TiPd- and TiPt-Based High-Temperature Shape Memory Alloys: A Review on Recent Advances

Abstract

In this paper high-temperature shape memory alloys based on TiPd and TiPt are reviewed. The effect of the alloying elements in ternary TiPd and TiPt alloys on phase transformation and strain recovery is also discussed. Generally, the addition of alloying elements decreases the martensitic transformation temperature and improves the strength of the martensite and austenite phases. Additionally, it also decreases irrecoverable strain, but without perfect recovery due to plastic deformation. With the aim to improve the strength of high-temperature shape memory alloys, multi-component alloys, including medium- and high-entropy alloys, have been investigated and proposed as new structural materials. Notably, it was discovered that the martensitic transformation temperature could be controlled through a combination of the constituent elements and alloys with high austenite finish temperatures above 500 °C. The irrecoverable strain decreased in the multi-component alloys compared with the ternary alloys. The repeated thermal cyclic test was effective toward obtaining perfect strain recoveries in multi-component alloys, which could be good candidates for high-temperature shape memory alloys.

1. Introduction

Shape recovery in shape memory alloys (SMAs) occurs during a reverse martensitic transformation from martensite to austenite phases. Thereafter, the SMA operating temperature is related to the martensitic transformation temperature (MTT). High-temperature shape memory alloys (HT-SMAs) are defined as SMAs that can recover their shapes at temperatures above 100 °C. Several applications of HT-SMAs have been proposed. For example, Ni₃₀Pt₂₀Ti₅₀, whose MTTs include austenite start temperature, A_s: 262 °C; austenite finish temperature, A_f: 275 °C; martensite start temperature, M_s: 265 °C; and martensite finish temperature, M_f: 240 °C, was applied for active clearance control actuation in the high-pressure turbine section of a turbofan engine [1]. This indicates that the design can offer a small and lightweight package without requiring motion amplifiers that cause efficiency losses and introduce an additional failure mode [1]. Another example is the helical actuators for surge-control applications in helicopter engine compressors [2]. In this application, Ni_19.5Ti_50.5Pd₂₅Pt₅, whose MTTs comprise A_s: 243 °C, A_f: 259 °C, M_s: 247 °C, and M_f: 228 °C, was applied because the alloy exhibited good work capabilities, a 2.5% recoverable strain, and a work output of 9.45 J/cm³ at 400 MPa [2]. Several SMA applications, such as the active jet engine chevron, springs and wires for a general class of high-temperature actuators, oxygen mask deployment latch, SMA-activated thermal switch for lunar surface applications, variable geometry chevrons, and gas turbine variable area nozzles, have also been proposed [3]. Here, HT-SMAs, Ni_19.5Ti_50.5Pd₂₅Pt₅ or Ni_50.3Ti_29.7Hf₂₀ are used only in springs and wires for a general class of high-temperature actuator. Furthermore, NiTi-based SMAs that can actuate in the temperature range of 70–90 °C are used in other applications.

Raising MTTs is necessary for the development of HT-SMAs. In addition, improving SMA strength is also important because plastic deformation easily occurs at high temperatures, thereby resulting in incomplete shape recovery. Several studies have been conducted to increase MTTs by adding alloying elements such as Hf, Zr, Pd, Pt, and Au, to NiTi [4,5,6,7,8,9,10]. Their MTTs successfully increased by adding an alloying element, but a perfect shape recovery was not obtained. Recently, research of NiTi alloys has shifted to Ni_50.3Ti_29.7Hf₂₀, which is strengthened by nano-size precipitates called the “H phase” [11,12,13,14,15,16,17,18,19,20,21,22,23,24]. The austenite finishing temperature A_f of Ni_50.3Ti_29.7Hf₂₀ is typically 166 °C under unloading conditions [13], but it rises to 270 °C under tensile loading conditions at 500 MPa [16]. Furthermore, ageing increased the work output due to the higher transformation strain and the work output under 500 MPa was 16.45 J/cm³ at A_f of 270 °C [16]. High strength Ni-rich Ni_51.2Ti_28.8Hf₂₀ was also developed and its work output was 23 J/cm³ under 1700 MPa at A_f of approximately 100 °C, 27 J/cm³ under 1500 MPa at A_f of approximately 220 °C, and 15 J/cm³ under 1000 MPa at A_s of 151 °C (A_f was not clearly shown) [19]. The effect of 2000 thermal training cycles under 300 MPa of Ni_50.3Ti_29.7Hf₂₀ was also investigated and it was found that the stable cyclic strain recovery with the almost constant transformation strain [24]. The work output under 300 MPa was approximately 7.5 J/cm³ at A_f of approximately 220 °C [24].

Another approach to increasing MTTs is using other alloys with MTTs higher than those of NiTi alloys. Therefore, TiPd, TiAu, and TiPt have been studied because they exhibit a martensitic transformation from a B2 to a B19 structure, and their MTT values are higher than 500 °C [25,26]. For example, typical martensitic twin structures were observed in TiPt, whose high potential as an HT-SMA has been established [27,28]. The first investigation on strain recovery at high temperatures was performed for TiPd [29]. A binary TiPd sample was deformed at 500 °C, and the change in its length after it was heated above the A_f was investigated to measure strain recovery [29]. However, only a 10% strain was recovered owing to plastic deformation at 500 °C [29]. The shape recovery behavior of the TiAu alloy was investigated through thermomechanical analysis measurements after compressive deformation at 500 °C [30]. It was found that an 80% strain recovery occurred after a 5% deformation [30]. The effects of both Zr [31] and Ag addition [32] into TiAu on the martensitic transformation and strain recovery was investigated; the MTT decreased by 10% with the addition of Zr and Ag. For example, A_f decreased from 624 °C for TiAu to 511 °C and 594 °C for Ti–50Au–10Zr and Ti–40Au–10Ag, respectively [31,32]. Compressive deformation was applied at a test temperature of 50 °C below M_f, and the deformed samples were heated above A_f to measure the strain recovery. The strain recovery ratios of the Ti–50Au, Ti–50Au–10Zr, and Ti–40Au–10Ag alloys were 68%, 82%, and 76%, respectively [32]. The addition of Zr and Ag was effective in improving the strain recovery of TiAu.

