The Ferromagnetic Heusler alloy Ni$AnGa is known to undergo a structural phase transformation of marten&c type. Thermoelastic nature, shape memory effect ...
Scripta h4etdlurgica et Materialia, Vol. 33. No. 8. lq 1UP1244,199S ElswierScicnccLtd Copyi&, 1995 ActaMetall~ca Inc. FhtdUltlXUSAAUli&tS09%716x/95 $9.50 + .oo
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TH:E DEVELOPMENT OF NEW FERROMAGNETIC SHAPE MEMORY ALLOYS IN Ni-Mn-Ga SYSTEM V. A. Chernenko, E. Cesari*, V. V. Kokorin, I. N. Vitenko Institute of Metal Physics, Vemadsky str. 36, Kiev, 252680, Ukraine *Departament de Fisica, Universitat de les Illes Balears, Crta. de Valldemossa km 7.5, E-07071 Pahna de Mallorca, Spain (Received March 22,1995) IDtroduction
The Ferromagnetic Heusler alloy Ni$AnGa is known to undergo a structural phase transformation of marten&c type. Thermoelastic nature, shape memory effect @ME) and superelasticity were found in (1) to be intrinsic to this transformation. Hence the martensitic transformation (MT) in NiwGa appeared to be a subject of further intensive investigations last few years. It tumcd out that the Ni&lnGa alloy was an affluent object to search ftx new phenomena such as magnetoelastic behavior in the vicinity of MT (2) and martensite lattice softening while approaching the intermartensitic transformation temperature during heating (3). In fact all the data published before were obtained using two Ni&lnGa samples of near stoichiometric composition. Despite the small composition difference the two mentioned Ni&lnGa alloys (alloys 3 and 10 in Table 1) studid e.g. in (Z!,3), possess entirely Werent martensitic temperatures, Iv&.So, the high sensitivity of M. to the deviation ILi-om the stoichiometric composition can be drawn as an important conclusion. In this work we present the results of the investigation of the following problems: how M,, the thermal hysteresis, Curie temperature, transformation heat are a&&d by the composition variation in the Ni-Mn-Ga alloy system in a concentration interval for each component of about 10 at. %. This work was performed to make sure that lhe new family of Ni-Mn-Ga based shape memory alloys @MA) with a wide variety of structural and magnetic properties is actually elaborated. ExDerimental Procedure
Polyqstalline ingots of 120 g weight ofNi-Mn-Ga alloys were prepared by repeated induction melting under argon atmosphere and cast into a copper mold Twenty-three different compositions were studied and they are given according to X-ray spectral analysis in Table 1. Uncertainty in determining the concentration of each element was about f 0.3 at_%. Single crystals of alloys 3,6,10,12,18 were grown from the initial ingots by Bridgtnan methal All the specimens for low field magnetic susceptibility tests, X-ray *actometry, electron microscopy, DSC measurcments,compression and bending tests were cut from the ingots by spark-machining and in some cases by low speed diamond saw. Marten&tic tran&ormation temperatures were measured by using low field magnetic susceptibility measurement technique (induction method (1) ) and diff&rential scanning calorimetry (DSC) . The temperature range for magnetic susceptibility tests, x (T), was f?om 4.2 K to 480 K The DSC measurementsunder controlled coolin&eating rate of 1OWminwere performed in Perkin Elmer DSC-4 and DSC-7 thermal analyzers in the temperature range 100 K-480 K and 290 K-870 K, 1239
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TABLE 1 Composition (in at. %), Martensitic Transformation Temperature, K, Thermal Hysteresis, AT, Transformation Q, Curie Temperature, T,, and Lattice Parameter, a, for Ni-Mn-Ga Alloys (SC: Single Crystal)
Heat,
Q. J/g
‘L,K
20
1.6
376
183
20
1.6
376
26.0
175
15
1.6
