Iron Based Transforming Single Crystals Huseyin Sehitoglu, C. Efstathiou, H. J. Maier, Y. Chumlyakov

Transcription

1 Iron Based Transforming Single Crystals Huseyin Sehitoglu, C. Efstathiou, H. J. Maier, Y. Chumlyakov University of Illinois, Department of Mechanical and Industrial Engineering, Urbana, IL Presented at ASME New Orleans, November 19, 2002 Supported by the Air Force Office of Scientific Research, Aerospace and Materials Sciences

2 Presentation Outline Relevancy Background Theoretical Calculations of Transformation Strains Shape Memory Behavior in Tension and Compression Temperature Cycling under Stress Potential Applications

3 Background The objective is to develop iron based transforming (shape memory) materials with high strength combined with high resistance to slip and fatigue. Previous iron based transforming materials (Fe-Mn-Si) do not possess high strength and exhibit small recoverability of strains. The NiTi materials have the recoverability but lower strength. The FeNiCoTi demonstrate enormous potential when both strength and recoverability are considered. We are systematically measuring transformation stresses and strains, electrical resistance to develop optimum FeNiCoTi compositions and aging treatments. We calculate the transformation behavior based on lattice parameters.

4 Recent Developments We demonstrate theoretically that these alloys can exhibit high transformation strains and high stresses. We have discovered Fe SMA compositions which produce high strength and good recoverability levels which have not been attained before. By varying the titanium content, we intend to minimize slip and enhance transformation characteristics. m b 1-f n Twinned Martensite a Detwinned Martensite f 1-f CVP

5 Stress-strain Response under Compression T ( o C) Fe 29Ni 18Co 4Ti Polycrystal min 550-3hrs Loaded at various temperatures in compression Upon heating Strain (%)

6 z, Z xyz- crystal axis in the austenite phase XYZ-crystal axis of BCT martensite a o X Y y x Z Z a o c X a o 2 a o 2 Y X a a Y

7 Three Lattice Correspondences of Martensite Variant 1 Variant 2 Variant 3 [100] T 1 2 [110] 1 C 2 [011] 1 C 2 [101] C [010] T 1 2 [ ] C 2 [ ] C 2 [ 101 ] C [001] T [001] C [100] C [010] C a a 0 a 0 a 0 F 1 = a a 0 = Deformation Gradient a 0 a 0 c 0 0 a U 1 = 0 1 0, U = , U = = symmetric part of F 1 = 2a a 0, 2 = c a 0

8 Schematic of Variants, Habit Planes, Twins, CVPs m b Twinned Martensite m= habit plane normal b=transformation shear of martensite f 1- f n a Habit Plane a o = 3.61A,a =2.8A,c= 3.01A f= volume fraction of twins within the martensite n=twin plane normal a= twinning shear m= {0.744,0.2189, } b= {0.1241, ,0}

9 Deformation Gradient Associated with CVP Formation F M = R h [ f R ij U j + (1 f )U i ) R h = relative rotation between the twinned martensite and the parent phase R ij = relative rotation between the two variants Strain associated with CVP Formation = 1 2 (F T M F M I ) = 1 [b m + m b + (b b)m m] 2 Deformation Gradient Associated with Detwinning F M dt = R h R ij U j = R h (U i + a n) Additional strain associated with Detwinning = 1 2 (F dt T M F dt M I) = 1 [b m + m b + (b b)m m] 2

10 Theoretical Calculations of CVP and Detwinning Strains

11 Schematic of Stress-Strain Response and Shape Recovery in FeNiCoTi alloys, T<M f Martensite Rearrangement (Detwinning) E m Stress Heat above A f Strain Inelastic Strain SME Strain Reverse Detwinning Elastic Strain

12 Schematic of Stress-Strain Response and Shape Recovery in FeNiCoTi alloys, T>A s Elastic Behavior Austenite to Martensite Transformation Slip of austenite Stress E A E m Heat above A f Strain Inelastic Strain SME Strain PE Strain Elastic Strain Recovarable Strain

13 Stress-strain Response under Tension T ( o C) -125 C -100 C -90 C -77 C C -59 C C -50 C -47 C -36 C -5 C Fe 29Ni 18Co 4Ti Polycrystal min 550-3hrs Loaded at various temperatures in tension Upon heating Strain (%)

14 Stress-strain Response under Tension(ctd.) T ( o C) 2.7 C 25 C 48 C 50 C 61 C 75 C 98.5 C 124 C Fe 29Ni 18Co 4Ti Polycrystal min 550-3hrs Loaded at various temperatures in tension Upon heating 1 Strain (%) 2 3

15 Transformation Characteristics of FeNiCoTi M A, Temperature ( o C) Heating Heating 550 o C 3 hr M A o C 85 min Cooling A M Heating M A Cooling A M A M, Temperature ( o C) Cooling dσ dt = s tra M

16 Fine Precipitates and Interface Dislocations 600 C 85min 550 C 3hours

17 Critical Stress as a Function of Temperature Fe 29Ni 18Co 4Ti Polycrystal min 550-3Hrs min min MARTENSITE Slip of A 400 Stress-Induced A-M Transformation 200 Martensite Rearrangement AUSTENITE Temperature ( C)

18 Temperature Cycling under Three Different Stress Levels Constant Stress 300 MPa 325 MPa 340 MPa Fe 29Ni 18Co 4Ti Polycrystal min 550-3hrs Loaded in tension Tmperature ( o C)

19 Changes in Electrical Resistance under Temperature Cycling Fe29Ni18Co4Ti [CR] 1150 o C 10 Min o C 85 Min + WQ Thermal Cycle, σ = 400 MPa Ro=2.98 mω Temperature o C

20 Lattice Softening Near the Transition Temperature Lattice Softening Fe 29Ni 18Co 4Ti Polycrystal 1150 C-10min 550 C -3 hrs Tension Compression Temperature ( o C)

21 Recovery as a Function of Temperature Fe 29Ni 18Co 4Ti Polycrystal 1150 o C-10min 550 o C-3hrs 600 o C 85min Tension (2%) 550 o C 3hrs Tension (2%) 550 o C 3hrs Compression (4%) Temperature ( o C)

22 Evidence of Planar Slip

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24 Summary The Fe - based materials have higher specific strength, higher specific modulus compared to NiTi alloys. With texture control higher transformation strains can be attained.