The effects of an alloying element on MTTs, strain recovery, as well as strength of the martensite and austenite phases in TiPd and TiPt alloys, have been investigated by my group [33,34,35,36,37,38,39,40,41,42,43,44,45,46,47,48,49,50] and are reviewed herein. In addition to the TiPd and TiPt alloys, high- and medium- entropy SMAs (HEAs or MEAs) are also appraised because HEAs and MEAs have been attracting considerable attention as new SMAs. Notably, HEAs and MEAs are multi-component equiatomic or near-equiatomic alloys, which have garnered much interest as new generation structural materials because their high-entropy effects, such as severe lattice distortion and sluggish diffusion, are expected to improve the high-temperature strength of alloys [51,52]. As already shown, it is difficult to achieve perfect strain recovery in HT-SMAs because of the easy introduction of plastic deformation at high temperatures. Furthermore, improvement of strength of SMAs is a key issue for HT-SMAs. Application of HEAs and MEAs to HT-SMAs is expected in this area for which results on multi-component alloys, in particularly MEAs and HEAs, are presented in this paper.

2. Martensitic Transformation Temperature

The MTTs of the TiPd and TiPt alloys measured through differential scanning calorimetry (DSC) (DSC3200, NETZSCH Japan, Yokohama, Japan) are shown in

Table 1. Martensitic transformation temperature of ternary alloys (°C).

Alloy	Austenite Start Temperature As	Austenite Finish Temperature Af	Martensite Start Temperature Ms	Martensite Finish Temperature Mf	Temperature Hysteresis Af–Ms	Average Decrease in Ms per 1 at% ΔMs/at%	Ref.
Ti50Pd50	568	587	527	515	60	-	[33]
Ti48Pd50Hf2	538	570	494	472	76	-
Ti45Pd50Hf5	481	505	460	429	45	−15	[34]
Ti40Pd50Hf10	227	306	276	188	30	-
Ti40Pd55Hf5	528	551	471	487	80	-	[35]
Ti47Pd50Zr3	520	533	491	470	42	-
Ti45Pd50Zr5	492	508	467	445	41	−15	[33]
Ti43Pd50Zr7	491	506	464	443	42	-	[36]
Ti40Pd50Zr10	256	308	221	204	87	-	[33,36]
Ti50Pd45Zr5	324	374	344	312	30	-
Ti47Pd48Zr5	429	450	370	331	80	-
Ti43Pd52Zr5	482	529	393	366	136	-
Ti40Pd55Zr5	530	593	451	438	142	-	[35]
Ti45Pd50V5	508	523	462	440	61	−15	[34]
Ti40Pd55V5	437	559	442	398	117	-	[35]
Ti45Pd50Nb5	486	508	431	408	77	−20	[34]
Ti40Pd55Nb5	487	535	409	360	126	-	[35]
Ti45Pd50Ta5 *	561	579	512	489	67	−4	[34]
Ti45Pd50Cr5	268	356	347	293	9	−37	[34]
Ti45Pd50Mo5	267	383	401	268	−18	−27	[34]
Ti45Pd50W5 *	558	576	513	492	63	−4	[34]
Ti50Pd48Ru2	561	576	516	510	60	-	[37]
Ti50Pd46Ru4	440	482	436	393	46	−23	[37]
Ti50Pd42Ru8	298	354	333	275	22	-	[37]
Ti50Pd37.5Ru12.5	-	-	-	-	-	-	[37]
Ti50Pd25Ru25	-	-	-	-	-	-	[37]
Ti50Pd48Ir2	556	581	508	500	73	-	[37]
Ti50Pd46Ir4	525	552	510	484	42	−4	[37]
Ti50Pd42Ir8	478	521	479	439	43	-	[37]
Ti50Pd38Ir12	471	508	452	395	56	-
Ti50Pd37.5Ir12.5	478	492	452	433	39	-	[37]
Ti50Pd25Ir25	-	-	-	-	-	-	[37]
Ti50Pd12.5Ir37.5	-	-	-	-	-	-	[37]
Ti50Pd48Co2	523	545	478	464	67	-	[37]
Ti50Pd46Co4	473	493	455	427	38	−18	[37]
Ti50Pd42Co8	375	398	259	327	34	-	[37]
Ti50Pt50	1000	1057	989	963	68	-	[38]
Ti50Pt45Ir5	1050	1081	1030	1008	51	-	[39,40]
Ti50Pt37.5Ir12.5	1036	1103	1000	978	103	-	[38]
Ti50Pt25Ir25	1121	1190	1110	1068	80	-	[38]
Ti50Pt20Ir30	1159	1189	1145	1127	44	-	[39]
Ti50Pt12.5Ir37.5	1175	1218	1184	1169	34	-	[38]
Ti55Pt35Ir10	931	959	-	-	-	-	[41]
Ti50Pt45Ru5	925	975	913	856	62	−20	[40,42]
Ti45Pt50Zr5	939	985	897	840	88	−23	[40,42]
Ti50Pt45Co5	959	1003	913	855	90	−15	[40,43]
Ti45Pt50Hf5	952	996	905	855	91	−17	[40,44]

*: melting was not perfect.

. The temperature hysteresis, A_f–M_s, is also summarized in Table 1. Data without reference are from my group’s unpublished work. The addition of a third element, such as Hf, Zr, V, Nb, Ta, Cr, Mo, W, Ru, Ir, and Co, was performed for TiPd, while elements, including Ir, Ru, Co, Zr, and Hf, were added for TiPt. In most cases, the MTT decreased with the addition of an alloying element. In Figure 1a, A_f changes with increasing alloying element concentration are shown for some of the ternary alloys. Notably, A_f decreased rather linearly with increasing amounts of alloying elements. The same trend appeared for A_s, M_s, and M_f. The average decrease in M_s per 1 at% of an alloying element up to 5 at% addition is shown in Table 1. Ta and W did not melt well into TiPd, and their particles remained in the alloy, thereby making the matrix composition of Ti₄₅Pd₅₀W₅ and Ti₄₅Pd₅₀Ta₅ approximately close to that of TiPd. Consequently, the decrease in the MTT by W and Ta was small. For other alloying elements, the decrease in MTT was between 15–37 °C. In Figure 1b, the MTT change with respect to Pd concentration is observed for Ti_{(95 − x)}Pd_xZr₅; the MTT increased with an increase in the Pd concentration.

The MTT of TiPt was very high; for example, A_s and A_f were 1000 °C and 1057 °C, respectively, as shown in Table 1. Although the addition of Ru, Co, Zr, and Hf decreased the MTT of TiPt, the addition of Ir increased it.

The MTTs of multi-component alloys are summarized in

Table 2. Martensitic transformation temperature of multi-element alloys.

