EFFECT OF AGING AND ALLOYING ON WEAR CHARACTERISTICS … · Alloy NiTi CuZnAl CuAlNi Property Melting point (co) 1300 950-1020 1000-1050 Density (g/cm3) 6.45 7.64 7.12 Electrical

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Int. J. Mech. Eng. & Rob. Res. 2014 Yellappa M et al., 2014

ISSN 2278 – 0149 www.ijmerr.comVol. 3, No. 2, April, 2014

© 2014 IJMERR. All Rights ReservedResearch Paper

EFFECT OF AGING AND ALLOYING ON WEARCHARACTERISTICS OF NITINOL BASED ALLOYS

Yellappa M1*, G V Krishnareddy2, Uday M1, Gowtham K1 and Puneet U1

*Corresponding Author: Yellappa M, [email protected]

NiTiNOL is an alloy of roughly 50% Nickel and 50% Titanium. NiTiNOL derives its name from itschemical components and its founders: Ni (Nickel) + Ti (Titanium) + NOL (Naval OrdinanceLab). It has good biocompatibility and good magnetic resonance imaging opacity, making itideally suited for design of biomedical implantable. In the present work, alloy samples withvarying percentage of ternary alloys are vacuum cast in vacuum arc melting furnace and thealloy samples are heat-treated at different low temperatures keeping the time (1hour) constantand tested for hardness wear behavior of the samples. The wear characteristics are predictedfor as cast samples as well as heat treated samples by carrying out Adhesive wear as well asAbrasive wear tests as per ASTM Standards. The hardness seems to increase with differentaging temperatures (oC) conditions. The Vickers hardness is low for the as-cast samples ofboth the compositions but gradually increases with increase in the heat-treatment temperaturesup to 350oC possibly due to formation of transition precipitates but there is a decrease in thehardness at 400oC possibly because of NiTi formation. Abrasive specific wear rates show atrend that is exactly the inverse of Vickers hardness. The wear rates tend to decrease from as-cast condition up to 350oC heat-treatment, and increase in the samples heat-treated at 400oC.The Vickers hardness of the samples is seen to go up gradually with an increase in the aluminumcontent, also the effect of heat –treatment will reduce the internal stresses and eliminate coring.The marginal hardness increase may be due to the Ni-Ti composition moving towardsstoichiometry by precipitation of excess Ni or Ti in the form of suitable precipitates. Abrasivewear of the specimens is seen to increase both with an increase with the cooper content andthe heat-treatment temperature, for different reasons. The specific wear rate for the samples isseen to go up accordingly with the abrasive wear since the ductility of the samples may belowered considerably with an increased addition of aluminum to the NiTi matrix.

Keywords: Adhesive Wear, Abrasive Wear, Vickers Hardness, Heat-treatment

1 Assistant Professor, Mechanical Department, SJBIT, Bangalore-60, India2 Lecturer, Mechanical Department, Govt Polytechnic, Bangalore, India

INTRODUCTION

Nickel titanium, also known as NiTiNol, is ametal alloy of nickel and titanium, where thetwo elements are present in roughly equalatomic percentages.

NiTiNol alloys exhibit two closely relatedand unique properties: shape memory andsuper elasticity (also called pseudo elastic-ity). Shape memory refers to the ability ofNiTiNol to undergo deformation at one tem-

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Alloy NiTi CuZnAl CuAlNi

Property

Melting point (co) 1300 950-1020 1000-1050

Density (g/cm3) 6.45 7.64 7.12

Electrical Resistivity (µ Ω

cm) 70-100 8.5-9.7 11.0-13.0

Thermal Conductivity, RT

(W/m K) 18 120 30-43

Young's Modulus (GPa)

83

(austenite)

72 (beta

phase)

85 (beta

phase)

26-48

(marten

site)

70 (marten

site)

80 (marten

site)

Yield Strength (MPa)

195-690

(austenite)

350 (beta

phase)

400 (beta

phase)

70-140

(marten

site)

80 (marten

site)

130

(marten

site)

Ultimate tensile strength

(MPa) 895 600 500-800

Shape memory strain (%) 8.5 4 4

Transformation range (o

C) <110 <120 <200

Transformation hysteresis

(o C) 30 – 50 15 – 25 15 – 20

Table 1: Shape Memory Alloys Commonly Usedperature, and then recover its original,undeformed shape upon heating above its“transformation temperature”. Super elastic-ity occurs at a narrow temperature range justabove its transformation temperature; in thiscase, no heating is necessary to cause theunreformed shape to recover, and the mate-rial exhibits enormous elasticity, some 10-30times that of ordinary metal.

