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Wear behavior of a TiNi alloy
WEAR ELSEVIER
Wear198 (1996) 236-241
Wear behavior of a TiNi alloy Y.N. Liang *, S.Z. Li, Y.B. Jin, W. Jin, S. Li Instituteof MetalResearch,AcademiaSinica,D,enyang 110015,People'sRepublicof China
Received8 September1995;accepted10March1996
Abstract The wear behaviorof a TiNi alloy after various heat treatmentshas been studied in three conditions:sliding wear, impact abrasion and sand-blasting erosion.The results show that for all three conditions,the TiNi in a pseudoplasticstate shows much better wear resistanceas comparedwith that in a pseudoelastiestate. The wear resistanceof a TiNi alloy is mainlydependenton the recoverablestrain limit, i.e. the sum of the pseudoelasticand pseudoplastiestrain limits. Keywords: TiNialloy:Pseudoelasticity;Pseudoplasticity;Strainlimit;Wear
1. Introduction Near-equiatomic TiNi alloys possess a unique shape memory effect and transformation pseudoelasticity. The pseudoelastic strain can reach to 7-20%, two orders of magnitude larger than ordinary elasticity [ 1,2]. Studies [ 3-8] also indicated that TiNi alloys exhibits good wear resistance in sliding wear, and water jet, slurry and cavitation erosions, where the resistance of TiNi alloys is comparable to that of Hasteltoy and S tellite alloys. Ball attributed the wear properties of TiNi simply to the work-hardening [3], while in most of the previous reports, the pseudoelasticity has been considered as the main factor responsible for the good wear resistance of TiNi [5,6], and thus, efforts to obtain a TiNi alloy in a pseudoelastie state at room temperature by adjusting the composition and heat treatments have been xaade [7]. A recent study showed that martensitic TiNi (in a pseudoplastic state) gave better cavitation-erosion resistance as compared with austenitie TiNi (in a pseudoelastie state) [9]. It therefore secm~ unreasonable to emphasize simply the role ofpseudoelasticity in wear behavior of TiNi alloys. The studies, as mentioned above, were in general carried out by selecting only one TiNi alloy sample as the test material and adopting only one tribologi.eal method, and the results cannot be generalized. In the present work, wear behavior of Ti-50.99 at% Ni alloy after various heat-treatments has been studied under three tribological conditions: (a) sliding wear, (b) impact abrasion, (c) sand-blasting erosion. The aim is to make a * Correspondingauthor. 0043.t64s/961515.00 © 1996ElsevierScienceS.A.Allrightsreserved P/I S0043-1648 (96) 06989-X
systematic assessment of the wear behavior of TiNi alloys and provide a better explanation to the wear re--istance.
2. Expedmental 2.1. Materials
Ti-50.99 at% Ni alloy plates were used as the test materials. These materials were divided into four groups from TiNi-1 to TiNI-4 and heat-treated to different conditions, as shown in Table 2, in order to obtain TiNi samples with or without pseudoelasticity and pseudoplasticity, respeetively, at room temperature. The soft 2024AI aluminum alloy and hard Ni-hard-4 iron were also used as comparative materials, whose compositions and microhardnesses are shown in Table 1.
2.2, Measurements of transformation temperatures and microhardness
The transformation temperatures of the TiNi alloys, namely the M~ (martensite start temperature on cooling), Mf (martensite finish temperature). A~ (reverse transformation- start temperature on heating) and Af (finish temperature), were measured on a Rigaku dilatometer. The test materials were taken from the four groups of TiNi samples and machined into specimens with the dimensions of ~5 × 0.45 ram. The microhardness values of the TiNi were measured,
Y)!. Lic,n.g: t al. I Wear If8 (1996) 236-241
237
Table I The composition and hardnessof the comparative materials Materials
Composition (wt.%)
MicrohardnessHv~ (kg ram- =)
2024AI
Cu 4,20
Mg 1.47
Mn 0,56
Zr 0,02
Si 0.40
Fe 027
Ni-hard-4
C 3.20
Si I A6
Mn 0.61
Cr 8.57
Ni 5.23
Fe lest
2.3. Bending test
AI rest
930
2
I
A simple bending test was carded out to determine qualitatively deformation capacities of the TiNi samples. As comparison, 2024AI and Hi-hard.4 were also tested. The bending device consisted of one upper bar and two base bars, as shown in Fig. l (a). The dimension of the platelike specimens was 25 ×6.5 × !.3 tara. The ratio of width/ thickness was 5:1 in order to keep the specimens in a near plane-strain state during the test. The specimen was placed horizontally on the two base bars with the length edges v~rti¢al to the axis of the bars, and then pressure was applied to the upper bar. When the specimen contacted completely with the upper-bar surface in the effective arc-length range, as shown in Fig. l(b), the bending strain ~:was:
a/2
8=~
123
(a)
3
4
5
40 P
(])
rmo
where a is the thickness of the specimen, r,~,, radius of the neutral plane. After unloading, the bending deformation recovered p ~ly [Fig. 1(c) ] and the remained deformation was the plastic strain ap:
a12 a[3 [[
ep . . . .
ro
a~
Ltr , + - 2)2 s i n -
,(
i
~-i-'
. . [2(r.+rb+a)JJ
(2)
where rm is the radius of neutral plane after unloading, ru and rb the radii of the upper and base bars, I the effective bending arc-length, and ,8 the bending angle after unloading. The elastic strain eo was: ~o=~-%
(3)
The bent specimens were then heated in hot water, and its pseudoplastic deformation recovered. The final remaining plastic strain ep' after heating could be determined by substitution of the final bending angle ,8' after heating in Eq. (2), and thus the transformation pseudoplastic strain er~ was obtained: £pp = Cp -- Ep~'
(4)
The sum of the elastic strain ~e and pseudoplastic strain %p was defined here as recoverable strain ~, of TiNi alloy: e,=e=+epp
(5)
Frem three to five replicate tests were done for each specimen. By changing the radius of the upper bar from 80 to
(e)~
.