Alloy	As	Af	Ms	Mf	Af–Ms	ΔSmix	Alloy Classification	Ref.
Ti45Zr5Pd45Ir5	457	475	422	398	53	1.0R	Low-entropy alloy LEA	-
Ti45Zr5Pd35Ir15	313	348	295	256	53	1.2R	Medium-entropy alloy MEA	-
Ti45Zr5Pd25Ir25	-	-	-	-	-	1.2R	MEA	-
Ti45Zr5Pd45Pt5	497	511	464	439	47	1.0R	LEA	[45]
Ti45Zr5Pd35Pt15	539	557	485	471	72	1.2R	MEA	[45]
Ti45Zr5Pd25Pt25	628	648	571	539	77	1.2R	MEA	[45]
Ti45Zr5Pd15Pt35	725	804	713	590	91	1.2R	MEA	[46]
Ti45Zr5Pd5Pt45	896	935	774	713	112	1.0R	LEA	[46]
Ti40Zr10Pd25Pt25	390	430	245	220	288	1.3R	MEA	[47] ST
Ti45Zr4V1Pd50	497	508	409	384	99	0.9R	LEA	[36]
Ti45Zr2.5V2.5Pd50	523	546	393	368	153	0.9R	LEA	[36]
Ti45Zr1V4Pd50	496	518	441	424	77	0.9R	LEA	[36]
Ti45Zr5Pd45Ni5	348	378	328	297	50	1.0R	LEA	[47]
Ti45Zr5Pd40Ni10	216	258	206	177	52	1.1R	MEA	[48]
Ti45Zr5Pd37Ni13	114	230	206	108	24	1.2R	MEA	[47] ST
Ti42Zr8Pd40Ni10	-	-	-	-	-	1.2R	MEA	[47]
Ti45Zr5Pd30Ni20	-	-	-	-	-	1.2R	MEA	[48]
Ti45Zr5Pd40Co10	303	361	339	298	22	1.1R	MEA	[48]
Ti45Zr5Pd40Ni8Co2	201	273	212	199	61	1.2R	MEA	[48]
Ti45Zr5Pd40Ni5Co5	-	-	-	-	-	1.2R	MEA	[48]
Ti45Zr5Pd40Ni2Co8	143	189	135	108	54	1.2R	MEA	[48]
Ti35Zr15Pd20Pt15Ni15	-	-	-	-	-	1.5R	High-entropy alloy HEA	[49]
Ti40Zr10Pd20Pt15Ni15	-	-	-	-	-	1.5R	HEA	[49]
Ti45Zr5Pd20Pt25Ni5	559	598	502	432	96	1.3R	MEA	[49]
Ti45Zr5Pd25Pt20Ni5	390	430	245	220	185	1.3R	MEA	[47] ST
Ti45Zr5Pd20Pt20Ni10	374	442	337	256	105	1.3R	MEA	[49]
Ti35Zr15Pd20Pt15Au15	-	-	-	-	-	1.5R	HEA	[50]
Ti35Zr15Pd20Pt15Co15	-	-	-	-	-	1.5R	HEA	[50]
Ti45Zr5Pd25Pt20Au5	520	590	515	453	75	1.3R	MEA	[50]
Ti45Zr5Pd25Pt20Co5	419	538	431	324	107	1.3R	MEA	[50]
Ti16.7Zr16.7Hf16.7Ni25Cu25	184	338	226	112	112	1.6R	HEA	[53]
Ti16.7Zr16.7Hf16.7Ni25Co10Cu15	−23	71	36	−80	35	1.8R	HEA	[54]
Ti30Hf20Pd15Ni35	537	686	525	479	161	1.3R	MEA	[55]
Ti25Hf25Pd25Ni25	680	720	620	580	100	1.4R	MEA	[55]
Ti16.7Zr16.7Hf16.7Pd25Ni25	740	780	660	620	120	1.6R	HEA	[55]

. Data without reference are from my group’s unpublished work. The multi-component alloys were designed based on TiPd as follows: Pd, a group 10 element in the periodic table, was replaced by Co and Ir, which are group 9 elements; by Ni and Pt which are group 10 elements; and by Au, a group 11 element. Furthermore, Ti, a group 4 element, was replaced by Zr and Hf, which are also group 4 elements. Evident DSC peaks were not observed in the three alloys, Ti₄₀Zr₁₀Pd₂₅Pt₂₅, Ti₄₅Zr₅Pd₃₇Ni₁₃, and Ti₄₅Zr₅Pd₂₅Pt₂₀Ni₅, and their MTTs were estimated from the strain–temperature (S–T) curves during the thermal cyclic test under constant stress.

Multi-component alloys are classified according to the mixing entropy, ΔS_mix, using the following equations [51].

Here, ΔS_mix is defined by the following equation:

$$\Delta S_{\mathrm{mix} }= -R{\displaystyle \sum }_{i=1}^n{x}_i{lnx}_i$$

(1)

where x_i is the mole fraction of component i, n is the number of constituent elements, and R is the gas constant (8.314 J/Kmol).

HEA:

$$\Delta S_{\mathrm{mix} }\ge 1.5R$$

(2)

Medium-entropy alloy (MEA):

$$1.0R \le \Delta S_{\mathrm{mix} }\le 1.5R$$

(3)

Low-entropy alloy (LEA, conventional solid-solution alloy):

$$\Delta S_{\mathrm{mix} }\le 1.0R$$

(4)

The calculated mixing entropies are presented in Table 2. Based on the mixing entropies, the alloy classification is also shown in Table 2. Most quaternary alloys are classified as MEAs, while some are classified as LEAs, i.e., conventional solid-solution alloys. Notably, HEAs have been identified in some multi-component alloys with five or six constituent elements.

The MTTs of the five quaternary alloys, TiZrPdIr, TiZrPdPt, TiZrPdNi, TiZrPdCo, and TiZrVPd, are shown in Table 2. The combination of TiZrPdIr, TiZrPdNi, and TiZrPdCo decreased the MTTs compared with those of Ti₄₅Zr₅Pd₅₀. This is because the phase transformations of TiIr, TiNi, and TiCo are different from those of TiPd. For example, TiIr undergoes a two-step phase transformation from the B2 structure at high temperature to a tetragonal structure (distorted B2 structure) at middle temperature, and finally to an orthorhombic structure with space group 65 at low temperature [56]. The MTT depends on Ir concentration. Notably, TiNi undergoes martensitic transformation from the B2 to the B19′ structure [4]. The B2 structure is very stable in TiCo from room temperature to the melting temperature [25]; the addition of these elements is considered to stabilize the B2 structure. However, TiZrPdPt increased the MTT relative to Ti₄₅Zr₅Pd₅₀. This is because the same martensitic transformation from B2 to B19 structures, as well as TiPd, occur in TiPt, whose MTT is very high, approximately 1000 °C, as shown in Table 1. Furthermore, Pt and Pd exhibit perfect solubility in both austenite and martensite phases. Consequently, MTT gradually increased with Pt content. In the case of TiZrVPd, the total amounts of Zr and V were maintained at 5 at%, and the values of A_s and A_f were approximately the same as those of Ti₄₅Zr₅Pd₅₀, while the M_s and M_f values of TiZrVPd were lower than those of Ti₄₅Zr₅Pd₅₀.