NiTiNol is an alloy of roughly 50% Nickeland 50% Titanium. NiTiNol derives its namefrom its chemical components and itsfounders: Ni (Nickel) + Ti (Titanium) + NOL(Naval Ordinance Lab).It has goodbiocompatibility and good magnetic reso-nance imaging opacity, making it ideallysuited for design of biomedical implantabledevices. More importantly are two character-istics that make NiTiNol extra ordinarilyunique from other bioengineering metallicalloys, namely its super elasticity and shape-memory, which are described in further.These two unique properties are attributedto a first-order phase transformation from theparent austenite phase to the daughter mar-ten site phase which can occur via the addi-tion of an energy source either in the form ofheat or stress. Nitinol has been used for avariety of applications: pipe couplings, braunderwires, earthquake dampeners, eye-glass frames, orthodontic wires, mobilephone antennas, micro-actuators, and a va-riety of biomedical devices. Recently, how-ever, the NiTiNol community's trend has beento turn its efforts to the biomedical device field.

SHAPE MEMORY ALLOYS

(SMAS)

Smart materials are a class of materials thatexhibits a coupling between multiple physicaldomains. Common examples of smartmaterials include those that convert electricalsignals into mechanical deformation and vice

versa. Smart materials incorporated into asystem are called a smart system that utilizesthe coupling properties of smart materials toprovide functionality.

Over the last ten to twenty years, a numberof materials have been given the term smartbased on their interesting materialsproperties. Some of these materials exhibita volume change when subjected to anexternal stimulus such as an electricpotential, some shrink or expand whenheated or cooled whereas are materials thatproduce electrical signals when bent orstretched.

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Based on their properties smart materialsare classified into the following categories:-

1. Piezoelectric materials - These are aclass of materials that convert energybetween mechanical and electricaldomain.

2. Shape memory alloys - These refer tosecond class of smart material that isthermo-mechanical materials whichdeform when heated a cooled.

3. Electro active polymers- This class ofmaterials exhibits electromechanicalcoupling. These are functionally similarto piezoelectric materials but exhibitmuch different electro mechanicalresponse characteristics Final Stage

If the SMA encounters any resistanceduring this transformation, it cans extremelylarge forces. This phenomenon provides aunique mechanism for remote actuation.

The below table 1.1 shows shape memoryalloys commonly used.

It is seen from the table 1.1 that are threeclasses of shape memory alloys, wherein NiTihas got the lowest density that gives it anadded advantage in terms of lightweight andpossesses a high value of shape memorystrain of about 8%.

The major applications for these alloys arein the field of medicine and orthodontics witha few other areas of significance such aseyeglass frames, cellular phones antennas,and automotive devices.

Some of the applications are given below:

1. Aircraft2. Micro-gripper3. Eye glass frames4. Industrial robot5. Endovascular stent6. Endodontic

NiTiNol alloys exhibits two closely relatedand unique properties: shape memory andsuper elasticity (also called pseudo elasticity).Shape memory refers to the ability of NiTiNolto undergo deformation at one temperature,and then recover its original, undeformedshape upon heating above its transformationtemperature. Superelasticity occurs at anarrow temperature range just above Attemperature; in this case, no heating isnecessary to cause the undeformed shapeto recover, and the material exhibitsenormous elasticity, some 10-30 times thatof an ordinary metal. Nitinol's extraordinaryability to accommodate large strains, coupledwith its Physiological and chemicalcompatibility with the human body has madeit one of the most commonly used materialsin medical device engineering and design.

EXPERIMENTAL DETAILS

Alloy Composition

Choosing the composition of the alloy withvarying Aluminium contents.

