Study of Tribological Characteristics of Nitife Shape Memory Alloys for Varying Aging Conditions

Study of Tribological Characteristics of Nitife Shape Memory Alloys for Varying Aging Conditions

Sunil Sarangamatp, Ramesh Gowda N.R 2, Chandrakumar D3, Yellappa M4 , Santhosh N5, Srinivas M .T6 1P.G.Student, Department of Mechanical Engineering, Dr. Sri Sri Sri Shivakumara Mahaswamy College of Engineering 2,4Assistant Professor, Department of Mechanical Engineering, Rajarajeswari College of Engineering,

3,6Assistant Professor, Department of Mechanical Engineering, Dr. Sri Sri Sri Shivakumara Mahaswamy College of Engineering

5Assistant Professor, Department of Aeronautical Engineering, Nitte Meenakshi Institute of Technology

Abstract Shape memory alloys are those groups of alloys which have a characteristic phenomenon of exhibiting the property of remembering its shape upon deformation and returning to its original shape when heated. These materials change their properties, for example vibration resonance frequency or modulus in response to a temperature variation. When heated by direct electrical current above the transformation temperature, the pre-deformed shape memory alloy (SMA) try to recover their shape and since they are restrained, a stress is created. The stress thus created can be relieved by aging of the samples at different temperature ranges. Ni Ti Shape memory alloys have a unique behavior of exhibiting maximum super elasticity as compared to other shape memory alloys. However there are certain elements such as Fe, Cu, Al, and Mo which are added to the NiTi as ternary additions to enhance the transformational behavior of these alloys. Alloying Fe is found to have major implications on the shape memory effect of Ni Ti (Nitinol) based shape memory alloys. The transformational behavior of these shape memory alloys are unique and found to have an impact on biomedical and structural applications. In our work, tribological characteristics of NiTiFe were effectively studied for varying compositions and it was found that hardness increases with the ternary addition of Fe from 3% to 9% and wear drastically reduces as a result of increase in hardness. This is attributed to the fact that the increase in addition of Fe causes precipitation hardening which will ultimately result in improved tribological characteristics. Also in our present work, the samples are subjected to heat treatment and subsequent aging, which has resulted in betterment of wear characteristics, that are compared with the as cast samples.

KeywordsShape Memory Alloy, Nitinol, Wear, Hardness, Tribological Characteristics.

1. INTRODUCTION

   Shape Memory Alloys (SMAs) are metallic alloys that undergo a solid-to-solid phase transformation which can exhibit large recoverable strains. Shape-memory alloys (SMAs) possess an array of desirable properties: high power to weight (or force to volume) ratio, thus the ability to recover large transformation stress and strain upon heating and cooling, pseudo elasticity (or super elasticity), high damping capacity, good chemical resistance and biocompatibility [1,2]

   Shape-memory alloys are functional materials with a variety of applications [3, 4]. Their mechanical properties and their

   microstructural changes at various strain rates and temperatures have been of considerable interest. The static superelastic properties of shape-memory alloys have been extensively studied [5-7]. In quasi-static loading conditions, the transition stress for stress-induced martensite formation increases with an increase in the strain rate. In addition, in the stress-induced martensite formation regime, the work- hardening rate increases with the increasing strain rate, due to the latent heat of transformation and the heat of deformation [8-10]. Their dynamic properties, however, have not been fully explored, especially their strain-rate sensitivity and their high strain-rate microstructural changes, due to the difficulty in controlling the strain rate [11].

   Since the last decade, TiNi shape memory alloy has attracted increasing interest from tribologists due to its high resistance to wear. The excellent performance of this alloy largely arises from its special deformation behaviour, the so-called pseudo elasticity, caused by a thermo elastic martensitic transformation [12-13]. The phase transformation involves a structural change from the parent phase 32(), to a martensitic phase (monoclinic) [14]. Prior to the martensitic transformation, another phase transformation may also occur, depending on the composition of the alloy and heat treatment. This transformation involves a structural change from the Body centred (B2) phase to a rhombohedral phase (R phase) [15-16].

   These two transformations are reversible and can be induced either by changing temperature or by applying stress [17]. The high wear resistance of TiNi alloy has been well demonstrated [18]. A number of researchers have investigated the wear behaviour of TiNi alloy in different wear conditions and compared it to conventional engineering materials such as steels, Ni-based and Co-based tribo-alloys [19-20]. It is observed that TiNi alloy performs better than these conventional wear-resistant materials.

   In addition, TiNi alloy also exhibits high resistance to corrosion [21] and this makes TiNi alloy attractive for application in wet or corrosive environments, such as cavitations and liquid impact [22].

