Effect of high pressure torsion processing on precipitate containing alloys

includes bibliographical references and index

PhD; 2023; Department of Materials Engineering

The effect of severe plastic deformation (SPD) on enhancing the strength – ductility synergy
in pure metals and single-phase alloys has been studied in great details since past three decades.
However, the metallurgical mixing resulting from SPD processes has come into focus only
recently. Most of the investigations pertaining to compositional modulation achieved during
SPD has been performed for binary alloys. The effect of SPD on multicomponent systems,
containing precipitate phases, has rarely been reported in literature. Hence, a systematic study
on the effect of SPD on the structure -property relationship of different alloy systems, that are
likely to undergo microstructural changes along with changes in micro-chemistry, is highly
desirable.
The present thesis focuses on severe plastic deformation by high pressure torsion (HPT)
of precipitate containing two ductile alloy systems, namely the niobium-zirconium alloy (BCC)
system and the aluminum-lithium alloy (FCC) system. The objectives of the investigation are
to examine the effect of severe plastic deformation on the pre-existing precipitates, and to
investigate the status of precipitation after large deformation. The first two chapters of the
thesis deals with a rigorous survey of literature in the domain of investigation, motivation, and
scope of the present thesis as well as the details of experimental methodology.
In the first part of the investigation, presented in chapter 3, pure Nb (as a control
material), Nb-1Zr and Nb-1Zr-0.1C (all in wt. pct.) were subjected to severe plastic
deformation by high pressure torsion up to a shear strain g = 90 (5 rotations). It was observed
that the composition of carbide precipitates was modified due to dissolution of the existing Nbrich carbides and their reprecipitation into Nb-lean carbides. The strength of alloys was
measured as function of incremental plastic strain. The contributions from various
strengthening mechanisms were subsequently partitioned by utilizing the results obtained from
multiscale characterization techniques, namely, X-ray diffraction, scanning electron
microscopy, transmission electron microscopy and atom probe tomography. The primary
strengthening contributor in Nb alloys was found to be the substructures formed by
dislocations.
The Nb-1Zr alloy is also a material for nuclear reactors. However, this alloy, when
irradiated in the annealed state shows poor mechanical properties due to the generation of
excess point and line defects during irradiation, that causes irradiation embrittlement. It was
envisioned that the availability of excess free volume in the form of grain boundaries in a bulk nanocrystalline microstructure could act as sinks for defects generated by irradiation.
Therefore, in the second part of this investigation, included as chapter 4 of the thesis, the
different types of defects formed due to irradiation of the annealed and the HPT processed Nb1Zr alloy were examined for this phenomenon and a model for the measured mechanical
properties was proposed, and further corroborated by multiscale characterization techniques.
In the third part of the thesis, that is chapter 5, the effect of Sc addition in the
strengthening of the third generation Aluminum-Lithium (Al-Li) alloy Al-4Cu-1.2Li-0.4Mg0.4Ag-0.12Zr (AA 2195) was examined, after deformation by high pressure torsion, in order
to understand the metallurgical mixing produced by severe plastic deformation. Addition of
scandium to aluminum alloys has been found to be greatly beneficial in terms of enhancing the
mechanical properties. It was envisaged that scandium addition to aluminum–lithium alloys
will be useful in overcoming limitations pertaining to strength-ductility combination these
alloys. In this regard, it is expected that the evolution of microstructures at nanoscale, as
obtained post HPT, would dictate the mechanical response in scandium added Al-Li alloys. In
the present investigation, three alloy compositions of Al-Li alloy, namely Al-4Cu-1.2Li0.4Mg-0.4Ag-0.12Zr (as a control material), Al-4Cu-1.2Li-0.4Mg-0.4Ag-0.12Zr-0.025Sc, and
Al-4Cu-1.2Li-0.4Mg-0.4Ag-0.12Zr-0.25Sc (all in wt. pct.) have been investigated. Each of
these alloys have been subjected to high pressure torsion up to a shear strain of 90. The role of
excess vacancies, dislocations and non-equilibrium grain boundaries were investigated by
theoretical calculations and corroborated experimentally by atom probe tomography to
understand the primary and secondary contributors to compositional modulations of various
alloying additions. A clear role of precipitate dissolution and re-precipitation resulting from
solute re-distribution on the mechanical property, particularly strength, has been elucidated.
In the last chapter of the thesis (chapter 6), generic conclusions have been drawn
highlighting the effect of severe plastic deformation on the modification of microstructure and
micro-chemistry for precipitate containing alloys as well their overall impact on mechanical
properties.