Damping properties of Ti₅₀Ni_50−xCu_x alloys utilizing martensitic transformation

Abstract

The internal friction (IF) and elastic modulus of rigidity were investigated for Ti₅₀Ni_50−xCu_x shape memory alloys for the composition range of 8≦x≦20 by utilizing a low frequency inverted torsion pendulum method. The observed extraordinarily large internal friction and large softening in rigidity are discussed, paying attention to low twinning shears of B19 (orthorhombic) martensite. The attained large internal friction Q⁻¹=0.2 for Ti₅₀Ni₃₀Cu₂₀ at peak temperature, and the shoulder temperature of IF indicate that the alloy system is a good damping material which can be used at temperatures higher than room temperature.

Introduction

The vibration problem is a serious one in the modern world e.g. in the aerospace, transportation, and manufacturing industries as well as in environmental engineering. Thus various damping materials are being developed. Among many damping mechanisms, the one utilizing the martensitic/displacive transformation in shape memory alloys is useful, since the martensitic transformation introduces a high density of mobile twins in martensites [1], and mobile phase boundaries between the parent phase and martensite, which contribute to the internal friction. Among many shape memory alloys, Ti–Ni alloys are attracting much attention from researchers, since these alloys provide high damping as well as high strength, which is crucial for structural materials. The internal friction in Ti–Ni alloys was studied by many researchers [2], [3], [4], [5], [6], [7], and the general trend, with some exceptions (which depend upon thermomechanical treatments), is that IF consists of two peaks: a sharp transient peak due to the martensitic transformation and a very broad peak so-called ‘relaxation type’, whose peak usually situated at around 200 K [8], [9]. The relaxation type peak is usually considered to be due to the twin boundary movement under stress. In practical applications this relaxation type IF is more important than the transient IF due to the martensitic transformation, since the IF in the former is stable at constant temperature, while that in the latter decreases rapidly when the temperature is held at a constant temperature, as Delorme’s theory [10] predicts. Although Ti–Ni alloys are good damping materials, one of the shortcomings is that high damping characteristics are obtained at rather low temperatures (i.e. below room temperatures) as the peak temperature of the broad peak of 200 K indicates. To resolve the problem and to obtain higher damping characteristics, we undertook the present work.

In binary Ti–Ni alloys quenched specimens transform from B2 parent to B19′ (monoclinic) martensite, while Ni-rich specimens, which are aged at proper temperatures (say 673–873 K), transform to R-phase (trigonal) followed by the subsequent transformation to B19′ martensite [11]. The Ni-rich aged specimens usually show lower transformation temperatures, thus are not useful in increasing transformation temperature. In the present investigation we investigated Ti₅₀Ni_50−xCu_x (8≦x≦20) alloys for two reasons. Firstly the transformation mode changes into B2–B19 (orthorhombic) rather than B2–B19′, which may be followed further by B19–B19′ transformation upon further cooling, depending on composition, when the Cu content is increased to higher than x=7.5 [12]. Secondly the first transformation temperature (i.e. that for B2–B19) increases gradually with increasing Cu content [12]. There are some pioneering works [13], [14] on internal friction of Ti₅₀Ni₄₀Cu₁₀ alloy already, which will be referred to in Results and discussions, but in the present work we will investigate much wider composition ranges, as indicated in the above, to understand the damping characteristics of the alloy system.