   The contribution of pseudo elasticity to the wear resistance has been directly and indirectly confirmed. Shida and Sugimoto

   1. observed remarkable erosion resistance of TiNi alloy during a water-jet erosion test. They found that the optimal

      composition that corresponded to the minimum erosion was in the range from Ti55 wt. % Ni to Ti56.5 wt. % Ni, where the TiNi alloy behaves pseudo elastically. Liang et al. [24] observed that the specimens with pseudo elasticity had higher wear resistance than those with little pseudo elasticity.

      Attempts have been made to apply TiNi alloy as a tribo material with success in chemical plants and power stations as reported [25]. Since TiNi alloy is relatively expensive, considerable efforts have been made to develop TiNi coatings using various processes, such as sputtering, plasma spray, explosive welding and plasma ion plating [26].

2. METHODOLOGY

   1. Production of Nickel, Titanium and Iron chips by suitable machining process.

   2. Weighing of Nickel, Titanium and Iron for varying atomic percentage.

   3. Melting of the Weighed proportions of Nickel Titanium and Iron in Vacuum arc remelting furnace to obtain buttons.

   4. Suction drawing the buttons in Vacuum suction casting furnace to a diameter of 6 mm and length of 60 mm.

   5. Heat treatment of the samples for different aging temperatures (i.e. 300oC, 350oC and 400oC).

   6. Abrasive wear test of the samples as per ASTM standards on an alumina (60) grinding wheel from M/s carborundum.

   7. Hardness test of the samples was carried out as per ASTM Standards on Vickers hardness tester configured and calibrated.

   8. Thorough evaluation of the results was carried out and suitable conclusions were drawn.

3. COMPOSITION OF THE ALLOY

   The composition of the shape memory alloy is fixed by varying the percentage of iron in increments of 3% up to 9%. But however in this paper, the results for only 3% composition of Fe are discussed.

   Element	Ti	Ni	Fe	Total
   Alloy	At %	At %	At %	Total (%)
   NiTi	50	50	0	100
   NiTiFe	50	47	3	100
   NiTiFe	50	44	6	100
   NiTiFe	50	41	9	100

   Table 1: Proportion of different alloying elements

   To weigh out the materials, conversions from at (%) to wt (%) are done by using following relation below.

   Wt %A =

   Table 2 gives atomic weight of different elements chosen for the project.

   Element	Atomic Weight
   Ti	47.88
   Ni	58.69
   Fe	55.84

   Table 2 Atomic weight of constituent elements

   III ABRASIVE WEAR TEST

   Abrasive wear test for the samples of three different compositions was carried out for both the as-cast and heat- treated samples for different load conditions and track diameter using pin-on-disc apparatus fitted with an AA60 alumina (60) grinding wheel from M/s carborundum. The Speed, time and load for which wear test is done is tabulated as shown in table 3.

   Sample	Load (kg)	Speed (rpm)	Time in Seconds
   NiTiFe (As Cast)	1	300	600
   NiTiFe (HT 300oC)	1	300	600
   NiTiFe (HT 350oC)	1	300	600
   NiTiFe (HT 400oC)	1	300	600

   Table 3: Speed, Wear and Load Conditions

4. VICKERS HARDNESS TEST

   The Vickers hardness test method consists of indenting the test material with a diamond indenter, in the form of a right pyramid with a square base and an angle of 136º between opposite faces subjected to a load of 5 kg. This load is reasonably big and should give an acceptable hardness for a cast metal. The 3mm diameter castings could not be tested by Brinell. Further the high hardness also precludes the use of Brinell which is the suggested method for cast materials. The full load is normally applied for 10 to 15 seconds. The two diagonals of the square indentation left in the surface of the material after removal of the load are measured using a microscope and their average calculated. The area of the sloping surface of the indentation is calculated. The Vickers hardness is the quotient obtained by dividing the kg load by the area of indentation (in square mm). The Vickers Diamond Pyramid harness number is the applied load (kg) divided by the

   surface area of the indentation (mm2) Fig 1 shows the Vickers hardness tester used for finding the Vickers hardness number of the given specimens.