Although the MTTs of multi-component alloys depend on the combination of elements, there are some trends whereby those of TiZrPdPtNi and TiZrPdPtAu are relatively high, whereas that of TiZrPdNiCo is relatively low.

The potential of HEAs as SMAs was presented for the first time by Fistov et al., wherein Ti_16.7Zr_16.7Hf_16.7Ni₂₅Cu₂₅ was investigated and its A_s and A_f were 184 and 338 °C, respectively [53]. The addition of Co to Ti_16.7Zr_16.7Hf_16.7Ni₂₅Cu₂₅ was investigated, but its MTT drastically decreased and A_f was –23 °C [54]. The MTTs of these alloys are summarized in Table 2. The potential of HEAs as HT-SMAs was first shown by Canadinc [55]. The MTT of their alloys was obtained from the DSC curves in reference [55] and is summarized in Table 2. The A_s and A_f of Ti_16.7Zr_16.7Hf_16.7Pd₂₅Ni₂₅ were 740 and 780 °C, respectively. In my group’s experiments, HEAs and some MEAs did not show martensitic transformation up to 700 °C during DSC measurement. However, martensitic transformation may be possible for these alloys at temperatures higher than 700 °C. The MTT measurement of my group’s HEAs is presently undergoing ultra-high temperature DSC. We found that some alloys have a high MTT, approximately close to 1000 °C in multi-component alloys.

Notably, the temperature hysteresis of the multi-component alloys becomes larger relative to that of ternary alloys. The number of alloys with temperature hysteresis exceeding 100 °C and their ratios are summarized in

Table 3. Ratio of alloys with temperature hysteresis over 100 °C (%).

Alloys	Number of Alloys with Temperature Hysteresis over 100 °C	Number of Tested Alloys	Ratio of Alloys with Temperature Hysteresis over 100 °C (%)
Ternary alloy in Table 1	5	42	12
LEA in Table 2	2	7	28
MEA and HEA in Table 2	8	20	40

. The ratio of alloys with temperature hysteresis exceeding 100 °C in ternary alloys is 12% and it increases to 28% in LEAs and to 40% in MEAs and HEAs. It indicates that the ratio of alloys with temperature hysteresis exceeding 100 °C increased with increase of amount of constituent elements in alloys. In multi-component alloys, in particularly in MEAs and HEAs, the amount of constituent elements is equivalent or near-equivalent; therefore, it is expected that severe lattice distortion, which is considered to obstruct martensitic transformation, will occur. In many MEA cases, a drastic decrease in M_s was observed compared with A_f. When SMAs are used as actuators, a smaller temperature hysteresis is necessary to quickly respond to the surrounding environment.

3. Strain Recovery Determined by Compression Test

In the early stage of my group’s research [33,34,35,37,38,39,40,41,42,43,44], the recovery strain was estimated by measuring the sample length before and after the compression test using a strain rate of 3 × 10⁻³/s, and after heat treatment above A_f. A sample was deformed by approximately 5% at the test temperature. The deformed sample length (L’) was measured after cooling to room temperature. Thereafter, the sample was re-heated over A_f, cooled to room temperature, and the recovered sample length (L”) was measured. When the initial sample length is L₀, the applied strain ε_a is defined by the following equation:

ε_a = 100 × ((L₀ − L’)/L₀)

(5)

Recoverable strain ε_r is defined according to the following equation:

ε_r = 100 × ((L” − L’)/L₀)

(6)

Strain recovery ratio, i.e., shape memory effect (SME) was evaluated using the following equation:

SME (%) = ε_r/ε_a × 100

(7)

The applied and recoverable strains, as well as the strain recovery ratios of the ternary TiPd and TiPt alloys, are summarized in

Table 4. Strain recovery of ternary alloys.