Ni Ti Al Vd Total

Alloy Wt(gms) Wt(gms) Wt(gms) Wt(gms Wt(gms)

NiTi 55.076 44.932 0 0 100

NiTiAl,Vd 46.16 49.78 2.09 1.97 100

Table 2: Selected Alloy Composition(atomic %) for Experimental Work

To weigh out the materials conversion fromat (%) to wt (%) is done by using followingrelation.

atom% A x at wt AWt%A = –––––––––––––––––––––––––––––––––––––––––––––

(Atom % A x at wt A) + (Atom % B x at wt B) + (Atom)

Table 3: Atomic Mass of the RawMaterials (Ti, Ni, Al and Vd)

NiTiAlVd

58.6947.8826.9850.94

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Table 4: Alloy Composition wt (%)Selected for Experimental Work

Ni Ti Al Vd

Alloy Wt (%) Wt (%) Wt (%)

Wt (%)

NiTi 55.076 44.932 0 0

NiTiAlVd 46.16 49.78 2.09 1.97

MELTING AND CASTING OF

ALLOY

Melting of the alloy is carried out in theVacuum arc-melting furnace. The pure metalrods (Ni, Ti, Al and VD) were cut into verysmall bits, weighed using an electronicbalance of 0.001 grams accuracy and thencharged into furnace for button meltingoperation. Vacuum is created using thestandard procedure up to 10-5 mbar andARGON gas is purged into chamber. The gasis flushed out by the rough pumping thesystem and the procedure for vacuumcreation is carried out to achieve an ultimatevacuum of 10-5 mbar and then followed bythe backfilling of ARGON gas. This is donein order to remove any impurities present inthe chamber. The melting machine is put ONand melting of the four ingots is carried outto form buttons. The melting and remeltingof the buttons is done a few times in order toachieve homogeneity of the alloycompositions. After achieving thehomogeneity, the set-up is changed for thesuction (rod drawing) operation and a buttonis charged into the copper crucible for melting.Vacuum is created using the same standardprocedure up to 10-5 mbar; argon gas ispurged followed by flushing it out and re-creation of vacuum with the same procedurefollowed by again by back filling of Argon gas.The suction and drawing of the rod is carriedout after string and arc to render the alloy tomolten form. An alloy rod of 3mm diameterand 60mm length forms by suction and quick

cooling of the molten metal into crucible. Fourrods were melted and cast in the meltingfurnace for each of the two composionsNiTi,NiTiAl, VD.

Table 5: Final Composition Selectedfor Experimental Work

AGING OF THE ALLOYS

The alloy samples are heat-treated atdifferent low temperatures keeping the time(1hour) constant. This is done in order tomanipulate the transformation temperatures(At) of the alloys to achieve both super elasticand shape memory properties needed formedical applications such as orthodonticwires stints etc and medical appliances.

The table below shows the temperaturesat which samples are aged.

Table 6: Heat Treatment Temperaturesfor the Alloy Samples

Aging treatment is given for the alloysamples in an INDFURR LTD furnace withARGON atmosphere since titanium is highlypruned to oxidation when heated to hightemperatures (>600.c). The alloy rods arecharged into the furnace in groups for eachheat treatment. The samples are heated at aconstant rate of 10. Per minute to the requiredtemperature and allowed to remain for thespecified period of one hour and then air-cooled to room temperature. The influences

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of aging process on the properties of thealloys are studied by performing test suchas microstructure, wear etc.

OPTICAL MICROSCOPY

Samples of small size approximately 5mmthickness are used for micro structuralexamination under optical microscopy.Samples were mounted using cold mountingacrylic resin and allowed to set. The surfacepreparation was done by successivelygrinding on 1/0, 2/0, 3/0 and4/0 emery papersfor getting a flat surface. Further groundsamples was polished on velvet cloth usingalumina suspension and final polish wasgiven using diamond paste on special velvetcloth meant for diamond polish to obtain amirror finish.

Etching was carried out on the sampleswith the following Keller's enchant.

2ml HF +10ml HN03 +20ml H20

The surface of the specimen swabbed fora few seconds until shinning surface turns todull finish. The specimen is immediatelywashed with water, then surface treated witha drop or two ethanol and then dried. Thesample is placed on specimen support of theoptical microscope (Dew inters) andstructures studied at suitable magnificationsand images grabbed.The same procedure isrepeated for the other samples to view andcapture the microstructure.

RESULTS AND DISCUSSION

Optical Microscophy

The optical micrographs of NITONOL andNiTiAlV in the as-cast and heat-treatedconditions are shown below.

