Abstract
Technically optimizing the processing cleanliness of Metal-Injection-Molded titanium alloys (Ti-MIM) is not economically feasible. This problem is common in the materials processing field. In the search for an alternative approach, the work tries to achieve superior high-cycle fatigue (HCF) performance while tolerating very high impurity levels. The concept arose from the large tolerance of β-class Ti-alloys for oxygen-solutes and the feasibility to mitigate detrimental effects of carbide-inclusions, under monotonous loading. In this paper, MIM β Ti-Nb-Zr biomaterials for fatigue-critical applications were deliberately produced with very high O-level and normal/very high C-levels. The impurity-resistant Ti-biomaterials exhibit superior HCF endurance limits beyond 600 MPa irrespective of processing cleanliness, being significantly higher than those of the α-β Ti-reference alloys produced with tight restrictions on impurity levels. This superior fatigue performance while tolerating amounts of impurities stems from the “weak”-microstructural features insensitive to impurities and increased resistance of the Ti-matrix against fatigue small-cracks. Furthermore, a conditional fatigue duality triggered by two competing mechanisms of crack initiation in certain cases, initiating at microscale pore α-platelets and at large pore TiC-inclusions can occur. The success of the present alloy-process development might greatly relax the processing demands for active metals.
Highlights
The field of material engineering has long resorted to more robust and advanced technologies to improve processing cleanliness or reduce impurity uptake
The mechanical property of active materials insensitive to impurities is capable of intrinsically overcoming low-cost and high-performance trade-off. This is economically important for some commercial processing technologies where the cost-effectiveness is an essential element, such as Metal-Injection-Molding (MIM) [1]
With regards to processing cleanliness, carbon and oxygen uptakes that are typical impurities originate mostly from starting powder, binder decomposition, and sintering furnace; the resulting impurity content of sintered parts is so high that MIM in essence has low cleanliness [3]
Summary
The field of material engineering has long resorted to more robust and advanced technologies to improve processing cleanliness or reduce impurity uptake. The mechanical property of active materials insensitive to impurities is capable of intrinsically overcoming low-cost and high-performance trade-off. This is economically important for some commercial processing technologies where the cost-effectiveness is an essential element, such as Metal-Injection-Molding (MIM) [1]. Due to its advantages of high mechanical performance, mass production capability, ability to form sophisticated shapes, and cost-savings, MIM technology to date has been used in industrialized applications in TMT (technology, media, and telecom), automotive, medical fields and so forth [2]. With regards to processing cleanliness, carbon and oxygen uptakes that are typical impurities originate mostly from starting powder, binder decomposition, and sintering furnace; the resulting impurity content of sintered parts is so high that MIM in essence has low cleanliness [3]. The nature of MIM technology currently confines its commercialization to ferrous metals, e.g., carbon steel, whereas great challenges still remain for impurityvulnerable active metals such as Ti, Nb, Zr, etc
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