Why is nitinol classified as an alloy

Nickel mixes easily with many other metals including chromium, iron, molybdenum, and copper alloys. This allows for a wide variety of alloys that have excellent corrosion resistance and high temperature scaling, exceptional high temperature strength, and other unique properties such as shape memory and low coefficient of expansion. Below is a simplified categorization of the different types of nickel alloys.

Wrought nickel

Pure nickel UNS N02200 is used in the chemical industry for its corrosion resistance - especially against alkalis. It is also used for its properties in shielding against electromagnetic interference and in transducers.

Nickel-iron alloys

These are used as soft magnetic materials, as glass-to-metal seals and as materials with defined thermal expansion properties.
Invar® (UNS K93600), with 36% nickel and the remaining iron, is unique in its almost zero coefficient of thermal expansion around room temperature. This makes it valuable where high dimensional stability is required, e.g. B. with precision measuring devices and thermostat sticks. It is also used at cryogenic temperatures because of its very low thermal expansion rates.
Alloys containing 72-83% nickel have the best soft magnetic properties and are used in transformers, inductors, magnetic amplifiers, magnetic shields and storage devices.

Nickel-copper alloys

These are very resistant to corrosion by alkaline solutions, non-oxidizing salts and sea water. The best known is Alloy 400.

Nickel-molybdenum alloys

These are very resistant to reducing acids in the absence of oxidizing ions such as iron and cupric or dissolved oxygen. The best known is Alloy B-2.

Nickel-chromium alloys

These are characterized by their high corrosion resistance at normal and high temperatures (scaling resistance), good high temperature resistance and high electrical resistance. There are three main groups of alloys:

Ni-Cr (and also Ni-Cr-Fe) alloys with high electrical resistance for heating elements, such as 70-30 (UNS N06008) and C-Grade (UNS N06004)

Ni-Cr alloys (with Fe and other alloy elements) with good corrosion resistance. The best known are Alloy 600 (UNS N06600) and Alloy 601 (UNS N06601)

Ni-Cr alloys with high temperature strength and creep resistance, mostly age-hardening, such as alloy X-750 (UNS N07750)

Nickel-chromium-iron alloys

There are basically two groups of alloys:

Ni - Cr - Fe alloys with excellent strength at high temperatures and the ability to withstand oxidation, gasification and other types of high temperature corrosion. The best known is the alloy 800 (UNS N08800) and its variants 800H (UNS N08810) and 800HT (UNS N08811). (Recently these alloys have been classified as stainless steels, reflecting their high Fe content))

Ni - Cr - Fe (with Mo and Cu) alloys with excellent corrosion resistance in specific applications. The best known is alloy 825 (UNS N08825), which offers exceptional resistance to sulfuric acid. Alloy G-3 (UNS N06985) offers exceptional corrosion resistance to commercial phosphoric acids as well as many complex solutions that contain highly oxidizing acids.

Nickel-chromium-molybdenum alloys

These are very resistant to corrosion, of which the alloy C-276 (N10276) is the best known. They offer exceptional resistance to reducing acids such as hydrochloric and sulfuric acid. There are a number of variants based on this composition that changed the Cr and Mo values ​​and in some cases added Cu or W to extend the corrosion resistance to conditions that are more oxidizing or reducing. These include Alloy C-22 (N06022), Alloy 59 (N08059), Alloy C-2000 (UNS N06200), and Alloy 686 (N06686).

Nickel-chromium-cobalt alloys

The addition of cobalt and molybdenum gives alloy 617 (UNS N06617) solid solution strength and high creep rupture strength. The addition of cobalt to HR-160 (N12160) provides excellent resistance to various forms of high-temperature corrosive attack, such as sulphidation and chloride attack in reducing and oxidizing atmospheres.

Nickel-titanium alloys

55% Nickel-Titanium Alloy (UNS N01555) (also known as Nitinol) has shape memory properties. If it is formed at one temperature and then deformed at a lower temperature, it will regain its original shape when heated again. The transition temperatures can be adjusted by careful control of the composition. Medical devices and special connectors are two specific uses. The same alloy can also undergo significant elastic deformation and still return to its original shape (super-elastic property). This property has been used in applications as diverse as eyeglass frames and shock absorbers that provide earthquake resistance in historic stone buildings.

Nominal composition of various nickel alloys:

Alloy name

US

% Ni

% Cr

% Fe

% Mon

% Cu

% Co

% Miscellaneous

200

N02200

99 min.







Invar

K93600

36


64





400

N04400

65

32






B-2

N10665

68


2

28




70-30

N06008

70

30






C class

N06006

60

16

Bal





600

N06600

76

16

8





601

N06601

60

23

Bal




Al - 1.3

X-750

N07750

70

15

7




Ti - 2.5

Al - 0.7

Nb - 1.0

825

N08825

42

21

25

3

2



G-3

N06985

Bal

22

20

7

2



800

N08800

32

21

45




W - 3.5

C-276

N10276

Bal

15

5

16




C-22

N06022

Bal

21

4

13




59

N06059

Bal

23

1.5 max.

16




C-2000

N06200

Bal

23

3 max

16

1.6



686

N06686

Bal

21

2 max

16



W - 3.7

617

N06617

44.5 min.

22

3 max

9


12


HR-160

N12160

Bal

28

3.5 max



30

Si - 2.75

Nitinol

N01555

55






Ti - 45

Alloy type

US

% Cr

% Ni

% Mon

% Cu

% Fe


304L

S30403

18

8

-

-

Bal

The most common stainless steel used for drinking water treatment and food contact.

316L

S31603

16

10

2.1

-

Bal

The addition of Mo offers greater resistance to chlorides and acidic solutions. Use in coastal regions, in water treatment and in the chemical processing industry.

2205

S32205

22

5.7

3.1

-

Bal

Even higher corrosion resistance than 316L. Can be used in place of 316L, but its greater strength can translate into weight reduction. Can be replaced with 316L if it fails in operation.

2507

S32750

25

7

4

-

Bal

Even higher corrosion resistance than 2205. Useful corrosion resistance against sea water.

254 SMO

S31254

20

18

6.1

-

Bal

Corrosion resistance similar to 2507, also with useful corrosion resistance against seawater.

Alloy 20

N08020

20

33

2.1

3.2

Bal

Cu bearing stainless steel with useful corrosion resistance to all concentrations of sulfuric acid.

310S

S31008

25

20