Engineering Applications and Devices: Session 4
Program Organizers: Alan Pelton, G.RAU Inc.
Thursday 10:20 AM
July 13, 2017
Location: Hyatt Regency Chicago
Session Chair: Eckhard Quandt, University of Kiel
The Use of Generation III Nitinol for Fatigue-Critical Medical Devices: Alan Pelton1; S.M. Pelton1; A. Sorg1; T. J÷rn1; J. Ulmer2; D. Niedermaier2; M.R. Mitchell3; P. Saffari4; 1G.RAU Inc.; 2EuroFlex GmbH Kaiser-Friedrich-Stra▀e; 3Mechanics and Materials Consulting, LLC; 4Engage Medical Device Services, Inc.
During the past decade, manufacturers have accelerated the pace to provide "ultra-clean" Nitinol to the medical device industry. The goal of these metallurgical and processing improvements is to improve in vivo device fatigue behavior. Robertson, et al. (2015) showed that the fatigue strain limit of Generation II Nitinol (40Ám inclusion length, area fraction <0.5%) is significantly greater than Generation I Nitinol (100Ám inclusion length, area fraction >1.5%). These results afford great insight into the performance of even higher purity microstructures required for fatigue-critical neurovascular and structural heart implants. Generation III Nitinol is manufactured from a unique VAR/EBR process with a resultant <7Ám inclusion length and <0.5% area fraction. Comparative fatigue testing with "diamond" coupons demonstrates a 100% improvement in the fatigue strain limit of Generation III compared with Generation II Nitinol. These results will be discussed in terms of finite element models, metallography and fracture analysis.
Microstructure and Mechanical Behaviors of Electron Beam Welded Ti44Ni47Nb9 shape memory alloys: Dan Yang1; Haichang Jiang1; Mingjiu Zhao1; Lijian Rong1; 1Institute of Metal research, Chinese Academy of Sciences
The microstructure and properties of the electron beam welding joints of TiNi binary, TiNiNb ternary and TiNiNbMo quarternary shape memory alloys have been studied. The coarse columnar structure formed in the fusion zone of TiNi binary shape memory alloy and grew towards the center line of the fusion zone with a <100> preferred orientation. The Nb and Mo elements in TiNiNb(Mo) alloy could refine the equiaxed grains of the fusion zone and the Mo would promote the supersaturation of Nb atoms in the matrix.The high cooling speed in the fusion zon led to the strain glass transition in the fusion zone. As a result of cointeraction of Nb and Mo elements, the tensile strength of welding joint of TiNiNbMo alloys at room temperature reached 94% of the base metal, the maximum recoverable strain reached 8.0%, and the strain recovery rate was 97% after 10% pre-deformation.
Elastocaloric Cooling: Mechanisms, Materials, and Systems: Huilong Hou1; Ichiro Takeuchi1; 1University of Maryland, College Park
We are actively developing prototypes of elastocaloric cooling systems using mechanisms based on compression of shape memory alloys. We are compressing bundles of NiTi tubes while heat exchange fluid flows through the tubes. A key component is a heat recovery system implemented to maximize efficiency of heat exchange. We have observed cooling DeltaT of 4.5 K at 70 W, as directly measured in cooled water. A latest prototype was designed to deliver 400 W. We have demonstrated when properly loaded, NiTi tubes can survive up to at least ~ 0.3 million cycles without any degradation in cooling capacity. We carry out combinatorial investigation of ternary and quaternary alloys to identify new compositions with enhanced latent heat and reduced critical stress. Our material optimization processes include developing understanding the effect of atomic structure and microstructure on latent heat and fatigue. This project is funded by DOE ARPA-E and DOE EERE CaloriCool.
Relationship between Applied Stress and Hydrogen-related Fracture Behavior in Martensitic Steel: Yasunari Takeda1; Takashi Yonemura1; Yuji Momotani1; Akinobu Shibata1; Nobuhiro Tsuji1; 1Kyoto university
Hydrogen embrittlement is one of the serious issues for practical use of high strength martensitic steels. There are mainly two modes of hydrogen-related fracture in martensitic steel; one is intergranular fracture at prior austenite grain boundaries, and the other is quasi-cleavage fracture inside prior austenite grains. For understanding the underlying mechanism of hydrogen embrittlement, it is important to elucidate the distinct condition that decides whether quasi-cleavage fracture or intergranular fracture occurs. The present study investigated the relationship between applied stress and fracture mode of hydrogen embrittlement in 8Ni-0.1C martensitic steel. Hydrogen embrittlement properties were evaluated by constant load tests. Hydrogen was introduced into the specimens by electrochemical charging in a 3 % NaCl and 3 g L-1 NH4SCN aqueous solution before and during constant load tests. In the presentation, we will discuss the mechanism of quasi-cleavage fracture and intergranular fracture from the view point of applied stress.