Rg. I. Diagramof the bending device (a) and principlesof the bending test (b) and (c); a is the thickl~s of a specimen, rm the radius of the neutral plane on loading, r. and r bthe radii of the upper and base bats, Ithe e[fcclive bending ate.length, and ~,/3 and ~' the bending angles on loading, after unloading and after heating, (a) Diagram of the bending device: (!) base plate, (2) upper-bar, (3) bese-bar, (4) specimen, (5) welding seam; (b) on loading; (c) recovery of bending deformation
3.5 ~m, the total bending strain e could be adjusted from 1,6% to the strain limit ~, until the specimens were broken, so that the plastic strain limit e~, the elastic strain limit e~o, pseudoplastic strain limit et,t,oand the recoverable strain limit g,o could be determined qualitatively. The room temperature was about 26.5 °(2 during the tests.
2.4. Weartests Sliding wear test was performed on a reciprocating sliding tester, the slider was Si3Ni4 with a fiat top of 1.5 x0.8 ram. Sliding speed was 0.01 m s-~ and the load ranged from 4 to 12N. Impact abrasion was performed on a single pendulum scratch tester, which could simulate a single event of an abrasive element's scratching with impact across a surface
238
Y.N. Liang et aL / Wear 198 (1996) 236.-241
Table2 ThetransformationtemperaturesandhardnessofTiNt Materials
Heat treatments
Mr (°C)
TiNi-I TiNt-2 TiNt-3 TiNt-4
720 °C/40',W.Q. 720*c/an', A.C. 720°C140',W.Q.+400 *c/10', age 720 °C/40',W.Q.+ 500 °C/40',age
4
- 31
.,14,(*C)
Ar (°C)
A~ (*C)
.tJo~s (kg mm"t)
17 -8.5
- 85 - 60 21 33.5
- 26 - 17 44 53
350 415 463 396
Table3 Theresultsof the bendingtest Materials
Plasticstrainlimit,epo(%)
TiNi-I
TiNi-2 TiNt-3 TiNt..4 2024AI Ni-hard-4
Elasticstrainlimit,e~
Psettdopl~tics~in limit,er#
Rp.coverable strainlimit,e,o
3.6
5.3
0.0
5.3
2.3 2.1 9.9 ." 30 < 0.8
6.7 9.8 6.5 1.5 < 0.4
0.0 1.2 6.5 0.0 0.0
6.7 It.0 13.0 !.5 < 0.4
and allow measurement of the energy dissipated in a scratching event. The stylus, made of a hard alloy, was a taper with the top radius of 0.4 mm. The entrance speed was 2.1 m s- t. After testing, the TiNt specimens were heated in hot water to make the pseudoplastic deformation recover, and then the real volume of removal materials was ~stabli:'aed through the measureme::ts of groove dimensions. The details of the test are given in another report [ 10]. Erosion test was carried out on a sand-blasting erosion tester, The erosive was 150 #m corundum, the impingement speed about 30 m s- t and the impingement angles 200 and 90°. Gravimetry was used to determined erosion loss, which was then converted into volume 'oss.
in Table 2. It is seen tkiatAo an¢,A~of TiNi-3 vary around the room temperature (RT=26.5 °C), while its M~ and Mr are lower than RT, indicating that TiNi-3 could possess good transfo~ation pseudoelasticity and a certain pseudoplasticity. The M~and Mr of TiNi-4 are lower than RT, while its A~ and Ac are higher than RT, which means that TiNi-4 might have good pseudoplasticity. As for TiNi-I and TiNi-2, the A~ and Af arc much lower than RT, thus they have little pseudoplasticity or pseudoelastieity. The microhardness of TiNi-3 is highest among the TiNi specimens. The hardness of the TiNi alloys lies between that of 2024 AI and Ni-hard-4 iron (as shown in "Fable 1).
3.2. Results of the bending test 3. Results 3. L Transformation temperatures and micrahardness of TiNt The transformation temperatures and microhardnesses of the TiNt specimens under various heat treatments are shown ~.0, 25.0
0¢,0Ii~-t
/ -
The results of the bending test are shown in Table 3. TiNt3 exhibits the highest elastic strain limit among the TiNI specimens, indicating it has good pseudoelasticity. The TiNt4 alloy shows the highest pseudoplastie strain limit, confirming its good pseudoplasticity (as in Table 2), The recoverable strain limit of the TiNt specimens increases from TiNt-1 to TiNt-4. It is also seen that the elastic strain limits of all the TiNt specimens are much higher than those of 2024 AI and Ni-hard-4.
#
~20~0
3.3. Results of wear tests
E 15.0
lo.o 5.o.
Load. H
Fig. 2. The variation of cross-sectional area A of the tracks with toad in the sliding wear test.