366
0.584
24.9
24.0
196
20
1.5
378
0.583
50.9
23.4
25.7
113
30
387
0.580
6 (SC)
49.2
26.6
24.2
173
16
1.3
368
7
52.6
24.4
23.0
258
30
3.3
373
8
52.0
24.4
23.6
298
15
4.2
360
0.576
9
49.4
27.7
22.9
283
18
3.4
383
0.584
10 (SC)
51.5
23.6
24.9
278
10
4.2
368
0.582
11
51.2
24.4
24.4
280
25
4.1
358
12 (SC)
52.6
23.5
23.9
283
10
4.2
363
13
54.3
20.5
25.2
276
10
4.1
341
14
51.7
22.2
26.2
273
20
4.0
383
T,, K
(AT)*, K
Ni
Mn
Ga
1
47.7
30.5
2
45.6
3 (SC)
HOY
W2,K
AT,K
21.8
238
32.0
22.4
49.7
24.3
4
51.1
5
a, nm
0.578
15
53.1
26.6
20.3
366
7
7.1
373
0.591
16
53.8
23.7
22.5
382
13
6.3
368
0.579
17
45.7
37.2
17.1
390
15
8.5
353
0.597
18 (SC)
51.2
31.1
17.7
446
8
11.0
356
0.590
19
59.0
19.4
21.6
465
40
8.3
346
0.589
20
58.3
15.9
25.8
494
27
7.8
333
0.587
21
58.4
25.3
16.3
626
9.5
308
0.586
22
47.6
25.7
26.7
4.2
-
-
380
0.585
21.9
28.5
4.2
-
-
356
0.584
53
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mpectively. The transformation heats
were directly calculated from the areas under DSC peaks by means of equipment software package. X-ray diffractometer DRON-3M was used for measuring lattice spacings at ambient temperature within an error about 0.001 nm. In order to check shape memory effect (SME) in polycrystahine and single crystal samples bend and compression tests were used, respectively. Specimens of 16x2x0.4 mm3were bent elasticallyby applying appmpm& load according to the three-point contacts scheme (1). cooled down and unloaded at 77 K. During heating recovery strain of tensed layer of specimen was obtained. The compressions of the cylindrical (0 2.5x4 mm) single crystal specimens with appropriate orientation were perhormed using an elaborated device at 77 K. Measurements of their elongation during heating after decompression were carried out by means of a Perkin Ehner TMA-7 dilatometer. Results and Discussion
The concentration intervals of each element in the used Ni-Mn-Ga alloys (Table 1) were chosen as to avoid any considerable decomposition. The results below seem to support the idea about single bee parent and/or martensitic phase state of these alloys at room temperature. (Only in some cases, X-ray diffraction pattern shov&dweak additionalreflections which may be attributedto the presence of a small amount of a precipitated phase). As the X-ray scattering amplitudes of Ni and Mn atoms differ only slightly, it was no possible to get information about the degree of chemical order by X-ray method. On the other hand, electron m-action patterns of(1OO)orientedfoilsofthesinglecrystalsofalloys3,1O, 18clearlycon6rmedL2,orderingofthese alloys. Concem& the homogeneity, preliminary measurements on several samples cut from top and bottom of the ingots indi&ed M, ditZrences lesser than 10 K. The results presented here are taken from samples cut from the top of the ingots. In Table 1 the composition of the studied alloys is listed, together with some characteristics of MT, Curie temperature and lattice parameter. Note that the transformation heats correspond to an average of the absolute values measured in the forward and reverse transformation for several runs. Due to the experimental limitations the calorimetric run for alloy 5 was not performed. Only two among the presented alloys do not transform martensitically during cooling down to 4.2 K.
50
60
Ni, at.% -W
Figurz 1.P&tions of kwtigated NiXn-Ga alloys, their numbers correspondto Table 1. Groupsaremarked by the diffknt symbols, Group I: 0; Group n: +; Group III: A.
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I
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I
I, K
Figure 2. Thermograms obtained for a single crystal of alloy 18 duringforward and reverse (2) transformations. The transfi~on tempemhe T.. is labelled.