Alloy	Test Temp., °C	The Difference from Martensite Transformation Temperature, °C	Applied Strain, %	Recoverable Strain, %	Strain Recovery Ratio, %	Ref.
Ti50Pd50	538	-	4.1	1.6	40	[33]
Ti50Pd50	485	Mf − 30	3.8	2.5	67	[34]
Ti50Pd50	380	-	3.7	0.46	13	[33]
Ti50Pd50	320	-	1.24	0.59	48
Ti48Pd50Hf2	380	-	2.8	0.4	14
Ti45Pd50Hf5	456	As − 30	2.9	2.1	74
Ti45Pd50Hf5	440	-	2.7	2.3	80	[34]
Ti45Pd50Hf5	380	Mf − 50	1.9	1.5	80
Ti45Pd50Hf5	200	-	5.25	4.1	78
Ti40Pd50Hf10	380	-	3.0	2.4	80
Ti40Pd55Hf5	457	Mf − 30	3.7	1.7	45	[35]
Ti50Pd45Zr5	210	-	4.28	0.7	17
Ti50Pd45Zr5	295	-	3.1	0.55	18
Ti50Pd45Zr5	380	-	3.2	1.47	46
Ti47Pd48Zr5	301	-	4.2	3.6	85
Ti47Pd48Zr5	399	-	8.3	5.7	69
Ti47Pd48Zr5	380	-	2.2	1.1	50
Ti48Pd50Zr2	400	-	6.5	3.4	53
Ti48Pd50Zr2	380	-	3.1	1.3	43
Ti47Pd50Zr3	490	As − 30	3.7	2.6	70
Ti47Pd50Zr3	415	Mf − 30	3.3	2.9	88
Ti47Pd50Zr3	380	-	3.2	2.8	87
Ti45Pd50Zr5	415	Mf − 30	4.4	3.5	81	[34]
Ti45Pd50Zr5	426	As − 30	3.1	2.58	84	[33]
Ti45Pd50Zr5	460	-	8	5.2	65
Ti45Pd50Zr5	380	-	3.5	2,2	62
Ti45Pd50Zr5	380	-	4.1	3.9	94	[33]
Ti43Pd52Zr5	380	-	2.2	1.75	81
Ti43Pd52Zr5	451	As − 30	4.6	4.1	90
Ti40Pd55Zr5	490	-	7.2	2.0	27
Ti40Pd55Zr5	450	-	1.2	0.39	34
Ti40Pd55Zr5	380	-	1.9	0.4	22
Ti43Pd50Zr7	461	As − 30	1.4	1.2	84
Ti43Pd50Zr7	413	Mf − 30	3.6	3.6	100
Ti43Pd50Zr7	380	-	3.2	2.9	90
Ti40Pd50Zr10	225	-	7.7	4.8	63	[33]
Ti40Pd50Zr10	380	-	1.4	1.4	100
Ti40Pd55Zr5	408	Mf − 30	4	1.2	30	[35]
Ti45Pd50V5	413	Mf − 30	2.4	1.2	60	[34]
Ti40Pd55V5	368	Mf − 30	3	0.3	10	[35]
Ti45Pd50Nb5	378	Mf − 30	3.1	1.1	37	[34]
Ti40Pd55Nb5	330	Mf − 30	3	0.3	10	[35]
Ti45Pd50Ta5 *	483	Mf − 30	3.0	1.1	35	[34]
Ti45Pd50Cr5	228	Mf − 30	3.3	0.4	12	[34]
Ti45Pd50Mo5	256	Mf − 30	2.8	0.4	14	[34]
Ti45Pd50W5 *	474	Mf − 30	2.3	0.5	22	[34]
Ti50Pd46Ru2	320	-	-	-	40	[37]
Ti50Pd46Ru4	320	-	3.7	2.0	52
Ti50Pd42Ru8	320	-	3.4	2.5	72
Ti50Pd48Ir2	320	-	2.6	1.22	47	[37]
Ti50Pd46Ir4	320	-	2.9	1.49	51	[37]
Ti50Pd46Ir4	454	Mf − 30	1.0	1.0	100
Ti50Pd46Ir4	427	Mf − 60	2.9	0.55	19
Ti50Pd42Ir8	320	-	2.17	1.25	57	[37]
Ti50Pd42Ir8	408	Mf − 30	1.5	1.3	87
Ti50Pd42Ir8	427	Mf − 60	6.36	2.05	32
Ti50Pd38Ir12	401	Mf − 30	1.4	1.01	72
Ti50Pd48Co2	320	-	2.35	1.36	58	[37]
Ti50Pd46Co4	320	-	3.2	1.4	47	[37]
Ti50Pd42Co8	320	-	3.3	1.0	31	[37]
Ti50Pt50	910	Mf − 50	6	0	0	[38]
Ti50Pt50	850	-	-	-	11	[39]
Ti50Pt45Ir5	850	-	-	-	10	[39,40]
Ti50Pt37.5Ir12.5	928	Mf − 50	17	2	12	[38]
Ti50Pt37.5Ir12.5	850	-	-	-	51	[39]
Ti50Pt25Ir25	1018	Mf − 50	17	4	23	[38]
Ti50Pt25Ir25	850	-	-	-	57	[39]
Ti50Pt20Ir30	850	-	-	-	36	[39]
Ti50Pt12.5Ir37.5	1119	Mf − 50	15	0	0	[38]
Ti50Pt12.5Ir37.5	850	-	-	-	21	[39]
Ti55Pt35Ir10	850	-	4.8	1	20	[41]
Ti56Pt22Ir22	850	-	2.5	1.4	56	[41]
Ti58Pt10Ir32	850	-	1.9	1.4	73	[41]
Ti50Pt45Ru5	802	Mf − 50	2.9	1.3	45	[40,42]
Ti45Pt50Zr5	790	Mf − 50	2.1	1.2	58	[40,42]
Ti50Pt45Co5	801	Mf − 50	2.5	0.9	36	[40,43]
Ti45Pt50Hf5	806	Mf − 50	2.4	1.4	58	[40,44]

*: melting was not perfect.

. Data without reference are from my group’s unpublished work. For the quaternary alloys, this method was used only for TiZrPdIr, and the results are summarized in

Table 5. Strain recovery of quaternary alloys.

Alloy	Test Temp., °C	The Difference from Martensite Transformation Temperature, °C	Applied Strain, %	Recoverable Strain, %	Strain Recovery Ratio, %
Ti45Pd45Ir5Zr5	368	Mf − 30	2.8	2.7	96
Ti45Pd35Ir15Zr5	226	Mf − 30	2.6	2.2	85

. Although it is difficult to compare the effect of alloying elements due to different applied strains, some alloys have a high strain recovery ratio of above 80%. The quaternary alloys also exhibited a high strain recovery ratio of above 80%, as shown in Table 5. In Figure 2, the strain recovery ratios of Ti₄₅Pd_{50 × 5} are plotted for the periodic table family; it can be observed that the addition of Zr and Hf is effective toward improving strain recovery. Since the atomic size (metallic) of Zr and Hf are 160 and 159 pm, respectively, and larger than those of Ti (147 pm) and Pd (137 pm) [57], the solid-solution hardening effects of Zr and Hf are considered to exceed those of the other alloying elements. Thereafter, the 0.2% flow stress of the austenite and martensite phases were investigated by compression testing using samples with dimensions of 2.5 × 2.5 × 5 mm³. The 0.2% flow stresses of ternary TiPd and TiPt alloys are summarized in

Table 6. Strength of the martensite and austenite phases of the binary and ternary alloys.