The results of the three wear tests are shown in Fig. 2 and Table 4. For different tests, different measurements were adopted to evaluate the wear rate or wear resistance. In sliding wear, the volume loss of materials can be presented by the cross-sectional area of the wear tracks (A) because of the same length of the tracks. The variations of the A of the tested materials with increasing loads in the sliding wear test are shown in Fig. 2. It can be seen that when the load is less than 7 N, wear of TiNt specimens is very low,
239
Y.N. Liang et al. / Wear 198 (1996) 236--241
Table4 Theresultsof impactabrasionandsaM-blastingerosiontests Malerials
TiNi-I 'Ih ;i-2 TiNi-3 TiNi-4 2024A1 Ni-hard.4
Specificenergy(J ram-3)
47 52 86 204 14 86
even lower than that of Ni-hard-4. When the load is more than 9 N, the wear of TiNi, except for TiNi-4, increases and is much higher than that of Ni-hard-4, although still much lower than that of 2024 Al. By comparison among the TiNi specimens, it is seen that the TiNi-4 in pseudoplastic state shows a wear resistance much better than the other T!Ni specimens; the TiNi-3 in the pseudodastic state exhibits a wen., ,esistanee better than TiNi-! and TiNi-2 at low load, but not at higher load. In the impact abrasion, specific energy e, defined as energy consumptioL for unit volume loss, can be used as measure of impact abrasive resistance of a materials [ 103 1]. The higher the e, the better impact abrasive resistance. As shown in Table 4, the specific energy e of TiNi-4 is much higher than those of the other specimens. The, of TiNy 3 is higher than that of TiNi-! and TiNi-2, and approximates that of Ni-bard-4. The soft 2024 AI shows the lowest wear resistance, The results of the sand-blasting erosion (in Table 4) shows that at an impingem.-.ntangle of 20°, the erosion rate of TiNi4 is similar to that of TiNi-3 and lower than TiNi- 1 and TiNi2. The hard Ni-hard-4 presents the lowest erosion rate, indicating that the hardness is also a important factor in erosion at low impingement angles. At the high impingement angle of 90°, the erosion rate decreases from TiNi-1 to TiNi4 regularly. Mos) of TiNi specimens, excel.:, for TiNi-1, exhibit better erosion resismr'ce than Ni-hard-4. The erosion resistance of 2024 A1 is the lowest in the two cases.
4. Discussions The hardness of TiNi alloys is not very high, but their wear resistance is comparable to that of typical wear-resistma materials [ 3-8]. The present study shows that the TiNi sampi,~spossess quite satisfactory wear resistance in sliding w~ar, impact abrasion and sand-blasting erosion. Ball has suggested that the good wear resistance of TiNi alloys, as well as Stellite and 304-type stainless steel could related to their work-hardening characteristics.That explanation seems unreasonable because the pseudoplastic deformation of a TiNi is quite different from the plastic deformation of such conventional metal materials as Stellite or 304-type stainless steel in micro-mechanisms, which will
Erosionrate ( × 10-3 mm3g- ,) a.~ 20o
~90 ~
2.31 1.95 1.85 I.g5 5.06 1A0
1.64 1.49 t.43 1.2g 2.47 1.57
be discussed in detail below, and the tensile stress-strain curve~ generally remain steady during pseudoplastie deformation [ 2] due to hardness of the martensites of TiNi being lower than that of the austenites and no obvious increase of dislocations can be produced. Another point has been accepted by most of the previous studies that the good wear resistance of TiNi alloys may be attributed to the pseudoelastieity. It is argued that TiNi alloys in a pseudoelastic state have great elastic accommodation and contacts elastically with the mating surface during the wear process, resulting in small material loss [5], but the study of Richman etal. indicates that martensitie TiNi alloy (i.e. in the pseudoplastic state) is more resistant to cavitation--erosion as compared with austenitic TiNi (i.e. in pseudoelastic state) [9]. The results of the present work also show that the wear resistance of the TiNi-4 in pseudoplastic state is generally better than that of TiNi-3 in the pseudoelastic state under various wear canditions. In fact, both pseudoelasticity and pseudoplastieity of TiNi alloys are a consequence of the reversible transformations between high.temperature austenite parent-phase and thermoe!~tic martensite variants. When the/if is a little lower than working temperature, TiNi usually presents good pseudoelasticity; when the A~is a little higher than working temperature, TiNi usually possesses good pseudoplasticity, i.e. shape memory effect. On loading a TiNi alloy in a pseudoplastic state, the self-accommodating martensites reorient along the direction of stress. When most or all the martensites arrange in one orientation, the alloy apparently deforms, which is the pseudoplastic deformation; on being heated above the Af, because the martensites remain coherent with parent phase during transformation, they will re,:erse eomplc~-ly into original parent structures and the deformation disappears. Thus the microstructure of the alloy is not changed permanently during the pseudoplastic deformation, which is distinctly different from the o~'dinary plastic deformation with dislocation-slip mechanism. Vingsbo [ 12] has pointed in his discussion on wear mechanisms that the basic phenomena of wear is fracture on a micro-scale, sometimes called "tribe fracture", which is generally nucleated whole dislocation density increasesobviously. In wear of a TiNi alloy, when pseudoelastie deformation occurs at the friction contacts by transformation of