   Fig 1 Vickers Hardness Testing Machine

5. FORMULAE USED FOR DATA ANALYSIS

   1. Sliding distance(S) = DNT/1000 (m) Where D = Diameter of Disc (mm)

      N = Speed in rpm

      T = Time in minutes

   2. Sliding speed (V) = DNT/ (1000×60) (m/s)

   3. Specific Wear Rate (SWR) = V/ (L×S) (mm3/N-m)

      Where V =Volume loss (mm3) = LXA

      Where L= Loss in length (mm), A = Area of specimen (mm2)

      Where D = diameter of the specimen, L= Load (N), S=Sliding distance (m)

   4. Hardness (HV) = (2Fsin 136/2)/d2, HV = 1.854 F/d2

   Where: F= Load in kg

   d = Arithmetic mean of the two diagonals, d1 and d2 in mm HV = Vickers hardness in kg/mm2

6. RESULTS

hardness value varies from 407 to 509 as the aging temperature vary from 300oC to 400oC

It is observed from each of the hardness values that addition of Fe to NiTi matrix has brought about a sufficient amount of solid solution hardening and there is increase in hardness.

Hardness VHN

Hardness VHN

Table 4 Hardness value for different aging temperatures

NiTiFe(3%)

600

500

400

300

200

100

0

As-Cast 300°C 350°C 400°C

Aging Temperature

NiTiFe(3%)

600

500

400

300

200

100

0

As-Cast 300°C 350°C 400°C

Aging Temperature

Fig 2 Variation of hardness values for different aging conditions

B. ABRASIVE WEAR

It is clearly observed from the table below that the wear gradually reduces with increase in aging temperature that is as the hardness of a sample goes up; the wear is seen to come down for same load, speed and time.

1. HARDNESS

   The hardness result for different samples in both as-cast and heat-treated condition is shown as in the table below.

   From the table it can be observed that, the hardness values tend to go up with an increase in aging temperature. Aging Temperature ranges from 300oC to 400oC, the hardness is found to increase as aging temperature is increased. The

   able 5 Abrasive wear results for different aging conditions

   NiTiFe(3%)

   200

   180

   160

   140

   120

   100

   80

   60

   40

   20

   0

   NiTiFe(3%)

   200

   180

   160

   140

   120

   100

   80

   60

   40

   20

   0

   Heat Treatment Temperature

   Heat Treatment Temperature

   Fig 3 Abrasive wear for 1 kg load for different aging conditions

   Basically wear reduces with increase in aging temperature predominantly due to the precipitation hardening that occurs in the samples.

   VII MICROSTRUCTURE

   In order to distinctly characterize the grain distribution, define the grain boundary and effectively find the inter atomic spacing of the given alloy samples, Micro structural evaluation of NiTi, NiTiFe (3%) as cast, and heat treated NiTiFe (3%) samples are carried out.

   Fig 4 SEM Micrograph of as-cast NiTiFe (3%) sample

   From the micrograph shown above it can be clearly seen that the addition of small quantities of Iron in terms of 3 percent, will cause the frmation of nodular grain structure. The atomic diameter of Iron is less than the interstices between nickel and titanium atoms and the Iron goes into solid solution of Nickel and titanium. As Iron dissolves in the interstices, it distorts the original crystal lattice of nickel and titanium.

   Fig 5 SEM Micrograph of NiTiFe(3%) sample heat treated at 300oC

   As-Cast 300°C

   As-Cast 300°C

   350°C

   350°C

   400°C

   400°C

   Wear in Microns

   Wear in Microns

   At 300oC, there is distortion of crystal lattice; this distortion of crystal lattice interferes with the addition of Iron to the crystal lattice that is the addition of Iron helps in blocking the dislocation of the crystal lattices. In other words, they avoid the dislocation of the crystal lattices. Obviously adding more and more Iron to NiTi matrix (upto solubility of Iron) results in lesser distortion of the crystal lattices hence increases wear and influences negatively with another important property of NiTi called the hardness. However, solubility of more Iron results in martensitic supression and results in reduction of transformation temperature and increases the superelasticity and shape memory effect.

   Fig 6 SEM Micrograph of NiTiFe(3%) sample heat treated at 350oC

   From the micrograph as shown in Fig 6, it can be clearly seen that the microstructure of NiTiFe(3%) gives an incipient information about the initial coring that takes place as the aging temperature increases from 300 to 350 degree Celsius, also the coarse grains tend to undergo segregation and result in the formation of dendrites which results in subsequent increase in hardness and reduction in wear of the sample.

   Fig 7 SEM Micrograph of NiTiFe(3%) sample heat treated at 400oC

   The microstructure given above is indicative of the fact that aging causes grain refinement and causes the grains to look like dendrites which eventually results in the formation of tree structure of fine dendrites around the periphery of which one can visualize the deposition of Iron atoms. It can be clearly be seen that the microstructure of NiTiFe(3%) gives a complete knowledge of the grain refinement that tends to take place after the sample is heat treated at 400oC.

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