Alloy	Test Temp., °C	The Difference from Martensite Transformation Temperature, °C	Detwining Stress, MPa	0.2% Flow Stress, MPa	Ref.
Ti50Pd50	617	Af + 30	-	82	[33]
Ti50Pd50	538	As − 30	231	293	[33]
Ti50Pd50	485	Mf − 30	249	617
Ti45Pd50Hf5	534	Af + 30	-	267	[34]
Ti45Pd50Hf5	456	As − 30	152	794	[34]
Ti40Pd55Hf5	581	Af + 30	-	341	[35]
Ti40Pd55Hf5	441	Mf − 30	725	1300	[35]
Ti47Pd48Zr5	301	Mf − 30	301	1163
Ti47Pd48Zr5	550	Af + 30	-	628
Ti47Pd48Zr5	399	As − 30	302	1449
Ti48Pd50Zr2	400	As − 30	631	1504
Ti48Pd50Zr2	563	Af + 30	-	467
Ti47Pd50Zr3	563	Af + 30	-	183
Ti47Pd50Zr3	490	As − 30	219	666
Ti47Pd50Zr3	439	Mf − 30	234	863
Ti45Pd50Zr5	538	Af + 30	-	274	[33]
Ti45Pd50Zr5	426	As − 30	161	722	[33]
Ti45Pd50Zr5	415	Mf − 30	298	937
Ti43Pd50Zr7	536	Af + 30	-	245
Ti43Pd50Zr7	461	As − 30	255	733
Ti43Pd50Zr7	413	Mf − 30	227	954
Ti40Pd50Zr10	337	Af + 30	-	386	[33]
Ti40Pd50Zr10	225	As − 30	331	894	[33]
Ti43Pd52Zr5	451	As − 30	371	1080
Ti43Pd52Zr5	337	Mf − 30	568	1378
Ti40Pd55Zr5	623	Af + 30	-	386	[35]
Ti40Pd55Zr5	408	Mf − 30	1030	1492	[35]
Ti45Pd50V5	591	Af + 30	-	644	[34]
Ti45Pd50V5	413	Mf − 30	521	1252	[34]
Ti40Pd55V5	589	Af + 30	643	386	[35]
Ti40Pd55V5	368	Mf − 30	744	386	[35]
Ti45Pd50Nb5	625	Af + 30	-	389	[34]
Ti45Pd50Nb5	378	Mf − 30	329	836	[34]
Ti40Pd55Nb5	565	Af + 30	601	386	[35]
Ti40Pd55Nb5	330	Mf − 30	651	386	[35]
Ti45Pd50Ta5 *	625	Af + 30	-	287	[34]
Ti45Pd50Ta5 *	483	Mf − 30	462	941	[34]
Ti45Pd50Cr5	333	Af + 30	-	273	[34]
Ti45Pd50Cr5	228	Mf − 30	317	1000	[34]
Ti45Pd50Mo5	503	Af + 30	-	364	[34]
Ti45Pd50Mo5	256	Mf − 30	512	1239	[34]
Ti45Pd50W5 *	615	Af + 30	-	232	[34]
Ti45Pd50W5 *	474	Mf − 30	591	1008	[34]
Ti50Pd48Ru2	320	-	-	301
Ti50Pd46Ru4	520	Af + 30	-	150
Ti50Pd46Ru4	320	-	196	598
Ti50Pd46Ir4	580	Af + 30	-	100
Ti50Pd46Ir4	454	Mf − 30	260	740
Ti50Pd46Ir4	320	-	280	800
Ti50Pd42Ir8	551	Af + 30	65	150
Ti50Pd42Ir8	408	Mf − 30	320	720
Ti50Pd38Ir12	521	-	215	744
Ti50Pd38Ir12	401	Mf − 30	360	850
Ti50Pd46Co4	520	Af + 30	-	65
Ti50Pd46Co4	320	-	200	710
Ti50Pt50	1100	Af + 50	-	27	[38]
Ti50Pt50	910	Mf − 50	-	320	[38]
Ti50Pt37.5Ir12.5	1153	Af + 50	-	42	[38]
Ti50Pt37.5Ir12.5	928	Mf − 50	300	640	[38]
Ti50Pt25Ir25	1240	Af + 50	-	35	[38]
Ti50Pt25Ir25	1018	Mf − 50	230	450	[38]
Ti50Pt12.5Ir37.5	1268	Af + 50	-	170	[38]
Ti50Pt12.5Ir37.5	1119	Mf − 50	285	435	[38]
Ti50Pt45Ru5	-	Af + 50	-	100	[40,42]
Ti50Pt45Ru5	800	Mf − 50	355	712	[42]
Ti45Pt50Zr5	-	Af + 50	-	111	[40,42]
Ti45Pt50Zr5	790	Mf − 50	600	1468	[40,42]
Ti50Pt45Co5	-	Af + 50	-	72	[40,43]
Ti50Pt45Co5	-	Mf − 50	407	523	[40,43]
Ti45Pt50Hf5	-	Af + 50	-	149	[40,44]
Ti45Pt50Hf5	-	Mf − 50	565	1113	[40,44]

*: melting was not perfect.

. Data without reference are from my group’s unpublished work. When the martensite phase is deformed, a double yielding behavior often occurs. The first yielding behavior represents the rearrangement of the martensite variant and is referred to as the detwinning stress. The second yielding behavior represents the yield stress of martensite. In Table 6, the detwinning stresses of the martensite phase are also shown. In Figure 3, the 0.2% flow stress of the austenite and martensite phases of Ti₄₅Pd₅₀X₅ (X in group 4–6 elements) and Ti₅₀Pd₄₆Y₄ (Y in group 8–10 elements) are plotted for the periodic table family. Most alloying elements improved the strength of the austenite and martensite phases compared with those of Ti₅₀Pd₅₀. The strengthening behavior of the alloying element on the austenite and martensite phases was similar. Comparing Figure 2 and Figure 3, it is evident that high-strength alloys, such as Ti₄₅Pd₅₀V₅, have a lower strain recovery than Ti₄₅Pd₅₀Zr₅ and Ti₄₅Pd₅₀Hf₅ with a smaller strengthening effect than Ti₄₅Pd₅₀V₅. It is difficult to establish the correlation between strength and strain recovery ratio.

4. Strain Recovery Determined by Thermal Cyclic Test under Constant Stress

Recently, strain recovery was investigated through a thermal cyclic compression test under a constant load (Shimadzu AG-X, Shimadzu, Kyoto, Japan). The specimens were heated to 30 °C above A_f and then cooled to a temperature lower than M_f under a constant load of 15–200 MPa. Five thermal cycles under loads of 15, 50, 100, 150, and 200 MPa were applied for the same samples in the order given with heating and cooling rates of 50 °C/min. The work output (work per volume) was obtained from the product of the recovery strain and the applied stress. The strain temperature (S–T) curves of Ti₄₅Pd₅₀X₅ (X = Zr, Hf, V, and Nb) are plotted in Figure 4. The transformation strain increased with an increase in the applied stress. The transformation strains of Ti₄₅Pd₅₀Zr₅ and Ti₄₅Pd₅₀Hf₅ exceed 2% above 50 MPa, as shown in Figure 4a,b. Nonetheless, the transformation strains of Ti₄₅Pd₅₀V₅ are less than 1% at an applied stress of 200 MPa. Phase transformation was not evident in TiPdNb, even at 200 MPa. The perfect strain recovery was obtained under a small applied stress of less than 100 MPa in Ti₄₅Pd₅₀V₅, but the irrecoverable strain appeared at 150 MPa and increased at 200 MPa. In Ti₄₅Pd₅₀Zr₅ and Ti₄₅Pd₅₀Hf₅, the small irrecoverable strain was observed, even at 15 MPa, which increased with the applied stress. At 150 and 200 MPa in Ti₄₅Pd₅₀Hf₅, trumpet-shaped S–T curves were obtained, as shown by arrows in Figure 4b. This is because the compressive plastic deformation is larger than the expansion of the sample during heating above A_f.

The recoverable and irrecoverable strains, as well as the work output of Ti₄₅Pd₅₀X₅ (X = Zr, Hf, and V) are plotted as a function of the applied stress, as shown in Figure 5; for reference, those of Ti₅₀Pd₅₀ at 50 MPa are also plotted [58]. The recoverable strain increased with the applied stress; however, by increasing the irrecoverable strain, the recoverable strain started to decrease after the peak recoverable strain. Although the recoverable strain of Ti₅₀Pd₅₀ is similar to that of Ti₄₅Pd₅₀X₅ (X = Zr and Hf), the irrecoverable strain is very large compared to that of the ternary alloys. The irrecoverable strain is expected to increase drastically with an increase in the applied stress. It also indicates that the addition of an alloying element is effective toward decreasing the irrecoverable strain. The work output increased with applied stress. Although an irrecoverable strain appeared, a large work output of 7.6 J/cm³ was obtained for Ti₄₅Pd₅₀Zr₅.

