S.F Hsieh

A Study on Ternary Ti-rich TiNiZr Shape Memory Alloys

Abstract The martensitic transformation in Ti50.5-XNi49.5ZrXand Ti51.5-XNi48.5ZrX alloys (X = 0–25 at.%) was studied by using thermomechanical treatments. These alloys have a B2↔B19′ transformation sequence, and their transformation peak temperature M* can be raised to 50–450°C by different additions of Zr. Although a great many second-phase particles exist around (Ti,Zr)Ni grain boundaries, these alloys still exhibit ≥80% shape-memory recovery. Thermal cycling can depress the M* temperature more significantly in the Ti41.5Ni48.5Zr10 alloy than in the Ti40.5Ni49.5Zr10 alloy in the first ten cycles, owing to the former’s having greater hardness and more second-phase particles. Martensite stabilization can be induced by cold rolling at room temperature for Ti-rich ternary TiNiZr alloys. The strengthening effects of cold rolling and thermal cycling on Ms temperatures of these alloys were found to follow the expression Ms = T0-KΔσy, in which K values are related to the as-annealed hardness of these alloys. For the study of 400°C aging effects, the martensite stabilization appearing in the Ti26.5Ni48.5Zr15 alloy may be due to the pinning effect on the interfaces of martensite plates by the point defects.

A study on lattice parameters of martensite in Ti50.5−xNi49.5Zrx shape memory alloys

Abstract Ti50.5−xNi49.5Zrx (x=5∼20 at.%) shape memory alloys exhibit the characteristics of B2↔B19′ martensitic transformation and their transformation temperatures increase linearly with increasing Zr content. The lattice parameters of B19′ martensite in these alloys are determined by XRD, SADP of TEM and the Rietveld method. Experimental results show that all of the lattice parameters a and c, monoclinic angle β and unit cell volume V increase, but lattice parameter b decreases with increasing Zr content in Ti50.5−xNi49.5Zrx alloys. The results of the Rietveld method indicate that the martensite of Ti50.5−xNi49.5Zrx with x≤10 at.% has a structure similar to that of Ti50Ni50, but that of Ti50.5−xNi49.5Zrx with x≥15 at.% may change in structure.

Martensitic transformation of quaternary Ti50.5−XNi49.5ZrX/2HfX/2 (X=0–20 at.%) shape memory alloys

Abstract Martensitic transformation of Ti 50.5− X Ni 49.5 Zr X /2 Hf X /2 quaternary alloys ( X =0–20 at.%) is studied by different thermo-mechanical treatments. These alloys have one-stage B2↔B19′ transformation and exhibit ≥80% shape memory recovery. Their DSC forward transformation peak M * can be raised from 50°C to 323°C with transformation hysteresis being slightly larger than that of Ti 50.5− X Ni 49.5 Zr X alloys. In the early 10 cycles, thermal cycling can depress the M * temperature more significantly in Ti 35.5 Ni 49.5 Zr 7.5 Hf 7.5 than in Ti 35.5 Ni 49.5 Zr 15 due to the former alloy having higher hardness in the matrix. Martensite stabilization can be induced by cold rolling at room temperature. The strengthening effects of cold rolling and thermal cycling on Ms temperature are found to follow Ms= T o − K Δ σ y , in which K values are related to the as-annealed hardness of these alloys. Ti 30.5 Ni 49.5 Zr 10 Hf 10 alloy, aged in martensite phase can cause the phenomenon of thermal-induced martensite stabilization.

A study on a Ti52Ni47Al1 shape memory alloy

The Ti52Ni47Al1 alloy has 16% volume fraction Ti2Ni particles in the B2 matrix with Ti2Ni particles having a higher Al content than the B2 matrix. Transformation temperatures M* and A* of this alloy are lower than those of the Ti51Ni49 alloy due to the solid solution of the Al atoms. M* and A* decrease with increasing aging time at 400°C because the Al atoms diffuse slightly from the Ti2Ni to the B2 matrix. The hardness increment of this alloy is more than that of the Ti51Ni49 alloy under the same degree of cold rolling. At the same time, M* and A* of this alloy can be more depressed by thermal cycling than those of the Ti51Ni49 alloy, especially in the first ten cycles. All of these features result from the fact that this alloy has a higher inherent hardness due to the solid solution of the Al atoms. This also causes the R-phase transformation to be more easily promoted by both cold rolling and thermal cycling in this alloy. The strengthening effects of cold rolling and thermal cycling on the M* (Ms) temperature of this alloy follows the expression Ms = T0−KΔσy, in which K values are affected by different strengthening processes. It is found that the higher the inherent hardness of the TiNi and TiNiX alloys, the higher the K values they have.

