 | 0.0050 | < 0.0100 | 0.004 | 0.0100 | 0.0050 |
Co | 0.0009 | 0.0009 | 0.0050 | < 0.0100 | 0.003 | 0.0100 | 0.03 |
Cu | 0.0004 | 0.0004 | 0.0050 | < 0.0100 | 0.0010 | 0.0100 | 0.0050 |
H | < 0.0050 | < 0.0050 | 0.0015 | 0.0008 | 0.0011 | 0.0007 | 0.0008 |
Fe | 0.0068 | 0.0066 | 0.0050 | 0.0062 | 0.011 | 0.011 | 0.0063 |
Nb | 0.0001 | 0.0001 | 0.0050 | < 0.0100 | 0.0010 | 0.0100 | 0.0050 |
N | 0.0005 | 0.0012 | NR | NR | NR | NR | NR |
0 | 0.0152 | 0.0238 | NR | NR | NR | NR | NR |
N + O | 0.0157 | 0.0250 | 0.0274 | 0.0197 | 0.041 | 0.026 | 0.0063 |
Transformation temperature*, Af, °C | − 4 | − 3 | − 19 | − 10 | − 1 | − 9 | − 9 |
Maximum NMI length, μm | 9 | 15 | 38 | 17 | 7 | 19 | 15 |
Area fraction, % | 0.74 | 0.75 | 1.25 | 1.1 | 0.7 | 1.18 | 0.46 |
Tube Manufacturing Process TM-2
Material Impurities
Test Methodology
Study | Tube lot | Perm. set, % | Upper plateau, MPa | Lower plateau, MPa | UTS, MPa | Strain to fracture, % |
---|---|---|---|---|---|---|
Current | 1−1 | ≈ 0.00 | 408 | 126 | 1145 | 44.8 |
1−2 | 0.02 | 399 | 122 | 1138 | 45.0 | |
2−1 | 0.14 | 414 | 138 | 1144 | 43.7 | |
Robertson et al. [1] | Standard VAR | ≈ 0.00 | 438 | 188 | NR | NR |
Standard VIM-VAR | ≈ 0.00 | 420 | 170 | NR | NR | |
Standard VIM | ≈ 0.00 | 400 | 180 | NR | NR | |
PO VIM-VAR | ≈ 0.00 | 415 | 170 | NR | NR | |
HP-VAR | ≈ 0.00 | 420 | 188 | NR | NR |
Results
Microstructural Analyses
Source | Vascotube data (optical) | |||||||||
---|---|---|---|---|---|---|---|---|---|---|
Material | Std VIM-VAR | |||||||||
Tube manufacturing process | TM-1 | n/a | ||||||||
Sample ID | 1−1 | 1−1 | 1−2 | 1−2 | 2−1 | 2−1 | n/a | |||
Sample configuration | Finished tube | Hollow | ||||||||
Orientation | Longitudinal | Transverse | Longitudinal | Transverse | Longitudinal | Transverse | Longitudinal | Transverse | Longitudinal | Transverse |
L max [µm] | 10 | 6 | 10 | 6 | 13 | 6 | 11 | 6 | 10 | 6 |
L mean [µm] | 1.35 | 0.79 | 1.21 | 0.75 | 0.82 | 0.53 | 1.06 | 0.69 | 1.35 | 0.58 |
L median [µm] | 1.06 | 0.61 | 0.84 | 0.57 | 0.56 | 0.44 | 0.52 | 0.48 | 0.98 | 0.43 |
NMI + porosity [%] | 0.23 | 0.34 | 0.27 | 0.27 | 0.46 | 0.38 | 0.34 | 0.31 | 0.31 | 0.28 |