5. Effects of Training

It is known that repeated thermal cyclic tests, which is referred to as “training,” reduces the irrecoverable strain, and a perfect recovery is finally obtained [8,59]. For example, the irrecoverable strain of TiNi-based alloys became approximately 0 after 40 cycles under a load of 80 MPa. The training was effective for the TiNi alloys strengthened by the addition of Pd or Sc. Therefore, training was performed for Ti₄₅Pd₅₀X₅ (X = Zr and Hf) under a 50-MPa load. The irrecoverable strain vs. the number of thermal cycles is plotted in Figure 6. The irrecoverable strain of Ti₄₅Pd₅₀Zr₅ was saturated after 10 cycles, and it remained at approximately 0.1%. The irrecoverable strain of Ti₄₅Pd₅₀Hf₅ was approximately double that of Ti₄₅Pd₅₀Zr₅, and it was unsaturated. This indicates that the strain recovery of ternary alloys is unstable for repeated cycles. The effect of Zr content on the training effect was investigated for Ti₄₅Pd₅₀Zr₇ and Ti₄₅Pd₅₀Zr₁₀ [36]. By increasing Zr addition, the irrecoverable strain finally disappeared during training and perfect strain recovery was achieved [36]. This outcome can be attributed to two reasons: (1) the solid-solution hardening effect is larger at high Zr contents, and (2) MTT reduction advantageously affects the suppression of plastic deformation.

6. Strain Recovery of Multi-Component Alloys

A thermal cyclic test was performed on the multi-component alloys to investigate strain recovery. Some results have already been published [45,46,47,48,49,50]. The recoverable and irrecoverable strains, as well as the work output of some of the multi-component alloys are shown in Figure 7; the TiZrPdNi and TiZrPdPt alloys are shown in Figure 7a,c,e, and in Figure 7b,d,f, respectively, while Ti₄₅Pd₅₀Zr₅ is shown as a standard sample in all the diagrams. Furthermore, Ti₄₅Zr₅Pd₄₅Ni₅, Ti₄₅Zr₅Pd₃₇Ni₁₃, Ti₄₀Zr₁₀Pd₂₅Pt₂₅, and Ti₄₅Zr₅Pd₂₅Pt₂₀Ni₅ are the original data source in this study. In Figure 7a,c,e, the concentrations of Ti and Zr were maintained at 45 and 5 at%, respectively, in TiZrPdNi, and only the concentrations of Pd and Ni were changed. Among the tested alloys in Figure 7a,c,e, TiZrPdNi alloys exhibited a relatively high recoverable strain, although the recoverable strain of Ti₄₅Zr₅Pd₄₀Ni₁₀ [48] was similar to that of ternary Ti₄₅Pd₅₀Zr₅. Moreover, Ni addition seems to increase the recoverable strain of Ti₄₅Pd₅₀Zr₅. The recoverable strain of Ti₄₅Zr₅Pd₄₀Co₁₀ [48] was very small, thereby suggesting that Co addition drastically decreased the recovery strain. However, the recoverable strain of the multi-component alloys of TiZrPdNiCo was larger than that of Ti₄₅Zr₅Pd₄₀Ni₁₀ [48]. This indicated that the recoverable strain was increased by the effect of the multi-component alloy, i.e., the high-entropy effect. In Figure 7c, the irrecoverable strains of most of the tested alloys were smaller than 0.2%. Only the ternary Ti₄₅Pd₅₀Zr₅ and quaternary alloy of Ti₄₅Zr₅Pd₄₅Ni₅, which were defined as LEA, represented a large irrecoverable strain exceeding 0.2%. A decrease in the irrecoverable strain of MEAs is also considered to be consequent of the high-entropy effect. As a result of the large recoverable strain, the work output of TiZrPdNi-type multi-component alloys also becomes large, as shown in Figure 7e. The work outputs of some alloys exceeded 10 J/cm³. The repeated thermal cycling test, i.e., training was also applied for some alloys. For example, training under 300 MPa was performed for Ti₄₅Zr₅Pd₄₀Ni₁₀ for 100 cycles [48]. Although the transformation strain decreased, the irrecoverable strain decreased, and a perfect recovery was finally achieved during the thermal cyclic test. The final recovery strain was approximately 3.6% under 300 MPa. Thereafter, the work output was 10.8 J/cm³ at the A_f of 258 °C. For Ti₄₅Zr₅Pd₄₀Co₁₀, training was performed under 700 MPa. After nine cycles, the irrecoverable strain disappeared, and a perfect recovery was achieved. The final recovery strain was approximately 1%, and the work output was 7 J/cm³ at the A_f of 361 °C.

In Figure 7b,d,f, TiZrPdPt alloys are compared. Again, the concentrations of Ti and Zr were maintained at 45 and 5 at%, respectively, in most of the alloys. Compared with the recoverable strain in TiZrPdNi alloys, as shown in Figure 7a and in TiZrPdPt alloys, as presented in Figure 7b, those of TiZrPdPt alloys were smaller than 3% and less than those of TiZrPdNi alloys. This indicates that the addition of Pt decreases the recoverable strain. The same trend can be observed in quaternary alloys by comparing Ti₄₅Zr₅Pd₂₅Pt₂₅ and Ti₄₅Zr₅Pd₄₅Pt₅. A small recoverable strain was achieved in the alloy with high Pt content. The effect of Zr on the recoverable strain in quaternary alloys could be understood by comparing Ti₄₀Zr₁₀Pd₂₅Pt₂₅ and Ti₄₅Zr₅Pd₂₅Pt₂₅, and it was found that Zr addition increased the recoverable strain. In the multi-component alloys, it was found that high Pt addition decreased recoverable strain, as shown in Figure 7b. In TiZrPdPt alloys, it was found that the irrecoverable strain decreased in the multi-component alloys including the MEAs and HEAs, except for the LEA, Ti₄₅Zr₅Pd₄₅Pt₅, as shown in Figure 7d. The irrecoverable strain of Ti₄₅Zr₅Pd₂₅Pt₂₅ is also small, which may be due to high MTT. The work outputs of the TiZrPdPt alloys are shown in Figure 7f; they are all between 2–6 J/cm³ and smaller than that of Ti₄₅Pd₅₀Zr₅.