2
100
200
300
Figure 3. Low field magnetic susc@bility of alloy 18 as a hction of temperature.
T, K
of a single crystal
E+xuse of physical considerations (mainly with respect to Q and M, values) and for the convenience of the discussion, the alloys in Table 1 are divided into three groups, which are marked in Fig. 1 by different symbols. Group I includes Ni-Mn-Ga alloys exhibiting low transformation temperature M,, detlned as temperature at 5% of the overall x (T) change during the forward transformation. Matte&tic transformation going on in a ferromagnetic matrix enables to observe a sharp decrease on x (T) curve during cooling. An example of the x (T) behavior for alloy 3 is given in (1). Furthermore for group I alloys unlike groups 2 and 3, the lack of any anomaly on the electrical resistance vs temperature curve in the vicinity of M, was established (1,4). Group 11 consists of Ni-Mn-Ga alloys having ferromagnetic ordered parent phase like group I and according to M, and Q values can be treated as an intermediate group. M. values were determined in the same marmer as for group I. A representative of group II, namely alloy 10, was studied in detail elsewhere (2-5). Alloys horn group II, similarlyto group I, possess nearly the same values of AT and lattice spacings variation. On the other hand measured values of Q for alloys belonging to group II are about three times higher than those fi-om group I. Finally, group III is made up of a number of Ni-Mn-Ga alloys with relatively high values of M. and Q. Lattice spacing in this case ranges in the interval 0.580-0.597 run assuming that the lattice geometry is nearly cubic. Trar&&manon heats are about twice higher than the values of Q for group II alloys. Another important aspect concerning the properties of these alloys is that the martensitic transformation takes place in the paramagnetic parent phase because the condition T, < T*,holds (Table 1). It is assumed that the paramagnetic state of the matrix can induce some distinguishing features of MT. In this respect it is worth noting that the available method for the de&-m&ion of MT using X(T) runs fails unless being improved as to the sensitivity of x(T) measurements. Thus all the martensitic transformations of group III alloys were studied mainly by DSC method. Moreover, in this case the peak temperatures of the cooling thermograms are taken as the transformation temperatures. In Table 1 they are listed under the symbol Tt, . (AT)* , calculated as the temperature difibrence between the maximum and the minimum on heating and cooling thermograms, respectively, plays the role of the tran&xmation hysteresis . Typical pictures of DSC heating and cooling runs and x(T) dependence of one of the representative alloys of group III, namely a single crystal alloy 18, are presented in Figs. 2, 3. The magnetic susceptibility curve (Fig. 3) shows only one anomaly caused by ferromagntic lransformationof the marmnailicphase, whereas x(T) curves for low and middle transformation temperatures Ni-Mn-Ga alloys display two anomalies(1,2,4). From the comparison of x(T) anomalies caused by ferromagnetic transformation in au&mite (1, 2, 4) and those caused by the same transformation in
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1
I 0.02 mm .-0’ z ? ii3
200
280
360
440
T, K
Figure 4. SME- recovery of 5% strain on alloy 18, atIer removing compression performed at 77 K along [loo] axis, upon heating (1); dilatometric heating run of the same alloy in &ah-he. state (2) (1 -initial length of appropriate sample).
