Deformation of NiTiCu Shape MemorySingle Crystals

H. Sehitoglu, X. Zhang,T.Kotil,1 H. J. Maier, 2Y. Chumlyakov

University of Illinois, Department of Mechanical and Industrial Engineering,Urbana, IL

1University of Paderborn, Lehrstuhl f. Werkstoffkunde, Paderborn,2Siberian Physical-Technical Institute,Tomsk, Russia

Presentation at the ASME ESDA Conference, July 8-11, 2002

Supported by the National Science Foundation, Mechanics and Materials Program,Engineering Directorate, Air Force Office of Scientific Research, Aerospace and Materials

Sciences

Introduction

• What is Shape Memory?• Shape memory is the ability of the material to recover large strains through a phase

transformation from martensitic to austenitic crystal states upon heating

• Why ‘Single Crystals’?• Clear understanding of SME and pseudoelasticity without GB effects.• Permits study of the influence of plasticity in the B2 phase.• Many of the polycrystalline NiTiCu alloys exhibit significant texture.

• Why’ Seek Alternatives to NiTi’?• To alter strength, the transformation temperature range, and eliminate aging treatments

for pseudoelasticity.

• Why’ Measure electrical Resistance?• To separate transformation from detwinning effects

What are shape memory alloys?

Shape Memory T<As

Strain

Stre

ss

PseudoleasticityT>Af

Austenite>Martensiteor Martensite Reorientation

Austenite < Martensite

Heat above Af

Austenite>Martensite

Austenite<Martensite

What makes them work?

A thermal or stress induced martensitic transformation.

Self accomodation

Schematic Illustration of The Mechanism ofThe Shape-Memory Effect

Parent Phase

Twinned Martensite

Detwinned Martensite

Applied Stress or

Stress free

Adapted from Figure 1 in [1]

[1] Otsuka, K. and Kakeshita, T., MRS Bulletin, Feb. 2002, 91

10

8

6

4

2

0

100500-50-100Temperature [

oC]

Cooling

Heating

NiTi10Cu

As= 0oC

Af= 31oC

Ms= 21oC

Mf= -11oC

M → Α

M ← Α

NiTiCu Shape Memory Alloys

Major Features:Stable Martensite Start TemperaturesTwo Stage Transformation ( B2 to B19 to B19’) for near 10%CuLower Transformation strains compared to NiTiNarrower Transformation Temperature Range compared to NiTi

Missing Information:Transformation Strains ( Calculations and Experiments)Single Crystal Information- Tension Compression Asymmetry,Crystal Orientation DependenceClear Understanding of Two Stages of Transformation ( B2 B19 B19’)The Role of Detwinning of B19’

800

600

400

200

0450400350300250200

Temperature (K)

<100> <011> <111> <012> <123>

NiTiCuCompression

Ms

Austenite Slip

Md

Critical Stress versus Temperature ( NiTi10Cu)

H. Sehitoglu et al. Met. Mats. Trans.,2001, 32A, 477-489

M>MM>A

700

600

500

400

300

200

100

0

543210Strain [%]

NiTi10Cu [012]

Temperatue = 100oC

Temperatue = 20oC

Temperatue = -60oC

100oC

20oC

-60oC

0.6

0.5

0.4

0.3

0.2

0.1

0.0

543210Strain [%]

NiTi10Cu [012]

Temperatue = 100oC

Temperatue = 20oC

Temperatue = -60oC

100oC

20oC

-60oC

Two Stage Transformation in NiTiCu- Schematic

a

n

Twinned Martensite, B19’

Habit Plane

1-f

b

b

B19 Martensite

m

m

b

1

1

B19 Martensite

B2

B2

f

CUBIC

MONOCLINIC

0.2898nm

0.4265nm

0.302nm

ORTHOROMBIC

0.288nm

0.456nm

96°

1 =1

2(FOrth

T ⋅FOrth − I) =1

2[b ⊗ m + m ⊗ b + (b ⋅ b)m ⊗ m]

Cubic to Ortohorombic Transformation

2 =1

2[(FMonoFOrth

−1 )T ⋅(FMonoFOrth− 1 ) − I] =

1

2[b ⊗ ′ m + ′ m ⊗ b + (b ⋅ b ) ′ m ⊗ ′ m ]

Ortohorombic to Monoclinic Transformation

Tension

Compression

NiTiCu ( 10%Cu)

Sehitoglu et al. (2001), Acta Mater., 49, 3621-3634 (image visible upon printing)

Single Step Transformation (B2 to B19’)

Tension Compression

Schematic of Variants, Habit Planes, Twins,CVPs

Twinned Martensite

bm

na

CVP

1-f

f

1-f

Detwinned Martensite

Two Step Transformation Single StepTransformation

B19' Detwinning

B2→B19

cubic→orthorhombic

B19→B19'

orthorhombic→monoclinic

B2→B19'

cubic→monoclinic

SingleStep

TwoSteps

[111] 2.55 3.49 7.05 8.67 8.33

[001] 2.78 0.70 2.98 2.98 3.49

[122] 4.64 2.60 7.83 9.39 8.71

[012] 5.01 0.41 5.32 6.19 6.32

[011] 5.57 0.44 5.87 7.00 7.14

Table 1 Transformation Strains Under Tension( in percent)

Monoclinic/Orthorombic Structure inMartensite in NiTiCu

Internally Twinned Martensite

200

150

100

50

0

543210Strain [%]