Figure 1: 1NiTi heat treated (300°C)

Figure 2: NiTi heat treated (350°C)

Figure 3: NiTi heat treated (400°C)

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Figure 4: NiTiAlV as-cast Condition

Figure 5: NiTiAlV Heat Treated (300°C)



Vickers Hardness

The variation of Vickers hardness for eachof the as-cast and heat-treated conditions istabulated below keeping the load constantas 5 kg on Vickers hardness machine.

Table 7: Vickers Hardness Variationfor the Samples

The variation of Vickers hardness number forboth the as-cast and the heat-treatedcondition of all the different compositions isshown graphically below. The hardnessseems to increase with different agingtemperatures (°C) conditions.

Figure 8: Vickers Hardness Number forNiTi for all Aging Conditions

Figure 9: Vickers Hardness Number forNiTiAlV for all Aging Conditions

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It is seen from the above graphs that thehardness of NiTiNol increases with theincrease in aluminum content. The hardnessof a NiTiNol does not change very much atthe aging temperatures and times used here.The process of aging in three sets of samplesseems to be similar.

WEAR

Adhesive Wear

Adhesive wear was carried out on the entiresample as per ASTM_99-95a. The three mmdiameter specimens were tested at 1 and 2kg loads. The wear in them were 1 to 5microns only. With the list count in testing unitbeing a micron meaningful conclusion aredifficult to draw. The results are to beanticipated as the NiTi's are hard and workhardened very rapidly. The wear will afterevery test reveals a golden colored smearindicating formation of TiO2 coat fromoxidation of Ti from the samples as shown inthe figure below.

Figure 10: Aging Conditions

Hence it was thought better to get relativeevaluation of the alloys by abrasive wear test.

Further Nitinol's are increasing used asendodontic tools for root canal treatmentwhere they are to meet mild to strongabrasive environment abrasives wear testinglooks relevant. Abrasive wear was carried outaccording to ASTM-132.

Abrasive Wear

Abrasive wear for the two compositions iscarried out for both the as-cast and heat-treated samples tested for two loads usingpin-on-disc apparatus fitted with an AA60alumina (60µ) grinding wheel wheel from M/s carborundum.

The results for the three NiTi compositionsis tabulated below.

Table 8: Wear Results for NiTi CompositionsUnder Different Loading Conditions

Table 9: Wear Results for NiTiAlV CompositionsUnder Different Loading Conditions

Figure 11: Wear Graph NiTi (0.5kg)Obtained from Experiment

Load(kg) Speed(rpm) Time(sec) Wear(microns)

0.5 200 590 90 NiTi-as-cast 1 200 585 158

0.5 200 585 81 NiTi-HT 300 1 200 587 159

0.5 200 595 40 NiTi-HT 350 1 200 590 165

0.5 200 545 84 NiTi-HT 400 1 200 592 162

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Figure 12: Wear Graph NiTi (1kg)Obtained from Experiment

Figure 13: Wear Graph NiTiAIV (0.5kg)Obtained from Experiment

Figure 14: Wear Graph NiTiAIV (1kg)Obtained from Experiment

The above graph shows the wear of thetwo samples for 0.5 kg and 1 kg loadapplication. It is observed that all twomaterials show similar wear behavior.

The wear results are graphicallyrepresented here under

Figure 15: Wear of as-cast and HeatTreatment Specimens of NiTiAlV for

Both Loading Conditions

Effect of heat treatment on Specific WearRate for all composions

The effect of heat treatment on specificwear rates for two compositions is tabulated

Table 10: Specific Wear Rate for NiTiCompositions

Load(kg) Speed(rpm) Time(sec)

Specific Wear Rate( )

0.5 200 590 1.75 NiTi-as-cast 1 200 585 1.04

0.5 200 585 1.58 NiTi-

HT

300⁰C 1 200 587 1.53

0.5 200 595 1.85 NiTi-

HT

350⁰C 1 200 590 1.60

0.5 200 545 1.76 NiTi-

HT

400⁰C 1 200 592 1.56

Fig (a)

Fig (b)

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The results for specific wear rates areillustrated graphically below for bothcompositions and heat treated conditions

Figure 16: Specific Wear Rate of NiTiCompostion

Figure 17: Specific Wear Rate ofNiTiAIV Composition

CONCLUSION

The micro-structures of the specimens showfine dendrites of NiTi in the as-cast conditionbut grains start growing bigger and tendingto homogenize as the aging temperature isincreased.