marten& (Fig. 3) it can be deduced that the latter is spread over a wider temperature interval than the former. By the way, this can be one of the reasons for the lack of any anomaly on the DSC curve (Fig. 2) due to ferromagnetic ordering of martensite (unlike austenite (4)) at least for the used scanning rates. Fig. 4. contirms that the high temperature Ni-Mn-Ga alloys show perfect SME: a single crystal of alloy 18 preliminarily compressed along the [ 1001 axis at 77 K shows a sharp recovery of about 5% strain in the reverse MT during heating inside the dilatometer. As a comparison, the dilatometric run for a strain&ee sample is also presented in Fig. 4. Considering the whole set of alloys in Table 1 irrespective to outlined groups one can conclude that a new family of shape memory alloys is available having widely varied properties. Using data fi-om Table 1 and Fig. 1 it is not difficultto deduce the MT temperatures evolution as a function of composition. Some examples are presented in Fig. 5. The smooth character of the composition dependence of M, supports indirectly the idea about the quite homogeneous state of alloys. The high sensitivity of M, to composition is clear from Fig. 5. The
18
22
111111111111111
26
30
Mn, at.%
0 25Mn-
A 21 Ga -I- 50 Ni _ Y f G t
300-
Is I-”
18 Figure 5. Evohztionofthe tmnshhion can&ant values of the third element
20
22
24
Ga, at.%
teqembsiuNi-hbGaalloysasahctionofGa(1)andhIn(2,3)contentatapproximately
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electron structure change is most probably responsible for such a behaviour though the atomic structure and lattice parameter influence can not be ignored. It is worth to note that the other cross-sectional composition depemknces appeared not to be approximated by straight lines which are drawn in Fig. 5 because of several reasons, among them the expemnental di%culty of maintaining a constant value of the third element in alloys. Austenite stabilization in the whole temperature scale was observed for alloys 22,23. By the way, the extrapolation of the dependence 3 in Fig. 5 to 0 K results in a value of Ga content which is in a good agreement with alloy 23. So the high content of Ga does not favor the onset of MT in Ni-Mn-Ga alloys. The general view of transformation temperature shifts as a function of the element content in the alloys studied in the present work can be summarized as follows: - At a constant value of Mn content, Ga addition lowers M, temperature. - Mn addition (instead of Ga) at constant Ni concentration causes M, increase. - Substitution of Ni atoms by Mn at constant Ga content results in alloys with lower M,. Just mentioned l$ evolutions were drawn within the limits of the uncertain@ in the determination of the element concentrations in alloys and can be useful for design of Ni-Mn-Ga alloys with given M, values. An important aspect of the properties of these alloys that served as the prime criterion for dividing them into three groups is the essentially difFerem numerical value of transformation heat, and hence entropy change accompanying MT. This fact can be explained neither by the temperature shift of MT alone, nor by an incomplete transfotmation. As to the latter our investigations have shown that these materials transform completely into martensite. One of the possible reasons for the Q behaviour is the di&rent type of crystal structure of the martensites formed on cooling. A previous study (4) has shown that the crystal lattice of martensite in alloy 10 was tetragonal one modulated by waves ofatom displacements with (ITO) polarization propagating along the [ 1 lo] direction with a periodic@ of 5 atomic layers. Seven-layered martensite and tetragonal martensite without modulation induced by external uniaxial loading were found in alloy 10 (5). A variety of martensites are expected to be formed in Ni-Mn-Ga alloys as a result of composition change. Preliminary X-ray and electron microscopy measurements confirm this assumption. Conclusions A new famiIy of Ni-Mn-Ga shape memory alloys having a large variety of properties has been elaborated. The dependences of the transformation temperature, the lattice parameter, the transformation heat and the Curie temperature on composition have been elucidated. The results seem to indicate that several structural types of mar&site can be formed by cooling in this system. Based on received data one can prepare a Ni-Mn-Ga alloy with given properties.
V. A. Chemenko is grateful to the Univ. de les Illes Balears for financing his staying at the Dept. de Fisica, UIB. References 1. 2. 3. 4. 5.
V. V. Kokorin and V. A Chemenko, Phys. Met. Metall., 68,111 (1989). A N. Vasil’ev, V. V. Kokorin, Yu. I. Savchenko aud V. A Chemenko, Sov. Phys. JETP, 71,803 (1990). A N. Vasil’ev, A Kayper, V. V. Kokorin, V. A -0, T. Takagi and J. Tani, JETP L.&ten, 58,306 (1993). L K. ZGmdmk, V. V. Kokorin, V. V. Martynov, A V. Tkachenko aud V. A Chemenko, Fiz. Met. i Metalloved, N6,llO (1990). V. V. Kokorin, V. V. Martpov and V. A Chernenko, Ser. met mat_,26,175 (1992).