EN. Liang et al. I Wear 198 (1996) 236.-241
240
200pm Fig.3. Profilesof cross-sectionof a grooveofTiNi-4in the impactab~ion hefom(a) and ~er (b) heating. austenite to marcasite, it will recover immediately by the reverse transformation on unloading; when pseudoplastic deformation occurs, it will remain even after stress is removed, but permanent defects, such as dislocations and the "tribe fracture", cannot be created in the microstructure, because the deformation is only consequent on the reorientation of the self-accommodatingmartensites along the direction of the stress, and it can be confirmed by the fact that the deformation will recover by the transformation of the martensite to austenite if the specimen is heated above the At. The profiles of cross-section of a groove of TiNi-4 produced in the impact abrasion, before and after heating, are shown in Fig. 3(a) and (b). It can be seen that the groove deformation is obvious (Fig. 3(a)), but it is only pseudoplastic deformation and almost disappears when it is heated (Fig. 3(b) ), presenting little evidence of wear. It is obvious that both pseudoplasticity and pseudodasticity have equivalent effects on anti-wear of TiNi alloys. In other words, wear resistance of TiNi alloys may be dependent on the stun uf pseudoplasticity and pseudoelasticity, namely, the total recoverable deformation capability. Fig. 4 shows the comparison of relative wear resistance of TiNi specimens with hardness Hv, pseudoelastic strain limit e~ and recoverable strain limit e~o(the sum oftbe pseudoelastic and pseudoplastie strain lintits). The recoverable strain limit e~o of TiNi corresponds quite well with the wear resistance for the three wear conditions, while the hardness and the pseudoeiastic strain limit do not. Thus it is irrational to simply empha-
size the roll of pseudoetasticity in anti-wear of TiNi alloys. Generally, the recoverable strain limit of a TiNi alloy in a pseudoplastic state is always higher than that in a pseudoelastic state, because the transformation resistance of the thermal m~ensite decreases with increasing transformation temperatures in a certain range, which has been confirmed in the present work (Table 3). If we lower Af down to the working temperature to pursue pseudoelasticity, the recoverable strain limit, and therefore the wear resistance of TiNi alloys, will be reduced. It is because the wear resistance uf TiNi alloys is mainly determined by the recoverable strain limit that the wear behavior of TiNi is affected by the level of applied load (as shown in Fig. 2). At relatively low loads, the TiNi specimens mainly form contacts of recoverable deformation on the surface and exhibit good wear properties; when the load exceeds a critical value, the defurmatiun of the contacts exceeds the limit of the recoverable deformation and thus, causes obvious increase of dislocations or microfracture, resulting in obvious wear.
5,
Conclusions
1. TiNi alloys under various heat-treatments, despite not having high hardness, exhibit good wear resistance under sliding wear, impact abrasion and sand-blasting erosion. 2. In all the three wear conditions, the TiNi alloy sample in pseudoplastic state exhibits distinctly better wear resistance than the TiNi in pseudoelastic state, and these two TiNi samples show better wear resistance than those with little pseudoplasu,~ity or pseudoelastieity. 3. The wear resistance of a TiNi alloy is mainly dependent on the recoverable strain limit, i.e. the sum of the pseudoplastic and pseudoelastic swain limits.
1
23 Hv
1234 E~
9 Er0
12
4 I/A (l.oad~N)
123/4 1/Erosion rate
(z--9o.)
Fig. 4. Comparison of relative weberresistanceof TiNi specimenswith hardnessHv, pseudoelasficst~n I!rnit ecoand recoverable strain limit ~ro.The values in each group of piles have been normnqzed by that of TiNi.I, where A is the cross-sectionalarea of a track in sliding test, e is the specific energy in the abrasion test.
EN. Liang et al. / Wear 198 (1996)236-241
Acknowledgements This work was supported by the National Science Foundation of China (59375204) and Laboratory of Solid lubrication, Lanzhou Institute of Chemical Physics, Academia Sinica.
References [ I l F.~. Wang, W.J. Buehler and S.J. Piekart, Crystal structure and a unique "marteositic" transition of TiNi, J. Appl. Phys., 36 (1%5) 3232. [2] K. Olsuka and K. Shimizu, Pseuduelasticityand shape memnq~effects in alloys, Int. Metals Re~., 31 (3) (1986) 93. [31 A. Ball, On the impedance of work hardening in the design of wearresist~t materials, Wear, 91 (i983) 201.
241
[4] Y. Suzuki and T. Kuroyanagi, Development and application of inlermetalli¢compound FAEDIC-NT,Titanium~rcanium, 27 ( 1¢~79) 67. [5] Jin Jialiang and Wang Hongliang,Weatresistance of Ni-Ti alloy,Acta Met. Sin. (EnglishEdn), AI (1988) 76. [61 Y. Shida and Y. Sugimolo, Walerje~erosion behavior of Ti-Ni binary alloys, Wear, 146 (1991) 219. [7] P. Clayton, Tribological behavior of a titanium-nickel alloy, Wear, 162 (t993) 202. [8] J. Singh ~ A.T. Alpas, Dry sliding wear mechanisn~ in a Ti50Ni4~e3 intermetallic alloy, Wear, 181-183 (1995) 302, [9] R.H. Richman, A,S. Ran and D. Kung, Cavi~ion erosion of NiTi explosively welded to steel, Wear, 181-183 {1995) 80, [10] Y,N. Liang, S.Z. Li and S. Li, Evaluation of abradability of porous seal materials in a single pendulum scratch device. Wear, 177 (1994) 167. [11] O. Vingstm, and S. Hogmark, Single-pass pendulum grooving--a technique for abrasive testing, Wear, 100 (1984) 489. [12] O. Vingsbo, Wear and wear mechanisms, in K.C. Ludema, W.A. Glaeser and S.K. Rhee (eds), Wearof Mate~als, ASME, New York, 1979, p. b20.
