Author
Hans Conrad
Bio: Hans Conrad is an academic researcher from University of Kentucky. The author has contributed to research in topics: Titanium & Stress relaxation. The author has an hindex of 19, co-authored 49 publications receiving 1683 citations.
Papers
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660 citations
TL;DR: In this article, the stress drop in the initial elastic region of the stress-strain curve and the Joule heating derived from the current-time profile and a reasonable heat transfer coefficient were calculated and found to be about 50-70% of the total stress drops associated with these temperature rises.
181 citations
TL;DR: In this article, the effects of high density, direct current pulses (100 to 1000 A/sq. mm for periods of 55-120 microsecs) on the flow stress determined in an Instron tensile testing machine were studied in a number of polycrystalline metals representing various crystal structures.
155 citations
TL;DR: The theoretical and experimental aspects of the athermal component of flow stress in crystalline solids are reviewed in this paper, where the available data clearly support the existence of a long-range internal stress in deformed crystals.
90 citations
67 citations
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TL;DR: The mechanical properties of nanocrystalline materials are reviewed in this paper, with emphasis on their constitutive response and on the fundamental physical mechanisms, including the deviation from the Hall-Petch slope and possible negative slope, the effect of porosity, the difference between tensile and compressive strength, the limited ductility, the tendency for shear localization, fatigue and creep responses.
3,828 citations
TL;DR: In this paper, the complexity and variety of fundamental phenomena in this material system with a focus on phase transformations and mechanical behaviour are discussed. And the challenges that lie ahead in achieving these goals are delineated.
1,797 citations
TL;DR: In this paper, the recent progress on Ti6Al4V fabricated by three mostly developed additive manufacturing techniques-directed energy deposition (DED), selective laser melting (SLM) and electron beam melting (EBM)-is thoroughly investigated and compared.
1,248 citations
TL;DR: It is shown that oxygen can take the form of ordered oxygen complexes, a state in between oxide particles and frequently occurring random interstitials, which lead to unprecedented enhancement in both strength and ductility in compositionally complex solid solutions, the so-called high-entropy alloys (HEAs).
Abstract: Oxygen, one of the most abundant elements on Earth, often forms an undesired interstitial impurity or ceramic phase (such as an oxide particle) in metallic materials. Even when it adds strength, oxygen doping renders metals brittle1–3. Here we show that oxygen can take the form of ordered oxygen complexes, a state in between oxide particles and frequently occurring random interstitials. Unlike traditional interstitial strengthening4,5, such ordered interstitial complexes lead to unprecedented enhancement in both strength and ductility in compositionally complex solid solutions, the so-called high-entropy alloys (HEAs)6–10. The tensile strength is enhanced (by 48.5 ± 1.8 per cent) and ductility is substantially improved (by 95.2 ± 8.1 per cent) when doping a model TiZrHfNb HEA with 2.0 atomic per cent oxygen, thus breaking the long-standing strength–ductility trade-off11. The oxygen complexes are ordered nanoscale regions within the HEA characterized by (O, Zr, Ti)-rich atomic complexes whose formation is promoted by the existence of chemical short-range ordering among some of the substitutional matrix elements in the HEAs. Carbon has been reported to improve strength and ductility simultaneously in face-centred cubic HEAs12, by lowering the stacking fault energy and increasing the lattice friction stress. By contrast, the ordered interstitial complexes described here change the dislocation shear mode from planar slip to wavy slip, and promote double cross-slip and thus dislocation multiplication through the formation of Frank–Read sources (a mechanism explaining the generation of multiple dislocations) during deformation. This ordered interstitial complex-mediated strain-hardening mechanism should be particularly useful in Ti-, Zr- and Hf-containing alloys, in which interstitial elements are highly undesirable owing to their embrittlement effects, and in alloys where tuning the stacking fault energy and exploiting athermal transformations13 do not lead to property enhancement. These results provide insight into the role of interstitial solid solutions and associated ordering strengthening mechanisms in metallic materials. Ordered oxygen complexes in high-entropy alloys enhance both strength and ductility in these compositionally complex solid solutions.
874 citations
TL;DR: In this article, a concept for the design of novel nanocrystalline/amorphous composites is suggested based on recent theoretical and experimental results regarding the appearance of supermodulus phenomena in composition-modulated layered heterostructures and on a simple thermodynamic criterion for segregation in ternary nitride systems.
871 citations