Infrared thermography applied for experimental investigation of thermomechanical couplings in Gum Metal

Abstract

Results of initial investigation of thermomechanical couplings in innovative β-Ti alloy called Gum Metal subjected to tension are presented. The experimental set-up, consisting of testing machine and infrared camera, enabled to obtain stress-strain curves with high accuracy and correlate them to estimated temperature changes of the specimen during the deformation process. Both ultra-low elastic modulus and high strength of Gum Metal were confirmed. The infrared measurements determined average and maximal temperature changes accompanying the alloy deformation process, allowed to estimate thermoelastic effect, which is related to the alloy yield point. The temperature distributions on the specimen surface served to analyze strain localization effects leading to the necking and rupture. MotivationGum Metal, a new class of β-type multifunctional titanium alloys, has attracted considerable attention in the past decade due to its outstanding mechanical properties. The underlying mechanisms of its excellent performance are still unclear. The literature reports have not covered thermographic analysis of the alloy so far. Since infrared techniques and studies of thermomechanical couplings are a great tool for better understanding of mechanical behavior and phenomena occurring in materials the present research aimed at conducting thermomechanical and thermographic investigation of Gum Metal during tension. IntroductionGum Metal is a β-type Ti-based superalloy developed in Japan and reported first in 2003 [1]. Three electronic parameters were proposed for providing β phase stability: (1) a compositional average valence number (e/a) of about 4.24; (2) a bond order (Bo value) of about 2.87; and (3) a ""d" electron-orbital energy level (Md value) of about 2.45 eV. In addition, the oxygen concentration in the alloy is restricted to a range between 0.7 and 3.0 at.% [1]. Typical compositions meeting these requirements are: Ti-12Ta-9Nb-3V-6Zr-1.5O or Ti-20Nb-3.5Ta-3.4Zr-1.2O. The fabrication route of Gum Metal includes powder sintering technique and processing by cold rolling. Both the fabrication route and the chemical composition strongly influence the superalloy characteristics. Gum Metal is known by the following outstanding properties: ultra-low elastic modulus with very high strength, rubber-like Poisson"s ratio, superelastic nature -one digit higher in nonlinear elastic deformation (≈2.5%) when compared to other metallic materials, super-plastic nature allowing cold plastic working without hardening up to ≈90%, very low linear coefficient of thermal expansion (similar to Invar) and a constant elastic modulus in the temperature range from -200°C to +250°C (similar to Elinvar) [1][2][3][4]. The density of Gum Metal equals around 5.6 g/cc which locates it between steels and aluminium alloys. The amount of oxygen in Gum Metal significantly changes its microstructure, mechanical and thermomechanical properties [7,8,11,15]. Deformation mechanisms occurring in Gum Metal are unconventional and still unclear. First they wer...

Development of Strain Localization in a Beta-Titanium Alloy Gum Metal Analyzed by Infrared Camera and Digital Image Correlation for Various Strain Rates

“…Increase in the specimen temperature after it drops due to thermoelastic effect, started from point A ( Figure 1b) revealed dissipative character of the process. This takes place in the range of reversible deformation [1,2] and is caused by a stress-induced phase transformation of martensite-like orthorhombic α″ nanodomains [3,4]. From the points B and C the slope of ΔT vs. ε dependence increases significantly.…”

Thermomechanical Studies of Yielding and Strain Localization Phenomena of Gum Metal under Tension

“…are in line with the scheme shown in Figure 4 a and allowed us to indicate the material yield limit with a high accuracy [ 20 , 21 , 22 , 23 ]. In the case of Gum Metal, even the initial experiments conducted by the authors [ 24 , 34 ] showed that the maximal drop in temperature occurred significantly earlier (stage I) than the limit of its recoverable deformation (stages I and I ’ ) as demonstrated in Figure 4 b.…”

“…The temperature changes accompanying the material’s deformation enable investigators to determine its yield point [ 19 , 20 , 21 ], analyze the nucleation and development of the strain localization [ 22 , 23 , 24 ], and estimate the energy storage [ 25 ]. Thus, the thorough determination of the effects of thermomechanical couplings can contribute to the knowledge about the behavior of new materials [ 20 , 23 , 26 , 30 , 31 , 32 , 33 , 34 ]. This section concerns analysis of the effects of thermomechanical couplings monitored in the initial stage of Gum Metal loading.…”

Estudo do comportamento termomecânico do aço inoxidável AISI 430 por meio da termografia por infravermelho

“…Nessa etapa, a termografia por infravermelho revelou claramente um aumento mais acentuado na temperatura para todas as taxas de deformação. Segundo GOLASINSKI et al [19], na região III, a deformação progride de forma acelerada com o aumento da taxa de deformação e o processo tende a ser adiabático. Sob tais condições, a geração de calor devido à deformação, principalmente após a estricção localizada, foi mais rápida que a difusão de calor em toda a amostra, resultando em um aumento de temperatura substancialmente maior para os experimentos com taxas de deformação mais altas.…”