| dc.description.abstract | For structural applications, a major section of high-strength alloys, such as steels, superalloys,
etc., has a base element of iron, nickel, cobalt, titanium, aluminium or their combinations. Such
materials enable efficient, safe, and sustainable operations across various aerospace,
automotive, energy, and defence industries. However, the increasing demands for high
performance and efficiency in such applications necessitate further enhancements in the
mechanical properties of these alloys. Increasing the strength of the alloy largely relies on
imparting resistance to the motion of dislocations in their microstructure during external load.
The most conventional practised strengthening strategies comprise solid-solution hardening,
precipitation/dispersion hardening, grain boundary strengthening, and mechanical working. A
key area of focus in recent research is the interaction of solutes with defects in the crystal
lattice. Solutes, when segregate to these defects, alter the local stress fields and energy
landscapes, thereby affecting the mobility of dislocations. These interactions can lead to
significant changes in the tensile strength, work hardening behaviour and creep resistance of
the materials. Despite extensive studies, the atomic-scale mechanisms governing the
interaction of solutes with different types of defects and their influence on mechanical
properties remain incompletely understood, particularly in alloys with complex chemistry in
multi-component ordered and disordered alloys. This thesis aims to elucidate the atomic-scale
solute-defect interactions and their implications in the mechanical properties of two FCC
alloys: (a) disordered CoNiCrMo alloy and (b) Ordered Ni-base superalloy (CMSX-4).
In the first part, deformation behaviour and solute-defects interaction in a low stacking fault
energy (SFE) complex concentrated alloy (CCA) (Co35-Ni35-Cr20-Mo10) was investigated.
First, the tensile deformation of this alloy with varying grain sizes was studied at room
temperature. The yield stress (YS) exhibits the classical Hall-Petch relationship with a Hall
Petch coefficient exceeding ~1GPa µm-0.5 and friction stress of ~190 MPa. This indicates the
excellent grain boundary-strengthening and solid solution-strengthening capability of this
alloy. The High value of the Hall-Petch coefficient is attributed to the combined effect of large
friction stress, low stacking fault energy (SFE) and solute segregation on grain boundary. Also,
the alloy demonstrated remarkable tensile strength, ductility and strain hardening at room
temperature across all grain sizes. Post-deformation microstructure analysis revealed activation
of multiple deformation mechanisms such as dislocation slip, stacking faults (SF), twinning
induced plasticity (TWIP) effect and transformation induced plasticity (TRIP, HCP(ε)
martensite formation) effect and microbands formation. These mechanisms collectively
contributed to the alloy's excellent mechanical performance. Second, to investigate the role of
solute-defect interaction on mechanical properties, tensile deformation was conducted at 600°C
under two strain rates (10-3 and 10-5 s-1). The stress-strain curves exhibit serrations and
enhanced strain hardening with the ultimate tensile strength reaching approached room
temperature value, although the YS was lower than the room temperature. This behaviour is
attributed to the dynamic solute segregation to the defects formed during the deformation,
which inhibits the defect mobility. However, despite the good strain-hardening behaviour, the
alloy's relatively low YS limits its practical applicability.
In the second part, a different strategy is used to enhance the yield strength (0.2% YS) of the
alloy by exploiting the solute segregation on different types of defect structures. The alloy was
subjected to controlled cold-rolling reductions of 45% and 65% to introduce a distinct
deformation structure into the microstructure. SFs and nano-twins (NTs) mostly dominate the
deformation microstructure of the 45% cold reduction (45CR) sample, while the 65% cold
reduction (65CR) sample exhibits (HCP) ε-nano-martensite laths (NMLs) along with SFs and
NTs. These deformation structures play a pivotal role in the initial strengthening of the alloy
by impeding dislocation motion, resulting in yield strength of 45CR and 65CR reaching up to
∼1.3GPa and ∼1.5GPa, respectively. Post-deformation tempering at 600°C further enhanced
the 0.2% YS, reaching ∼1.5GPa for 45CR samples and ∼2GPa for 65CR samples. The atomic
scale structural and compositional analysis of these deformation structures reveals diffusion of
Mo and Co into the SFs, NTs, and NMLs, which is hence responsible for further hardening of
the alloys. The presence of NMLs in 65CRT (tempered) alloy also resists abnormal grain
growth and retains the strengthening effect even after 100 hours of tempering. We also reveal
that the control over the degree of hardening was found to depend on the fraction of
deformation-induced twins and martensite in the microstructure. More specifically, cryo-rolled
alloys having a higher fraction of NTs and NMLs show 0.2%YS values of ⁓ 2.3GPa.
In the third part, solute interaction with the defects formed during the creep deformation at two
conditions (intermediate temperature/high stress (800°C/800MPa), (b) high temperature/low
stress (1000°C/200MPa) was investigated in a multi-component single crystal γ/γ' Ni-base
superalloy (CMSX4). The microstructure of post crept samples was investigated using
scanning electron microscopy (SEM), transmission electron microscopy (TEM) and atom
probe tomography (APT). Controlled electron channelling contrast imaging (cECCI) was
iv
conducted to (1) characterize the deformed structure over the large area and (2) to prepare the
site-specific samples from the defect regions for correlative transmission electron microscopy
(TEM) / atom probe tomography (APT) experiments. The defect structures formed in γ' are
superlattice-extrinsic-stacking faults (SESFs), micro-twins, and anti-phase boundaries (APBs).
The former two were formed during creep at 800°C (applied stress 800 MPa), while APBs and
dislocation network were formed at 1000°C (applied stress 200 MPa). Atomic-scale
compositional and structural analysis of these defects reveals a clear dependence of segregation
behaviour on their local internal fault structure. The results indicate variation in Al composition
along the twin boundaries and higher composition of W/Ta confined at the Al-depleted portion
of the boundaries. Based on the segregation behaviour, an additional step of reordering in
Kolbe's mechanism of micro-twinning was introduced, that can be one of the rate-limiting steps
for creep deformation. At 1000°C, enrichment of Mo/W along with the Re at the core of
dislocations pinned at γ/γ' interfaces and APB's fault plane was observed, indicating the
occurrence of solute drag during the shearing of γ'. This emphasizes and reveals the importance
of the solutes Mo/W in contributing towards the "Re effect" during high-temperature creep.
This work demonstrates that solute-defect interactions can significantly influence the
mechanical properties of structural alloys. The findings provide a comprehensive
understanding of the atomic-scale mechanisms underpinning the deformation mechanism and
solute-defect interaction in a disordered Ni-Co base alloy and ordered Ni-base superalloy,
paying the way for the design of next-generation structural materials with superior performance
under diverse conditions. | en_US |
