Original Title: Failure Analysis Site Direct Attack — — Analysis of Fracture Causes of Nickel-Titanium Wire With the development of modern medicine, nickel-titanium alloy has been widely used in human body minimally invasive implant medical devices in the medical field because of its excellent biocompatibility, radiopacity, non-impact of nuclear magnetic resonance, mechanical properties, corrosion resistance, shape memory effect and superelasticity. Background of the event A batch of nickel-titanium alloy implantable medical devices were broken during X-ray examination after production and assembly. The production process of the nickel-titanium wire comprises the steps of braiding, heat treatment, setting, cleaning, coating, heat setting, self-inspection, and X-ray inspection. Therefore, a series of inspection methods are used to analyze the fracture causes of nickel-titanium wire, and the corresponding improvement measures are put forward. Physical and chemical test The chemical composition analysis, mechanical property test and cleanliness test were carried out on the nickel-titanium wire in the nickel-titanium alloy product. The results are shown in Table 1-3. It can be seen that the three indexes meet the technical requirements of GB 24627 2009. Table 1 Chemical composition (mass fraction) of nickel-titanium wire Table 2 Mechanical Property Test Results of Ni-Ti Wire Expand the full text Table 3 Test Results of Nickel Titanium Wire Cleanliness 1 Macro analysis The fracture of nickel-titanium wires in this batch of nickel-titanium alloy products mostly occurred near the small wave peak or trough, and there were many corrosion pits distributed in the crack source area and the surface near the non-fracture small wave peak or trough; yellow substances were found in the product envelope area; the fracture was a transverse flat fracture without obvious plastic deformation,nickel titanium wire, belonging to macroscopic brittle fracture, as shown in Figure 1. Fig. 1 Macroscopic morphology of fractured Ni-Ti wire 2 Microscopic analysis of fracture The fractured NiTi wire was immersed in ethanol solution and cleaned by ultrasonic vibration, and then the fracture and crack source area were observed by JSM-6510 scanning electron microscope. The crack source of the fractured nickel-titanium wire starts from the corrosion pit on the surface of the wire, as shown in Fig. 2; the crack propagation zone is a cleavage fracture, and the instantaneous fracture zone is a flat dimple, as shown in Fig. 3; there are many corrosion pits on the side of the crack source and on the surface of the wire near the small wave peak or trough, as seen in Fig. 4; There are obvious gray mud pattern corrosion products in the corrosion pits far away from the fracture,titanium filler rod, as shown in Fig. 5, with typical stress corrosion characteristics. Fig. 2 Micro-morphology of NiTi wire fracture Fig. 3 SEM morphology of Ni-Ti wire fracture 3. Analysis of chemical composition in microarea Energy spectrum (EDS) analysis was carried out on the crack source area of the fractured nickel-titanium wire, the gray corrosion products in the surface corrosion pits near the small wave peak or trough, the contaminated yellow substances on the envelope, the rusty substances on the braided die rod and the heat-shrinkable tube used for heat setting. The results are shown in Fig. 6 ~ 8. Fig. 4 Microscopic morphology of corrosion pits near the fracture of nickel-titanium wire Fig. 5 Microscopic morphology of corrosion pits of NiTi wire far away from fracture Fig. 6 EDS spectrum of crack source area of nickel-titanium wire Fig. 7 EDS spectrum of NiTi wire corrosion pit and coated yellow substance Fig. 8 EDS spectra of rust and heat-shrinkable tube on braided die rod The main chemical components of the yellow substance in the visible crack source area, corrosion pits and coating are bromine, nickel, titanium and a small amount of iron, manganese, chromium, vanadium, calcium, sodium, 3d titanium wire ,titanium exhaust tubing, chlorine, sulfur and other elements. Bromine comes from heat-shrinkable tubes, while iron, manganese, chromium, vanadium, calcium, sodium, chlorine and sulfur mostly come from rusty substances in moulds. Analysis and discussion 1 Fracture characteristics The fracture of the nickel-titanium wire in the failed product started from the corrosion pits on the surface of the nickel-titanium wire, the crack source area of the fracture was corrosion pits, the crack propagation area was cleavage, the instantaneous fracture area was flat dimples, and the fracture was brittle fracture.The corrosion products in the corrosion pits had the characteristics of stress corrosion mud pattern, so it could be judged that the fracture of the nickel titanium wire was a brittle fracture caused by stress corrosion cracking. 2 Stress source of stress corrosion cracking (internal cause) By analyzing the production process of the product, when weaving the wave crest or trough area, the way of heating and weaving is adopted, and the hot air gun is used to eliminate the wave crest stress, but when weaving the small wave crest or trough area, due to the limitation of the mold, the heating effect is slightly poor, and the stress can not be well eliminated by heating and weaving. As a result, there is a large residual tensile stress at the thermal stress boundary near the small wave peak or small wave trough (that is, the corrosion pit distribution area of the broken nickel-titanium wire).
3 Medium source of stress corrosion cracking (external cause) This batch of broken wire products is produced from August to October, which is a period of marine climate with high environmental humidity, and the metal weaving mould is easy to rust. It is observed on the spot that the mould rod has different degrees of rust. Due to the corrosion of the die rod, the braided nickel-titanium wire has different degrees of pollution, and the corrosion pollution can not be completely eliminated when the product is cleaned after heat treatment and shaping. In the process of coating heat setting, because the heat-shrinkable tube contains bromine element, after high temperature vaporization, it forms a composite corrosive medium with the rust substance attached to the surface of the product. 4 Simulated HBr stress corrosion test As is known to all, HF has a strong corrosive effect on nickel-titanium wires. In order to explore the influence of HBr on nickel-titanium wires, the following tests were carried out in a simulated stress corrosion environment: the tensile stress existing in the nickel-titanium wire was artificially created, and then the nickel-titanium wire was placed in HBr solution for 1 H, taken out and placed for 4 H, and then broken. The fracture was microscopically observed, and the results are shown in Figure 9. Fig. 9 Fracture morphology of nickel-titanium wire after HBr stress corrosion test It was found that HBr also had stress corrosion effect on NiTi wire, and the fracture morphology of NiTi wire was similar to that of the broken wire, which proved that the batch of products had stress corrosion cracking in the area where there were small wave peaks or troughs in the medium environment containing bromine compounds. Conclusions and recommendations During the braiding process, the stress of the nickel-titanium wire was not well eliminated, and the corrosion medium was introduced into the braiding die rod and the heat-shrinkable tube, which led to the stress corrosion cracking of the nickel-titanium wire in the nickel-titanium implant device. It is suggested that the small wave crest of the braided die rod should be designed into a large wave crest to better relieve the stress in the production process; the braided die rod should be dried in time, and the rusty substances on the surface of the braided pattern rod should be cleaned regularly to prevent the introduction of corrosive media. Author: Shi Xiaoli,titanium plate gr7, Senior Engineer, Shenzhen Leading Medical Service Co., Ltd. Return to Sohu to see more Responsible Editor:. yunchtitanium.com