A study on the nickel-rich ternary Ti–Ni–Al shape memory alloys

The transformation sequence and hardening effects of 400 °C aged Ti47.5Ni50.65Al1.85 and Ti49.5Ni50.13Al0.37 shape memory alloys have been investigated by electrical resistivity tests, internal friction measurements, hardness tests and TEM observations. Both solution hardening and precipitation hardening are found to occur in these alloys. The hardening effects of Ti47.5Ni50.65Al1.85 alloy are obvious and much higher than those of Ti49.5Ni50.13Al0.37 alloy due to the former having the larger Ni/Ti ratio and a higher Al solute content in its matrix. The transformation sequence of 400 °C aged Ti47.5Ni50.65Al1.85 alloy shows B2↔R-phase only for an ageing time of more than 10 h and that of 400°C aged Ti49.5Ni50.13Al0.37 alloy shows the sequence B2↔R-phase↔B19′ or B2↔R-phase with different ageing times. All of these characteristics are associated with Ti11Ni14 precipitates during the ageing process. These aged Ti–Ni–Al alloys exhibit very good shape memory effects, in which the maximal shape recovery occurs at the peak of hardness.

Transformation temperatures and second phases in Ti–Ni–Si ternary shape memory alloys with Si≤2 at.%

Abstract Effects of Si on transformation temperatures and second phases in Ti 50− x Ni 50 Si x , Ti 50 Ni 50− x Si x and Ti 51 Ni 49− x Si x shape memory alloys (SMAs) with x =1, 2 at.% are investigated. Three different second-phase particles located at grain boundaries are observed. They are χ -phase particles (Ti 5 Ni 4 Si 1 ), λ 1 -phase particles (Ti 2 Ni 3 Si 1 ) and Ti 2 (Ni, Si) particles. In addition to the formation of second phases, a small amount of Si remained in solid solution in the matrix of Ti–Ni–Si ternary SMAs. Experimental results show that, in the matrix, the effects of Ni+Si in combination on transformation temperatures of Ti–Ni–Si ternary SMAs are similar to Ni effects on those of as-quenched TiNi binary SMAs.

Martensitic transformation of a Ti-rich Ti51Ni47Si2 shape memory alloy☆

Abstract Ti 51 Ni 47 Si 2 alloy is similar to Ti 51 Ni 49 alloy with nearly equal transformation temperatures and having B2↔19′ martensitic transformation. Despite the existence of many second phase particles, this alloy still exhibits good shape recovery. The B19′ martensite structure in Ti 51 Ni 47 Si 2 alloy is calculated from SADPs as a =0.284 nm, b =0.412 nm, c =0.468 nm and β =98°. Martensitic transformation temperatures decrease with increasing aging time at 400°C. The hardness increment and transformation temperature depression of Ti 51 Ni 47 Si 2 alloy are more than those of Ti 51 Ni 49 alloy under the same degree of cold rolling and the same number of thermal cycles due to the former alloy having a higher inherent hardness from Si atoms solid-soluted in TiNi alloy. The strengthening effects of cold rolling and thermal cycling on M s temperature of Ti 51 Ni 47 Si 2 alloy follow the expression of M s = T o − K Δ σ y .

Lattice parameters of martensite in Ti50.5-xNi49.5Zrx/2Hfx/2 quaternary shape memory alloys

Abstract Ti50.5−xNi49.5Zrx/2Hfx/2 (x=10–20 at.%) shape memory alloys exhibit the characteristics of B2↔B19′ one-stage martensitic transformation. The lattice parameters of B19′ martensite in these alloys are determined by XRD, SADPs of TEM and Rietveld methods. Experimental results show that lattice parameters a and c, monoclinic angle β and unit cell volume V increase, but the lattice parameter b decreases with increasing x-content in Ti50.5−xNi49.5Zrx/2Hfx/2 alloys. The results of Rietveld method indicate that the martensite of Ti50.5−xNi49.5Zrx/2Hfx/2 with x=10–20 at.% has a structure similar to that of Ti50Ni50. The parameters a, c and β of B19′ martensite in Ti50.5−xNi49.5Zrx/2Hfx/2 alloys are slightly larger, but b is slightly smaller than those in Ti50.5−xNi49.5Zrx alloys for the same x-content. The B19′ martensite of Ti30.5Ni49.5Zr10Hf10 alloy is a monoclinic structure with a=0.3068 nm, b=0.4062 nm, c=0.4882 nm and β=101.2° and has a substructure of (001)M compound twin.