Wear198 (1996) 236-241
Wear behavior of a TiNi alloy Y.N. Liang *, S.Z. Li, Y.B. Jin, W. Jin, S. Li Instituteof MetalResearch,AcademiaSinica,D,enyang 110015,People'sRepublicof China
Received8 September1995;accepted10March1996
Abstract The wear behaviorof a TiNi alloy after various heat treatmentshas been studied in three conditions:sliding wear, impact abrasion and sand-blasting erosion.The results show that for all three conditions,the TiNi in a pseudoplasticstate shows much better wear resistanceas comparedwith that in a pseudoelastiestate. The wear resistanceof a TiNi alloy is mainlydependenton the recoverablestrain limit, i.e. the sum of the pseudoelasticand pseudoplastiestrain limits. Keywords: TiNialloy:Pseudoelasticity;Pseudoplasticity;Strainlimit;Wear
1. Introduction Near-equiatomic TiNi alloys possess a unique shape memory effect and transformation pseudoelasticity. The pseudoelastic strain can reach to 7-20%, two orders of magnitude larger than ordinary elasticity [ 1,2]. Studies [ 3-8] also indicated that TiNi alloys exhibits good wear resistance in sliding wear, and water jet, slurry and cavitation erosions, where the resistance of TiNi alloys is comparable to that of Hasteltoy and S tellite alloys. Ball attributed the wear properties of TiNi simply to the work-hardening [3], while in most of the previous reports, the pseudoelasticity has been considered as the main factor responsible for the good wear resistance of TiNi [5,6], and thus, efforts to obtain a TiNi alloy in a pseudoelastie state at room temperature by adjusting the composition and heat treatments have been xaade [7]. A recent study showed that martensitic TiNi (in a pseudoplastic state) gave better cavitation-erosion resistance as compared with austenitie TiNi (in a pseudoelastie state) [9]. It therefore secm~ unreasonable to emphasize simply the role ofpseudoelasticity in wear behavior of TiNi alloys. The studies, as mentioned above, were in general carried out by selecting only one TiNi alloy sample as the test material and adopting only one tribologi.eal method, and the results cannot be generalized. In the present work, wear behavior of Ti-50.99 at% Ni alloy after various heat-treatments has been studied under three tribological conditions: (a) sliding wear, (b) impact abrasion, (c) sand-blasting erosion. The aim is to make a * Correspondingauthor. 0043.t64s/961515.00 © 1996ElsevierScienceS.A.Allrightsreserved P/I S0043-1648 (96) 06989-X
systematic assessment of the wear behavior of TiNi alloys and provide a better explanation to the wear re--istance.
2. Expedmental 2.1. Materials
Ti-50.99 at% Ni alloy plates were used as the test materials. These materials were divided into four groups from TiNi-1 to TiNI-4 and heat-treated to different conditions, as shown in Table 2, in order to obtain TiNi samples with or without pseudoelasticity and pseudoplasticity, respeetively, at room temperature. The soft 2024AI aluminum alloy and hard Ni-hard-4 iron were also used as comparative materials, whose compositions and microhardnesses are shown in Table 1.
2.2, Measurements of transformation temperatures and microhardness
The transformation temperatures of the TiNi alloys, namely the M~ (martensite start temperature on cooling), Mf (martensite finish temperature). A~ (reverse transformation- start temperature on heating) and Af (finish temperature), were measured on a Rigaku dilatometer. The test materials were taken from the four groups of TiNi samples and machined into specimens with the dimensions of ~5 × 0.45 ram. The microhardness values of the TiNi were measured,
Y)!. Lic,n.g: t al. I Wear If8 (1996) 236-241
237
Table I The composition and hardnessof the comparative materials Materials
Composition (wt.%)
MicrohardnessHv~ (kg ram- =)
2024AI
Cu 4,20
Mg 1.47
Mn 0,56
Zr 0,02
Si 0.40
Fe 027
Ni-hard-4
C 3.20
Si I A6
Mn 0.61
Cr 8.57
Ni 5.23
Fe lest
2.3. Bending test
AI rest
930
2
I
A simple bending test was carded out to determine qualitatively deformation capacities of the TiNi samples. As comparison, 2024AI and Hi-hard.4 were also tested. The bending device consisted of one upper bar and two base bars, as shown in Fig. l (a). The dimension of the platelike specimens was 25 ×6.5 × !.3 tara. The ratio of width/ thickness was 5:1 in order to keep the specimens in a near plane-strain state during the test. The specimen was placed horizontally on the two base bars with the length edges v~rti¢al to the axis of the bars, and then pressure was applied to the upper bar. When the specimen contacted completely with the upper-bar surface in the effective arc-length range, as shown in Fig. l(b), the bending strain ~:was:
a/2
8=~
123
(a)
3
4
5
40 P
(])
rmo
where a is the thickness of the specimen, r,~,, radius of the neutral plane. After unloading, the bending deformation recovered p ~ly [Fig. 1(c) ] and the remained deformation was the plastic strain ap:
a12 a[3 [[
ep . . . .
ro
a~
Ltr , + - 2)2 s i n -
,(
i
~-i-'
. . [2(r.+rb+a)JJ
(2)
where rm is the radius of neutral plane after unloading, ru and rb the radii of the upper and base bars, I the effective bending arc-length, and ,8 the bending angle after unloading. The elastic strain eo was: ~o=~-%
(3)
The bent specimens were then heated in hot water, and its pseudoplastic deformation recovered. The final remaining plastic strain ep' after heating could be determined by substitution of the final bending angle ,8' after heating in Eq. (2), and thus the transformation pseudoplastic strain er~ was obtained: £pp = Cp -- Ep~'
(4)
The sum of the elastic strain ~e and pseudoplastic strain %p was defined here as recoverable strain ~, of TiNi alloy: e,=e=+epp
(5)
Frem three to five replicate tests were done for each specimen. By changing the radius of the upper bar from 80 to
(e)~
.