NMI density [#/mm2] | 2118 | 6097 | 2898 | 5362 | 12,544 | 18,186 | 4216 | 6392 | 3601 | 8292 |
Average grain size [µm] | 15 | 18 | 14 | 16 | 20 | 17 | n/a | n/a | n/a | n/a |
Area surveyed [mm2] | 1.72 | 1.70 | 1.70 | 1.72 | 1.72 | 1.72 | 0.57 | 0.57 | 0.57 | 0.57 |
Source | Published Robertson Data [1] (BSE) | ||||
---|---|---|---|---|---|
Material | Std VAR | Std VIM-VAR | Std VIM | PO VIM-VAR | HP-VAR |
Tube manufacturing process | TM-2 | ||||
Sample ID | n/a | ||||
Sample configuration | Finished tube | ||||
Orientation | Longitudinal | Longitudinal | Longitudinal | Longitudinal | Longitudinal |
L max [µm] | 101 | 81 | 50 | 20 | 40 |
L mean [µm] | 3.55 | 1.86 | 1.29 | 1.40 | 2.88 |
L median [µm] | 1.72 | 1.23 | 1.06 | 1.06 | 1.26 |
NMI [%] | 1.46 | 1.51 | 2.67 | 1.49 | 0.41 |
Porosity [%] | 0.12 | 0.05 | 0.05 | 0.01 | 0.02 |
Total [%] | 1.58 | 1.55 | 2.72 | 1.50 | 0.43 |
NMI density [#/mm2] | 3247 | 7272 | 21,483 | 12,702 | 1425 |
Average grain size [µm] | 14 | 17 | 12 | 14 | 17 |
Area surveyed [mm2] | 0.1 | 0.1 | 0.1 | 0.1 | 0.1 |
Source | G. Rau (BSE) | |||
---|---|---|---|---|
Material | Std. VAR | Std. VAR | Std. VIM-VAR | Std. VIM-VAR |
Tube manufacturing process | TM-2 | |||
Sample ID | TM-2G | |||
Sample configuration | Finished tube | |||
Orientation | Longitudinal | Transverse | Longitudinal | Transverse |
Lmax [µm] | 40 | 17 | 24 | 6 |
Lmean [µm] | 2.42 | 1.69 | 1.37 | 0.69 |
Lmedian [µm] | 1.63 | 1.35 | 1.11 | 0.60 |
NMI + porosity [%] | 0.56 | 0.60 | 0.45 | 0.45 |
NMI density [#/mm2] | 2573 | 4013 | 7393 | 14,088 |
Average grain size [µm] | n/a | n/a | n/a | n/a |
Area surveyed [mm2] | 1.14 | 1.14 | 1.14 | 1.14 |
-
Lmax: TM-2G: Standard VAR > TM-2G: Standard VIM-VAR > TM-1: 2−1 > TM-1: 1−1 = TM-1: 1−2.
-
Lmean: TM-2G: Standard VAR > TM-2G: Standard VIM-VAR > TM-1: 1−1 > TM-1: 1−2 > TM-1: 2−1.
-
Lmedian: TM-2G: Standard VAR > TM-2G: Standard VIM-VAR > TM-1: 1−1 > TM-1: 1−2 > TM-1: 2−1.
-
dmax: TM-2G: Standard VAR > TM-2: Standard VIM-VAR = TM-1: 2−1 = TM-1: 1−1 = TM-1: 1−2.
-
dmean: TM-2G: Standard VAR > TM-1: 1−1 > TM-1: 1−2 > TM-2G: Standard VIM-VAR > TM-1: 2−1.
-
dmedian: TM-2G: Standard VAR > TM-1: 1−1 > TM-2G: Standard VIM-VAR > TM-1: 1−2 > TM-1: 2−1.
