TY - BOOK

T1 - Atomic-scale characterization of severely deformed nanocrystalline alloys

AU - Guo, Jinming

N1 - no embargo

PY - 2018

Y1 - 2018

N2 - The usually immiscible composites in thermodynamic equilibrium condition can be forced into a metastable supersaturated state through deformation, which has received enormous scientific attention for the last decades. Severe plastic deformation processing has unique advantages, inducing dissolution of second phases and considerable grain refinement down to nanometer range. The as-generated materials have various processing-induced nanostructural features such as deformation twins, non-equilibrium grain boundaries, dislocation substructures, vacancy agglomerates, solute segregation and clusters. By specifically designing and controlling these features, many nanocrystalline materials show extraordinary mechanical and electrical properties compared to their coarse-grained counterparts. Despite the extensive studies on nanocrystalline materials, the underlying phenomena are still not well understood at an atomistic scale. For example, the deformation process, mechanism of forced intermixing and influence of light elements on microstructure and properties as well as thermal stabilities are barely reported in literature. To understand these phenomena in nanocrystalline alloys, this thesis mainly include 2 parts on the cases of Cu-Cr and Cu-Fe systems. In the first part, a dual-phase Cu-Cr composite with different initial phase hardness and ductility was deformed with controllable strains by high pressure torsion. Shear deformation processes were observed at the atomic scale to get insights into the grain refinement and intermixing in the Cu-Cr system at the early stage of deformation. It was found that the hard Cr phase underwent elongation until reaching extremely fine lamellar structures, embedded with 1 – 2 nm thick Cu layers at the phase boundaries. The Cr lamellae then necked and finally fractured via dislocation multiplication, forming almost equiaxed grains with saturated average size of 13.7 nm and reaching stable hardness of 480 – 495 HV after deformation to a strain of 1360. In addition, the dissolved Cu was surprisingly observed as nanoclusters with dimensions of about 2 nm inside the Cr grains, with Cu maintaining a body-centered cubic structure. The phase fraction change associated with Cu dissolution into Cr matrix during continuous deformation was measured and accurately calculated, indicating a negative exponential phase change mode. In the second part, Cu-Fe was mechanically alloyed directly from blended powders and from vacuum arc-melted bulk respectively which contain different contents of oxygen impurity, by means of high pressure torsion. All investigated compositions formed single-phase face-centered cubic supersaturated solid solutions after extremely straining, reaching strain-saturated states. The comparative investigations on a series of Cu-Fe nanocrystalline alloys reveal that oxygen facilitates grain refinement and lattice expansion, which in turn gives rise to higher hardness and poor ductility. Theoretical calculations indicate that the higher twin density in powder samples can be attributed to the reduction in stacking fault energy by oxygen, resulting in a mixed deformation mechanism. The behavior of oxygen during in-situ heating of highly-strained Cu-Fe powder alloys was investigated. Contrary to expectations, oxide formation occurred prior to the decomposition of the metastable Cu-Fe solid solution. This oxide formation commenced at relatively low temperatures, forming nano-sized clusters of firstly CuO and later Fe2O3. In summary, this thesis revealed the related deformation-induced phenomena and mechanisms at the atomic scale by taking use of the advanced transmission electron microscopy, and gained insights into the oxygen’s behavior in severely deformed nanocrystalline alloys. The findings provided the direct observations of oxide formation and offered a pathway for the design of nanocrystalline materia

AB - The usually immiscible composites in thermodynamic equilibrium condition can be forced into a metastable supersaturated state through deformation, which has received enormous scientific attention for the last decades. Severe plastic deformation processing has unique advantages, inducing dissolution of second phases and considerable grain refinement down to nanometer range. The as-generated materials have various processing-induced nanostructural features such as deformation twins, non-equilibrium grain boundaries, dislocation substructures, vacancy agglomerates, solute segregation and clusters. By specifically designing and controlling these features, many nanocrystalline materials show extraordinary mechanical and electrical properties compared to their coarse-grained counterparts. Despite the extensive studies on nanocrystalline materials, the underlying phenomena are still not well understood at an atomistic scale. For example, the deformation process, mechanism of forced intermixing and influence of light elements on microstructure and properties as well as thermal stabilities are barely reported in literature. To understand these phenomena in nanocrystalline alloys, this thesis mainly include 2 parts on the cases of Cu-Cr and Cu-Fe systems. In the first part, a dual-phase Cu-Cr composite with different initial phase hardness and ductility was deformed with controllable strains by high pressure torsion. Shear deformation processes were observed at the atomic scale to get insights into the grain refinement and intermixing in the Cu-Cr system at the early stage of deformation. It was found that the hard Cr phase underwent elongation until reaching extremely fine lamellar structures, embedded with 1 – 2 nm thick Cu layers at the phase boundaries. The Cr lamellae then necked and finally fractured via dislocation multiplication, forming almost equiaxed grains with saturated average size of 13.7 nm and reaching stable hardness of 480 – 495 HV after deformation to a strain of 1360. In addition, the dissolved Cu was surprisingly observed as nanoclusters with dimensions of about 2 nm inside the Cr grains, with Cu maintaining a body-centered cubic structure. The phase fraction change associated with Cu dissolution into Cr matrix during continuous deformation was measured and accurately calculated, indicating a negative exponential phase change mode. In the second part, Cu-Fe was mechanically alloyed directly from blended powders and from vacuum arc-melted bulk respectively which contain different contents of oxygen impurity, by means of high pressure torsion. All investigated compositions formed single-phase face-centered cubic supersaturated solid solutions after extremely straining, reaching strain-saturated states. The comparative investigations on a series of Cu-Fe nanocrystalline alloys reveal that oxygen facilitates grain refinement and lattice expansion, which in turn gives rise to higher hardness and poor ductility. Theoretical calculations indicate that the higher twin density in powder samples can be attributed to the reduction in stacking fault energy by oxygen, resulting in a mixed deformation mechanism. The behavior of oxygen during in-situ heating of highly-strained Cu-Fe powder alloys was investigated. Contrary to expectations, oxide formation occurred prior to the decomposition of the metastable Cu-Fe solid solution. This oxide formation commenced at relatively low temperatures, forming nano-sized clusters of firstly CuO and later Fe2O3. In summary, this thesis revealed the related deformation-induced phenomena and mechanisms at the atomic scale by taking use of the advanced transmission electron microscopy, and gained insights into the oxygen’s behavior in severely deformed nanocrystalline alloys. The findings provided the direct observations of oxide formation and offered a pathway for the design of nanocrystalline materia

KW - Hochverformung

KW - nanokristalline Legierungen

KW - Transmissionselektronenmikroskopie

KW - Auflösungsprozess

KW - Zwangslösung

KW - in-situ Heizversuch

KW - Einfluss von Sauerstoff.

KW - Severe plastic deformation

KW - nanocrystalline alloys

KW - transmission electron microscopy

KW - dissolution process

KW - forced chemical intermixing

KW - grain refinement

KW - in-situ heating

KW - oxygen’s effect.

M3 - Doctoral Thesis

ER -