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Orthopedic implants first started out as an all-metal hip joint replacement. However, poor design and machinability as well as unsatisfactory surface finish subjected the all-metal joint replacement to being superseded by a polyethylene bearing. Continued improvement in manufacturing techniques together with the reality that polyethylene wear debris can cause hazardous reactions in the human body has brought about the revival of metal-on-metal (MOM) hip joints in recent years. This has also led to a relatively new research area that links tribology and corrosion together. This article aims at reviewing the commonly used tribochemical methods adopted in the analysis of tribocorrosion and putting forward some of the models and environmental factors affecting the tribocorrosive behavior of CoCrMo alloys, a widely-used class of biomaterial for orthopedic implants. [ABSTRACT FROM AUTHOR]

One major hindrance that alloy design for additive manufacturing (AM) faces nowadays is hot tearing. Contrary to the previous works which either try to reduce solidification range or introduce grain refinement, the current work presents a new approach of employing segregation engineering to alter the residual stress states at the interdendritic and grain boundary regions and consequently prevent hot tearing. Here, in situ Al alloying is introduced into an existing hot-cracking susceptible high-entropy alloy CoCrFeNi. It is found that within a certain range of compositions, such as Al 0.5 CoCrFeNi, the hot crack density was drastically decreased. During the solidification of this specific alloy composition, Al is firstly ejected from the primary dendritic face-centred cubic (FCC) phase and segregates into the interdendritic regions. Spinodal decomposition then occurs in these Al-enriched regions to form the ordered B2 NiAl and disordered body-centred cubic (BCC) Cr phases. Due to the higher molar volume and lower homologous temperatures of these B2/BCC phases, the inherent residual strain is accommodated and transformed from a maximum 0.006 tensile strain in CoCrFeNi to a compressive strain of ~0.001 in Al 0.5 CoCrFeNi. It is believed that this grain boundary segregation engineering method could provide a new pathway to systematically counteract the hot tearing problem in additive manufacturing of metals and alloys, using available thermodynamic and kinetic database information. Image, graphical abstract [ABSTRACT FROM AUTHOR]

• The lamellar and cellular interfaces are semi-coherent obeying the KS and NW relationships. • The KS and NW relationships allow the slip transfer of dislocations. • The development of SF networks and LC locks contributes to sustained strain-hardening behavior. • The nano-bridging channels are the main microstructural origin for enhanced ductility. Limited tensile ductility usually restricts the practical applications of new classes of high-strength materials in many industrial fields. Therefore, in-depth understanding of the work hardening behavior and its underlying plastic deformation mechanism are critical for the newly developed high-entropy alloys (HEAs). In this work, a geometric atomistic model of face-centered cubic (FCC)/ordered body-centered cubic (BCC (B2)) interfaces and the evolution of dislocation substructures have been investigated to explore the microstructural origins of work hardening responses for two additively manufactured AlCoCrFeNi 2.1 eutectic high-entropy alloys (EHEAs) with the respective lamellar and cellular microstructures. Unlike the lamellar interphase interfaces with the most classical Kurdjumov-Sachs (KS) FCC-BCC relationship of { 111 } FCC ∥ { 110 } B 2 〈 011 〉 FCC ∥ 〈 111 〉 B 2 , the Nishiyama-Wassermann (NW) relationship, namely { 111 } FCC ∥ { 110 } B 2 〈 011 〉 FCC ∥ 〈 001 〉 B 2 , is observed to be dominant on the cellular interphase interfaces. Furthermore, our intermittent high-resolution transmission electron microscopy (HR-TEM) results directly show that the deformation of lamellar AlCoCrFeNi 2.1 alloy first proceeds with massive stacking faults (SFs) and then dislocation walls developed across phases interfaces, due to the effective dislocation transfer capability of lamellar boundaries. The large uniform elongation of the cellular AlCoCrFeNi 2.1 alloy is attributed to the stable and progressive strain-hardening mechanism that is stemmed from the activated multiple slip systems, deformation-induced SF networks, and the associated Lomer-Cottrell locks in the middle and later stages of plastic deformation. Moreover, the nano-bridging of FCC cells in the 3D-printed microstructure provides unique channels for dislocation movement, which offsets the "blocking effect" of cellular boundaries and thus suppresses the pre-mature fracture. [ABSTRACT FROM AUTHOR]