0.8

0.6

0.4

0.2

0.0

NiTi10Cu [012]

Tension at -60 oC

Stress-strain Electrical Resistance- Strain

Heating at Zero Stress

B'

D'

OAB : Martensite (-60 oC)

D' : Austenite ( 60 oC)

O,O' : Martensite (-60 oC)

BC

C'O, O'

A

A'

6

5

4

3

2

1

0

-1

-60 -40 -20 0 20 40 60Temperature [

oC]

0.6

0.5

0.4

0.3

0.2

0.1

0.0

-0.1

NiTi10Cu [012]Heating At Zero Stress

ReverseDetwinning∆ε ≅ 1.5%

M → Α∆ε ≅ 3.7%

Reverse Detwinning∆R/R ≅ 0.44

M → Α∆R/R ≅ 0.13

A'

D'

C

C'

Electrical Resistance

Strain

B'

A B

Temperature

Stra

in

Ms1

Transformation Strain

Af1M

f2 As

2

InelasticStrain

Step 1

As1

CUBICORTHORHOMBICMONOCLINIC

Step 2 B19->B19’

B2->B19

B19’Detwinning

8

6

4

2

0

-100 -50 0 50 100Temperature [

oC]

0.8

0.6

0.4

0.2

0.0

Strain Electrical Resistance

NiTi10Cu [012]

Martensite Detwinning∆R/R ≅ 0.44

M → Α∆ε ≅ 3.8%

Martensite Detwinning∆ε ≅ 1%

M → Α∆R/R ≅ 0.12

C

C'

D'

NiTiCu [111]

-8

-6

-4

-2

0

2

4

6

8

-140 -120 -100 -80 -60 -40 -20 0 20 40 60 80 100 120 140

Temperature (°C)

NiTiCu[111] OrientationStrain vs. Temperature

150

50

Cycle 1Cycle 4

50,75,100,150 MPa Tension Hold

100 MPa Compression Hold

2.55

3.49

-1.38

-1.18

Experimental strain - temperature hysteresis curves for a solutionizedTi-50.37at%Ni [123] single crystal

14

12

10

8

6

4

2

0

-2

-4

-6

-200 -160 -120 -80 -40 0 40 80Temperature (oC)

10.51% Detwinning Strain

-5.45% Detwinning Strain-5.53% CVP Strain

max strain

NiTi- Solutionized [123]

175 MPa

125 MPa

100 MPa

75 MPa25 MPa

0 MPa

-125 MPa

-170 MPa

-245 MPa

6.49% CVPStrain

Experimental strain - temperature hysteresis curves for an aged (1.5hrs @ 823K) Ti-50.37%Ni [123] single crystal

14

12

10

8

6

4

2

0

-2

-4

-6

-200 -160 -120 -80 -40 0 40 80Temperature (oC)

10.51% Detwinning + CVP Strain

-5.45% CVP Strain

-5.53% Detwinning + CVP Strain

NiTi- 823K 1.5Hrs [123]

210 MPa

100 MPa75 MPa

50 MPa

25 MPa

0 MPa

-175 MPa

-250 MPa

6.49% CVP Strain

170 MPa

Maximum Strain (%) vs Stress (MPa) Data for Solutionized (SL) &Over-aged (OA) NiTi

12

11

10

9

8

7

6

5

4

3

2

1

0

-600 -400 -200 0 200 400 600Stress (MPa)

10.51%10.27%

8.75%

7.55%

6.49%5.98%

5.21%4.82%

2.72%

5.53%5.45%5.06%

4.38%

3.58%

2.98%

CVP StrainDetwinning + CVP Strain

[012]-OA[012]-SL

[111]-OA[111]-SL

[011]-OA[011]-SL

[001]-OA[001]-SL

[123]-OA[123]-SL

Stage IElasticBehavior

Stage III

A>M

A>M;MartensiteElasticity

Slip ofMartensite

Elastic StrainPseudoelastic Strain

SME Strain

Inelastic Strain

Strain

Stre

ss

Em

EA

Recovarable Strain

Heat above Af

(B2 → B19 → B19' → B19' Detwinning

Stage II

1000

900

800

700

600

500

400

300

200

100

08.07.06.05.04.03.02.01.00.0

Strain (%)

8

6

4

2

0

86420Strain(%)

NiTi-Cu<001>Compression

Stress-Strain Response- Incremental Compression ,NiTi10Cu<100>

500

400

300

200

100

0

6543210Strain [%]

0.5

0.4

0.3

0.2

0.1

0.0

NiTi10Cu[012]

Stress-strain Electrical Resistance

Martensite Reverse Detwinning∆R/R ≅ 0.17

M → Α∆R/R ≅ 0.13

A → Μ∆R/R ≅ 0.13

Martensite Detwinning∆R/R ≅ 0.19

B'

B

P

P'

O O'

A'

A

Heating at Zero Stress

(1) In NiTiCu alloys with 10%Cu detwinning of the martensite phaseplays a significant role in deformation in addition to the two stagetransformation (B19 and B19’phases). (2) In the tensile experiments at 20 oC, the electrical resistancemeasurements confirmed that the end of the stress plateau region is the conclusion of the transformation and the onset of detwinning.(3) In the tensile experiments below martensite finish temperatures (-60°C) the detwinning of the martensite variants produces a largechange in resistance.(3) The relationship between strain and electrical resistance is notlinear when the electrical resistance change occurs due to bothdetwinning of the martensite variants and austenite to martensitetransformation.

Conclusions