Training was performed for Ti₄₅Zr₅Pd₂₀Pt₂₅Ni₅ and Ti₄₅Zr₅Pd₂₀Pt₂₀Ni₁₀ [49], and perfect recovery was achieved during the thermal cyclic test for approximately 100 cycles under loads of 200, 300, and 400 MPa, whereby the final work outputs were approximately 3.5, 3, and 2 J/cm³, respectively, for Ti₄₅Zr₅Pd₂₀Pt₂₅Ni₅. For Ti₄₅Zr₅Pd₂₀Pt₂₀Ni₁₀, final work outputs of 3 and 1.5 J/cm³ under loads of 200 and 300 MPa were obtained, respectively. The small work output for the large applied stress represents a drastic decrease in the transformation strain. It is necessary to keep the transformation strain of the multi-component alloys during the thermal cyclic test. The thermal cyclic test is considered as a kind of thermal fatigue test and the stable cyclic strain recovery with the constant transformation strain indicates the thermal fatigue life of SMAs and stability as SMA actuators.

The strength of the austenite and martensite phases of some of the multi-component alloys are investigated, and the results are summarized in

Table 7. Strength of the martensite and austenite phases of the multi-element alloys.

Alloy	Test Temp., °C	The Difference from Martensite Transformation Temperature, °C	Detwining Stress, MPa	0.2% Flow Stress, MPa	Ref.
Ti45Pd45Pt5Zr5	-	Af + 30	-	281	[45]
Ti45Pd45Pt5Zr5	425	Mf − 30	246	877	[45]
Ti45Pd35Pt15Zr5	-	Af + 30	-	315	[45]
Ti45Pd35Pt15Zr5	442	Mf − 30	387	1080	[45]
Ti45Pd25Pt25Zr5	-	Af + 30	-	231	[45]
Ti45Pd25Pt25Zr5	509	Mf − 30	436	1205	[45]
Ti45Zr5Pd20Ni5Pt25	628	Af + 30	-	322	[49]
Ti45Zr5Pd20Ni5Pt25	402	Mf − 30	-	1266	[49]
Ti45Zr5Pd20Ni10Pt20	472	Af + 30	-	610	[49]
Ti45Zr5Pd20Ni10Pt20	226	Mf − 30	-	1615	[49]
Ti45Zr5Pd25Pt20Au5	620	Af + 30	-	267	[50]
Ti45Zr5Pd25Pt20Au5	423	Mf − 30	-	1169	[50]
Ti45Zr5Pd25Pt20Co5	568	Af + 30	-	269	[50]
Ti45Zr5Pd25Pt20Co5	294	Mf − 30	-	741	[50]

. The strength of the austenite phases in the tested alloys was between 200 and 300 MPa, and they were similar to those of the ternary alloys. This is because the MTTs of the multi-component alloys are relatively high. Therefore, it is difficult to correlate the strength and strain recovery directly. Thereafter, the temperature dependence of the strength of the martensite and austenite phases was investigated for ternary and multi-component alloys [49]. The strength of the multi-component alloys was higher in both martensite and austenite phases when compared at the same temperature. The solid-solution strengthening effect of the multi-component alloys was more evident when the strength of the austenite phase was compared at the same test temperature of 700 °C. The strength of the multi-component alloys was higher than that of the ternary alloys or LEAs.

7. Change in Microstructure before and after Training

The microstructural changes during training were also investigated in Ti₄₃Zr₇Pd₅₀ and Ti₄₀Zr₁₀Pd₅₀ [36], Ti₄₅Zr₅Pd₃₅Pt₁₅ [45], as well as Ti₅₀Pd₅₀ [58]. A typical twin structure with multiple variants was observed in the heat-treated sample, but variant selection occurred during the thermal cyclic test, and the major variant was predominantly the [010] axis parallel to the compression axis in all observed alloys. It is suggested that a martensite variant is selected to obtain the largest constriction of the lattice during the compression test. The crystal orientation relationship between the B2 and B19 structures is shown as follows:

$${\left[110\right]}_{\mathrm{B}2}//{\left[100\right]}_{\mathrm{B}19}, {\left[010\right]}_{\mathrm{B}2}//{\left[010\right]}_{\mathrm{B}19}, {\left[\overline{1}10\right]}_{\mathrm{B}2}//{\left[001\right]}_{\mathrm{B}19}$$

The lattice parameters of the orthorhombic B19 structure of TiPd were a = 0.459, b = 0.28, and c = 0.484 nm [60]. The lattice parameter of b in the B19 structure was the smallest, and the reorientation of the martensite variant along the [010] direction is, therefore, considered to relax the compressive strain under compressive stress.

8. Potential of High-Temperature Shape Memory Alloys (HT-SMAs)

The work outputs obtained by the thermal cyclic test are plotted for A_f, as shown in Figure 8. The open symbols indicate the alloys with imperfect recovery after a single thermal cycle, while solid symbols indicate perfect recovery after training. The green square indicates the work output of Ni_50.3Ti_29.7Hf₂₀ with nano-sized precipitation, which is developed in the USA and is the most expected HT-SMA [16,19]. It is said that the work output of the commercially used TiNi is between 12–18 J/cm³ for operating temperatures under 100 °C [61]. The work output of Ni_50.3Ti_29.7Hf₂₀ is equivalent to that of TiNi alloys at higher A_f (220 °C) than TiNi. Although the work outputs of some alloys in my group’s research exceed 12 J/cm³ at approximately 200 °C [48], most of them underwent decreasing work outputs with increasing A_f. Nevertheless, it is notable that perfect recovery is achieved at temperatures between 200–600 °C because it is difficult to obtain above 200 °C. The work output obtained at 600 °C was approximately 3 J/cm³.

9. Conclusions

This paper reviewed TiPd- and TiPt-based HT-SMAs. The effects of alloying elements on phase transformation, strain recovery, and strength are shown in ternary TiPd and TiPt alloys. In most cases, MTT decreased through the addition of alloying elements, as well as an increase in the alloying element content. The alloying element improved the strength of the martensite and austenite phases by the solid-solution strengthening effect. However, it is difficult to correlate with solid-solution strengthening and strain recovery. A thermal cyclic test was performed to investigate the strain recovery. The irrecoverable strain decreased with the addition of an alloying element, but it was difficult to achieve perfect recovery, even after repeated thermal cyclic tests (training). Multi-component alloys were also investigated. Notably, MTT can be controlled by a combination of the constituent elements. For example, Zr, Ni, and Co decreased MTT, whereas Pt increased MTT. The effect of alloying elements on the recoverable strain differed from that of the MTT. For example, Ni increased the recoverable strain, while Pt and Co decreased it. The extra solid-solution hardening effect relative to ternary alloys was found in the multi-component alloys. Consequently, the irrecoverable strains were smaller in the multi-component alloys than those in the ternary alloys. In many cases, training was effective in achieving perfect recovery in the multi-component alloys; however, a decrease in the transformation strain also occurred. Multi-component alloys can be good candidates for HT-SMAs; the limitations that need to be overcome entail the suppression of the transformation strain reduction and temperature hysteresis increment.