Discussion
Probabilistic Analyses
Material | ε
a
50
@ 107 cycles, % | ε
a
5
@ 107 cycles, % | ε
a
1
@ 107 cycles, % |
---|---|---|---|
TM-1: Standard VIM-VAR; 1−1
|
1.75
|
1.13
|
0.85
|
TM-1: Standard VIM-VAR; 1−2
|
1.71
|
1.19
|
0.92
|
TM-1: Standard VIM-VAR; 2−1
|
2.25
|
1.78
|
1.51
|
TM-2: HP-VAR | 1.71 | 1.33 | 1.11 |
TM-2: PO VIM-VAR | 1.56 | 1.23 | 1.04 |
TM-2: Standard VIM | 1.00 | 0.71 | 0.54 |
TM-2: Standard VIM-VAR
|
0.83
|
0.59
|
0.47
|
TM-2: Standard VAR | 0.61 | 0.34 | 0.22 |
-
TM-1: Standard VIM-VAR; 2−1 exhibits superior fatigue life at all probabilities to all materials and processes.
-
TM-1: Standard VIM-VAR; 1−1 and 1−2 exhibit the second-best fatigue life and are virtually identical.
-
TM-2: HP-VAR exhibits the third best fatigue life converging with the TM-1: 1−1 and 1−2 curves at 107 cycles.
-
TM-2: PO VIM-VAR exhibits the fourth best fatigue life and is slightly inferior to the HP-VAR.
-
TM-2: Standard VIM, TM-2: Standard VIM-VAR, and TM-2: Standard VAR exhibit increasingly worse fatigue lives, respectively.
-
TM-1: Standard VIM-VAR; 2−1 exhibits the best fatigue life.
-
TM-1: Standard VIM-VAR; 1−1, TM-1: Standard VIM-VAR; 1−2, TM-2: HP-VAR, and TM-2: PO VIM-VAR, exhibit the second-best fatigue lives.
-
TM-2: Standard VIM, TM-2: Standard VIM-VAR, and TM-2: Standard VAR exhibit increasingly worse fatigue lives, respectively.
Direct Comparison of TM-1 and TM-2
Correlation with Microstructures
Material | Standard VAR | Standard VIM-VAR | Standard VIM | PO VIM-VAR | HP-VAR |
---|---|---|---|---|---|
Lmedian, μm | 1.72 | 1.23 | 1.06 | 1.06 | 1.26 |
d
median
est
, μm | 1.65 | 1.18 | 1.02 | 1.00 | 1.21 |
Particle Size and Matrix Healing
Conclusions and Future Efforts
-
For 50, 5, and 1% probabilities of fracture, the probabilistic fatigue endurance limit (FEL), ε a P at 107 cycles, of diamond samples made from the Standard VIM-VAR grade of Nitinol TM-1 tubing is about two to three times greater than ε a P for diamonds made from the same tubing processed using TM-2 techniques [1].
-
For all probabilities and despite using a standard grade of Nitinol, diamonds made from TM-1 tubing exhibit markedly superior fatigue performance than different material lots processed using TM-2, and similar fatigue behavior to the cleaner HP-VAR and PO VIM-VAR materials also processed using TM-2.
-
For the six tube lots made using standard grades of Nitinol and within the confines of these analyses, median probability fatigue life, ε a 50 , exhibits a strong dependence on median NMI inclusion transverse diameter following an inverse power-law function with an exponent close to unity consistent with model proposed by Murakami for steels. This type of analysis may provide a basis for development of a predictive relationship between starting hollow microstructure, tube manufacturing technique, and probabilistic fatigue strength of a finished component.
-
Based on Murakami’s model, and substantiated by FE modeling of primary inclusion clusters, inclusion stringer length per se is expected to have little effect on fatigue life in longitudinal loading conditions as are encountered by the majority of stressed material in a cardiovascular implant.
-
Void closures at particle-void assemblies were observed in TM-1 processed samples with concurrent relatively small measured inclusion dimensions. Such closures can facilitate matrix healing and rebonding of the particle–matrix interface with notable improvement of high-cycle fatigue life.
-
Future testing and simulations are needed to investigate conjectured attributes of tube processing techniques that may promote matrix healing and reduced NMI size.