Rg. I. Diagramof the bending device (a) and principlesof the bending test (b) and (c); a is the thickl~s of a specimen, rm the radius of the neutral plane on loading, r. and r bthe radii of the upper and base bats, Ithe e[fcclive bending ate.length, and ~,/3 and ~' the bending angles on loading, after unloading and after heating, (a) Diagram of the bending device: (!) base plate, (2) upper-bar, (3) bese-bar, (4) specimen, (5) welding seam; (b) on loading; (c) recovery of bending deformation
3.5 ~m, the total bending strain e could be adjusted from 1,6% to the strain limit ~, until the specimens were broken, so that the plastic strain limit e~, the elastic strain limit e~o, pseudoplastic strain limit et,t,oand the recoverable strain limit g,o could be determined qualitatively. The room temperature was about 26.5 °(2 during the tests.
2.4. Weartests Sliding wear test was performed on a reciprocating sliding tester, the slider was Si3Ni4 with a fiat top of 1.5 x0.8 ram. Sliding speed was 0.01 m s-~ and the load ranged from 4 to 12N. Impact abrasion was performed on a single pendulum scratch tester, which could simulate a single event of an abrasive element's scratching with impact across a surface
238
Y.N. Liang et aL / Wear 198 (1996) 236.-241
Table2 ThetransformationtemperaturesandhardnessofTiNt Materials
Heat treatments
Mr (°C)
TiNi-I TiNt-2 TiNt-3 TiNt-4
720 °C/40',W.Q. 720*c/an', A.C. 720°C140',W.Q.+400 *c/10', age 720 °C/40',W.Q.+ 500 °C/40',age
4
- 31
.,14,(*C)
Ar (°C)
A~ (*C)
.tJo~s (kg mm"t)
17 -8.5
- 85 - 60 21 33.5
- 26 - 17 44 53
350 415 463 396
Table3 Theresultsof the bendingtest Materials
Plasticstrainlimit,epo(%)
TiNi-I
TiNi-2 TiNt-3 TiNt..4 2024AI Ni-hard-4
Elasticstrainlimit,e~
Psettdopl~tics~in limit,er#
Rp.coverable strainlimit,e,o
3.6
5.3
0.0
5.3
2.3 2.1 9.9 ." 30 < 0.8
6.7 9.8 6.5 1.5 < 0.4
0.0 1.2 6.5 0.0 0.0
6.7 It.0 13.0 !.5 < 0.4
and allow measurement of the energy dissipated in a scratching event. The stylus, made of a hard alloy, was a taper with the top radius of 0.4 mm. The entrance speed was 2.1 m s- t. After testing, the TiNt specimens were heated in hot water to make the pseudoplastic deformation recover, and then the real volume of removal materials was ~stabli:'aed through the measureme::ts of groove dimensions. The details of the test are given in another report [ 10]. Erosion test was carried out on a sand-blasting erosion tester, The erosive was 150 #m corundum, the impingement speed about 30 m s- t and the impingement angles 200 and 90°. Gravimetry was used to determined erosion loss, which was then converted into volume 'oss.
in Table 2. It is seen tkiatAo an¢,A~of TiNi-3 vary around the room temperature (RT=26.5 °C), while its M~ and Mr are lower than RT, indicating that TiNi-3 could possess good transfo~ation pseudoelasticity and a certain pseudoplasticity. The M~and Mr of TiNi-4 are lower than RT, while its A~ and Ac are higher than RT, which means that TiNi-4 might have good pseudoplasticity. As for TiNi-I and TiNi-2, the A~ and Af arc much lower than RT, thus they have little pseudoplasticity or pseudoelastieity. The microhardness of TiNi-3 is highest among the TiNi specimens. The hardness of the TiNi alloys lies between that of 2024 AI and Ni-hard-4 iron (as shown in "Fable 1).
3.2. Results of the bending test 3. Results 3. L Transformation temperatures and micrahardness of TiNt The transformation temperatures and microhardnesses of the TiNt specimens under various heat treatments are shown ~.0, 25.0
0¢,0Ii~-t
/ -
The results of the bending test are shown in Table 3. TiNt3 exhibits the highest elastic strain limit among the TiNI specimens, indicating it has good pseudoelasticity. The TiNt4 alloy shows the highest pseudoplastie strain limit, confirming its good pseudoplasticity (as in Table 2), The recoverable strain limit of the TiNt specimens increases from TiNt-1 to TiNt-4. It is also seen that the elastic strain limits of all the TiNt specimens are much higher than those of 2024 AI and Ni-hard-4.
#
~20~0
3.3. Results of wear tests
E 15.0
lo.o 5.o.
Load. H
Fig. 2. The variation of cross-sectional area A of the tracks with toad in the sliding wear test.