The Vickers hardness is low for the as-cast samples of both the compositions butgradually increases with increase in the heat-treatment temperatures up to 350oC possiblydue to formation of transition precipitates butthere is a decrease in the hardness at 400oCpossibly because of equilibrium precipitatesNiTi formation.

Abrasive specific wear rates show a trendthat is exactly the inverse of Vickershardness. The wear rates tend to decreasefrom as-cast condition up to 350oC heat-treatment, and increase in the samples heat-treated at 400oC. Again possibly due to theformation of equilibrium precipitates.

Load(kg) Speed(rpm) Time(sec)

Specific Wear Rate(

)

0.5 200 590 5.55 NiTiAlV

-as-cast 1 200 585 4.87

0.5 200 585 5.53 NiTiAlV-HT

300⁰C 1 200 587 3.62

0.5 200 595 5.05 NiTiAlV

-HT

350⁰C 1 200 590 2.54

0.5 200 545 4.08 NiTiAlV

-HT

400⁰C 1 200 592 2.06

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The material even when heat -treated forall the three compositions has shown verymarginal change i.e. increase in the attemperature since the heat treatmenttemperatures and times appear to be ratherlow.

The as-cast samples for all the threecompositions are observed to have very highinternal stresses in them.

The microstructures clearly show a reliefof the internal stresses in the alloy samplesas it is seen that the grains in the highestheat-treated condition are clearly seencompared to the as-cast ones. In the as-castsamples, the structures are seen to be in theform of dendrites that tend to change topolyhedral grains with an increase in the heat-treatment temperature to 4000 C for one hourholding.

The Vickers hardness of the samples isseen to go up gradually with an increase inthe aluminum contaent.also the effect of heat-treatment will reduce the internal stressesand eliminate coring. the marginal hardnessincrease may be due to the Ni-Ti compositionmoving towards stoichiometry by precipitationof excess Ni or Ti in the form of suitableprecipitates.

Abrasive wear of the specimens is seento increase both with an increase with thecooper content and the heat-treatmenttemperature, for different reasons.

The specific wear rate for the samples isseen to go up accordingly with the abrasivewear since the ductility of the samples maybe lowered considerably with an increasedaddition of aluminum to the NiTi matrix.

SCOPE FOR FUTURE WORK

The NiTi alloys contain higher aluminumconcentrations need to be investigated asconflicting reports exists about the properties

of NiTi alloys of higher aluminumconcentration.

Effects of addition of other element s suchas chromium, niobium etc to the NiTi matrixon the transformation temperature andmechanical properties can be studied.

The aging process can be investigated byvarying the aging time with the agingtemperature.

Influence of amounts of Ni3Ti and Ti2Nion the aging behavior and properties of theally need to be elucidated.

Wear test can be performed by varyingother parameters such as sliding speed andalso wear track diameter to study the effectson wear rate.

Micro-hardness of the alloy samples canbe carried out to study the hardness atdifferent locations at micron level.

REFERENCES

1. J A Shaw, C B Churchill, and M A Iadicola,— Tips and tricks for characterizing shapeM memory alloy wire, ExperimentalCharacterization of Active MaterialsSeries.

2. M Arciniegas, a, J Casalsa, J M Maneroa,J Penaa and F J Gila, (2008), “Study ofhardness and wear behavior of NiTishape memory alloys”, Conference Toolsfor 2011 TMS Annual Journal of Alloysand Compounds, Vol. 460, No. 1-2, 28July, pp. 213-219.

3. “Some studies on phase transformationand mechanical behavior if Ti-Ni Shapememory alloys” (2006), <NarendranathS, P.H.D. thesis,submitted, IIT Kharagpur.

5. Zheng, Y, et al., (2008), Effect of AgingTreatment on the Transformation, Ti-50.9at. % Ni alloy, Acta. Mater. 58, pp. 736 -745.

Documents

EFFECT OF AGING AND ALLOYING ON WEAR CHARACTERISTICS … · Alloy NiTi CuZnAl CuAlNi Property Melting point (co) 1300 950-1020 1000-1050 Density (g/cm3) 6.45 7.64 7.12 Electrical