The results of the three wear tests are shown in Fig. 2 and Table 4. For different tests, different measurements were adopted to evaluate the wear rate or wear resistance. In sliding wear, the volume loss of materials can be presented by the cross-sectional area of the wear tracks (A) because of the same length of the tracks. The variations of the A of the tested materials with increasing loads in the sliding wear test are shown in Fig. 2. It can be seen that when the load is less than 7 N, wear of TiNt specimens is very low,
239
Y.N. Liang et al. / Wear 198 (1996) 236--241
Table4 Theresultsof impactabrasionandsaM-blastingerosiontests Malerials
TiNi-I 'Ih ;i-2 TiNi-3 TiNi-4 2024A1 Ni-hard.4
Specificenergy(J ram-3)
47 52 86 204 14 86
even lower than that of Ni-hard-4. When the load is more than 9 N, the wear of TiNi, except for TiNi-4, increases and is much higher than that of Ni-hard-4, although still much lower than that of 2024 Al. By comparison among the TiNi specimens, it is seen that the TiNi-4 in pseudoplastic state shows a wear resistance much better than the other T!Ni specimens; the TiNi-3 in the pseudodastic state exhibits a wen., ,esistanee better than TiNi-! and TiNi-2 at low load, but not at higher load. In the impact abrasion, specific energy e, defined as energy consumptioL for unit volume loss, can be used as measure of impact abrasive resistance of a materials [ 103 1]. The higher the e, the better impact abrasive resistance. As shown in Table 4, the specific energy e of TiNi-4 is much higher than those of the other specimens. The, of TiNy 3 is higher than that of TiNi-! and TiNi-2, and approximates that of Ni-bard-4. The soft 2024 AI shows the lowest wear resistance, The results of the sand-blasting erosion (in Table 4) shows that at an impingem.-.ntangle of 20°, the erosion rate of TiNi4 is similar to that of TiNi-3 and lower than TiNi- 1 and TiNi2. The hard Ni-hard-4 presents the lowest erosion rate, indicating that the hardness is also a important factor in erosion at low impingement angles. At the high impingement angle of 90°, the erosion rate decreases from TiNi-1 to TiNi4 regularly. Mos) of TiNi specimens, excel.:, for TiNi-1, exhibit better erosion resismr'ce than Ni-hard-4. The erosion resistance of 2024 A1 is the lowest in the two cases.
4. Discussions The hardness of TiNi alloys is not very high, but their wear resistance is comparable to that of typical wear-resistma materials [ 3-8]. The present study shows that the TiNi sampi,~spossess quite satisfactory wear resistance in sliding w~ar, impact abrasion and sand-blasting erosion. Ball has suggested that the good wear resistance of TiNi alloys, as well as Stellite and 304-type stainless steel could related to their work-hardening characteristics.That explanation seems unreasonable because the pseudoplastic deformation of a TiNi is quite different from the plastic deformation of such conventional metal materials as Stellite or 304-type stainless steel in micro-mechanisms, which will
Erosionrate ( × 10-3 mm3g- ,) a.~ 20o
~90 ~
2.31 1.95 1.85 I.g5 5.06 1A0
1.64 1.49 t.43 1.2g 2.47 1.57
be discussed in detail below, and the tensile stress-strain curve~ generally remain steady during pseudoplastie deformation [ 2] due to hardness of the martensites of TiNi being lower than that of the austenites and no obvious increase of dislocations can be produced. Another point has been accepted by most of the previous studies that the good wear resistance of TiNi alloys may be attributed to the pseudoelastieity. It is argued that TiNi alloys in a pseudoelastic state have great elastic accommodation and contacts elastically with the mating surface during the wear process, resulting in small material loss [5], but the study of Richman etal. indicates that martensitie TiNi alloy (i.e. in the pseudoplastic state) is more resistant to cavitation--erosion as compared with austenitic TiNi (i.e. in pseudoelastic state) [9]. The results of the present work also show that the wear resistance of the TiNi-4 in pseudoplastic state is generally better than that of TiNi-3 in the pseudoelastic state under various wear canditions. In fact, both pseudoelasticity and pseudoplastieity of TiNi alloys are a consequence of the reversible transformations between high.temperature austenite parent-phase and thermoe!~tic martensite variants. When the/if is a little lower than working temperature, TiNi usually presents good pseudoelasticity; when the A~is a little higher than working temperature, TiNi usually possesses good pseudoplasticity, i.e. shape memory effect. On loading a TiNi alloy in a pseudoplastic state, the self-accommodating martensites reorient along the direction of stress. When most or all the martensites arrange in one orientation, the alloy apparently deforms, which is the pseudoplastic deformation; on being heated above the Af, because the martensites remain coherent with parent phase during transformation, they will re,:erse eomplc~-ly into original parent structures and the deformation disappears. Thus the microstructure of the alloy is not changed permanently during the pseudoplastic deformation, which is distinctly different from the o~'dinary plastic deformation with dislocation-slip mechanism. Vingsbo [ 12] has pointed in his discussion on wear mechanisms that the basic phenomena of wear is fracture on a micro-scale, sometimes called "tribe fracture", which is generally nucleated whole dislocation density increasesobviously. In wear of a TiNi alloy, when pseudoelastie deformation occurs at the friction contacts by transformation of
EN. Liang et al. I Wear 198 (1996) 236.-241
240
200pm Fig.3. Profilesof cross-sectionof a grooveofTiNi-4in the impactab~ion hefom(a) and ~er (b) heating. austenite to marcasite, it will recover immediately by the reverse transformation on unloading; when pseudoplastic deformation occurs, it will remain even after stress is removed, but permanent defects, such as dislocations and the "tribe fracture", cannot be created in the microstructure, because the deformation is only consequent on the reorientation of the self-accommodatingmartensites along the direction of the stress, and it can be confirmed by the fact that the deformation will recover by the transformation of the martensite to austenite if the specimen is heated above the At. The profiles of cross-section of a groove of TiNi-4 produced in the impact abrasion, before and after heating, are shown in Fig. 3(a) and (b). It can be seen that the groove deformation is obvious (Fig. 3(a)), but it is only pseudoplastic deformation and almost disappears when it is heated (Fig. 3(b) ), presenting little evidence of wear. It is obvious that both pseudoplasticity and pseudodasticity have equivalent effects on anti-wear of TiNi alloys. In other words, wear resistance of TiNi alloys may be dependent on the stun uf pseudoplasticity and pseudoelasticity, namely, the total recoverable deformation capability. Fig. 4 shows the comparison of relative wear resistance of TiNi specimens with hardness Hv, pseudoelastic strain limit e~ and recoverable strain limit e~o(the sum oftbe pseudoelastic and pseudoplastie strain lintits). The recoverable strain limit e~o of TiNi corresponds quite well with the wear resistance for the three wear conditions, while the hardness and the pseudoeiastic strain limit do not. Thus it is irrational to simply empha-
size the roll of pseudoetasticity in anti-wear of TiNi alloys. Generally, the recoverable strain limit of a TiNi alloy in a pseudoplastic state is always higher than that in a pseudoelastic state, because the transformation resistance of the thermal m~ensite decreases with increasing transformation temperatures in a certain range, which has been confirmed in the present work (Table 3). If we lower Af down to the working temperature to pursue pseudoelasticity, the recoverable strain limit, and therefore the wear resistance of TiNi alloys, will be reduced. It is because the wear resistance uf TiNi alloys is mainly determined by the recoverable strain limit that the wear behavior of TiNi is affected by the level of applied load (as shown in Fig. 2). At relatively low loads, the TiNi specimens mainly form contacts of recoverable deformation on the surface and exhibit good wear properties; when the load exceeds a critical value, the defurmatiun of the contacts exceeds the limit of the recoverable deformation and thus, causes obvious increase of dislocations or microfracture, resulting in obvious wear.
5,
Conclusions
1. TiNi alloys under various heat-treatments, despite not having high hardness, exhibit good wear resistance under sliding wear, impact abrasion and sand-blasting erosion. 2. In all the three wear conditions, the TiNi alloy sample in pseudoplastic state exhibits distinctly better wear resistance than the TiNi in pseudoelastic state, and these two TiNi samples show better wear resistance than those with little pseudoplasu,~ity or pseudoelastieity. 3. The wear resistance of a TiNi alloy is mainly dependent on the recoverable strain limit, i.e. the sum of the pseudoplastic and pseudoelastic swain limits.
1
23 Hv
1234 E~
9 Er0
12
4 I/A (l.oad~N)
123/4 1/Erosion rate
(z--9o.)
Fig. 4. Comparison of relative weberresistanceof TiNi specimenswith hardnessHv, pseudoelasficst~n I!rnit ecoand recoverable strain limit ~ro.The values in each group of piles have been normnqzed by that of TiNi.I, where A is the cross-sectionalarea of a track in sliding test, e is the specific energy in the abrasion test.
EN. Liang et al. / Wear 198 (1996)236-241
Acknowledgements This work was supported by the National Science Foundation of China (59375204) and Laboratory of Solid lubrication, Lanzhou Institute of Chemical Physics, Academia Sinica.
References [ I l F.~. Wang, W.J. Buehler and S.J. Piekart, Crystal structure and a unique "marteositic" transition of TiNi, J. Appl. Phys., 36 (1%5) 3232. [2] K. Olsuka and K. Shimizu, Pseuduelasticityand shape memnq~effects in alloys, Int. Metals Re~., 31 (3) (1986) 93. [31 A. Ball, On the impedance of work hardening in the design of wearresist~t materials, Wear, 91 (i983) 201.
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[4] Y. Suzuki and T. Kuroyanagi, Development and application of inlermetalli¢compound FAEDIC-NT,Titanium~rcanium, 27 ( 1¢~79) 67. [5] Jin Jialiang and Wang Hongliang,Weatresistance of Ni-Ti alloy,Acta Met. Sin. (EnglishEdn), AI (1988) 76. [61 Y. Shida and Y. Sugimolo, Walerje~erosion behavior of Ti-Ni binary alloys, Wear, 146 (1991) 219. [7] P. Clayton, Tribological behavior of a titanium-nickel alloy, Wear, 162 (t993) 202. [8] J. Singh ~ A.T. Alpas, Dry sliding wear mechanisn~ in a Ti50Ni4~e3 intermetallic alloy, Wear, 181-183 (1995) 302, [9] R.H. Richman, A,S. Ran and D. Kung, Cavi~ion erosion of NiTi explosively welded to steel, Wear, 181-183 {1995) 80, [10] Y,N. Liang, S.Z. Li and S. Li, Evaluation of abradability of porous seal materials in a single pendulum scratch device. Wear, 177 (1994) 167. [11] O. Vingstm, and S. Hogmark, Single-pass pendulum grooving--a technique for abrasive testing, Wear, 100 (1984) 489. [12] O. Vingsbo, Wear and wear mechanisms, in K.C. Ludema, W.A. Glaeser and S.K. Rhee (eds), Wearof Mate~als, ASME, New York, 1979, p. b20.