| dc.description.abstract | A large number of advanced structural materials are based on metallic materials
where alloying additions play a key role in imparting the required properties.
Most of the commercially important aluminium alloys are classified by the
nature of the alloying additions. Among them the 2219, 2618, 5086, and 7075
are important class of lightweight alloys that plays critical role in modern
engineering application. However, despite having a series of commercially useful
aluminum alloys for commercial applications the increasing need of improved
performance requires newer development in particular for applications that
require high strength at elevated temperatures and performance at extreme
environments.
Precipitations of the intermetallic compounds containing copper during thermal
treatments play a very important role in developing high strength aluminium
alloys. Although,these precipitates are stable at fairly high temperatures, the
rapid coarsening of these second phase precipitates (e.g. Al2Cu), leads to loss of
strength at elevated temperature. Several approaches are explored to overcome
this problem. One of them is to utilize non-equilibrium solidification route, which
can increase solid solubility and hence increasing the precipitate density. Nonequilibrium processing can also alter the selection pathway of the competitive
phases and evolution of the microstructure. Recently, non–equilibrium solidification by suction casting technique is becoming increasingly popular for casting of metallic materials of any shape. In this technique solidification is effected by sucking the molten alloy into water cooled copper mold using a suction force resulting from the differences between the melting chamber in
Argon gas pressure and casting chamber under vacuum.
The present thesis aims to develop a set of newer alloys with small amount of
alloying additions primarily based on nickel that can retain reasonable strength
at high temperature by utilizing the non-equilibrium solidification route. In
addition to Ni (≤ 0.10at.%), the thesis present results of the effect of minor
addition of Sc and Zr as ternary and quaternary additions. Following a short
review in chapter 2, Chapter 3 presents the experimental techniques adopted for
both preparation of alloys and their characterization.
Chapter 4 deals with the results of alloying of aluminum with minor amount of
nickel. The Ni in the range of 0.05-0.20at% was used to develop a high temperature template, containing a set of hardening intermetallic compounds to increase the strength of the host matrix. The microstructural investigations of
the suction cast alloys reveal a characteristic feathery microstructure. At higher magnification the microstructure reveals the presence of fine dispersions of a
second phase. Both x-ray and transmission electron microscopy confirms the phase in the dispersions to be primarily crystalline Al9Ni2 phase having a monoclinic crystal structure. This phase does not exist in equilibrium phase diagram. Only at higher concentration one can observe equilibrium Al3Ni (Orthorhombic) particles. The size of the particle ranges from 50-200nm.
Beyond~0.5at%Ni, the microstructure changes to normal cellular type solidification morphology with interdendritic space decorated by the eutectic network of Al-Al3Ni having a rod eutectic morphology. A careful observation of alloys with small amount of Ni reveals that the feathery structure is associated with the thin cells, which have grown by continuously splitting the tip yielding a fractal like dendritic morphology. The dispersoids form at the intercellular
regions. We have presented clear evidence of their origin from the interdendritic
liquid, which most likely underwent Rayleigh instability. The random distribution reflects the nature of the dendritic growth. We have argued that these inter-dendritic liquid droplets, which are enriched with Ni, get
undercooled. The metastable Al9Ni2 phase nucleates and grows in this liquid. In
order to confirm this scenario, we have carried out a phase field simulation for
dendritic growth of aluminium solid solution in the alloy melt both under the
condition of constraint growth and free growth. The observed distribution of the
dispersoid is well reflected in the phase field simulation. The chapter also report
the response of effect of direct ageing of suction cast alloy as one expect an
extension solid solubility of Ni in Al. A small increase in hardness could be
observed during ageing treatment.
In order to determine the thermal stability of the intermetallic particles, the
samples of the suction cast alloys were exposed at 200°C for 200h and 500°C for
100h respectively. No change in the microstructure could be observed excepting
a slight coarsening indicating the dispersed particles are thermally stable. After
exposure at two different temperatures the maximum retained hardness was measured to be 350MPa. We have also attempted to correlate the hardness with coarsening behavior of particles.
The feathery morphology of the cast structures and fine dispersion of the
intermetallic phase is expected to improve the tensile strength of the alloy. The
tensile yield strength of cast alloys was determined to be 150MPa ± 20 for Al-
0.09at%Ni alloy. We have tried to estimate the expected strength of the alloy
from quantitative microstructural parameters using possible hardening mechanism. The estimates are in good agreement to the observed values.
The chapter 5 reports attempts to develop thermally stable precipitation strengthened aluminum alloys by retaining the dispersion template developed earlier alloyed with Ni. Then, the binary alloys were added with extremely low diffusivity element Zr. The element Zr is traditionally added in the aluminium alloys as grain refiner and as a powerful agent for inhibiting recrystallization especially for high strength aluminium alloys. However, in this work we have alloyed Zr for imparting precipitation hardening.
An amount of 0.15at%Zr was added to the suction cast alloys of Al-0.05, 0.09 and
0.20at%Ni. The first two alloys exhibit the formation of metastable phase Al9Ni2
during solidification stage. Increase the concentration of the alloy to Al-0.20at%
Ni with 0.15at%Zr additions exhibits combination of both stable Al3Ni and Al9Ni2
metastable phases. Microstructures of these alloys show columnar cells of
~200μm with dispersions of spherical nodules of Al9Ni2 and Al3Ni with varying
size ranges from 200-500nm. Particle size distribution of Zr containing aluminium alloys with 0.05at% Ni is 595nm ± 20 while the alloy having the 0.09 at% Ni has the optimum size of 290nm. Further increase of Zr composition above 0.20at % led to columnar to equiaxed transition.
The as cast alloys containing Zr does not show the improvement with limited yield strength of the order of 150MPa. The equivalent hardness of the samples
has been measured to be about 370-420MPa. Heat-treated alloys however show the presence of Al3Zr (L12) precipitates with ~20nm size that are coherent with the matrix. Binary suction cast Al-0.15at%Zr alloy after ageing exhibits tensile yield strength of ~200MPa. With ternary aluminium alloy with minor additions Ni and Zr, The strength increases to ~300MPa. Additionally, the alloy continue
retain a maximum hardness of 870-920MPa even after long hours of aging. The Zr containing alloys were proved to be stable. When the tests were carried out on a nominally alloyed sample of Al-0.09at%Ni-0.15at%Zr peak aged and exposed to 250°C for 200h, the yield strength under compression tests was found to be 280MPa.
The chapter 6 of the thesis discusses the role of Sc with the ternary Al-alloys with
Ni and Zr. Addition of small quantities 0.1 and 0.2at%Sc substantially reduces
the inter-particle distance of precipitates by increasing volume fraction and
number of nano-sized particles. It has been observed and presented in this thesis
that the Sc addition provides the highest incremental strengthening per atom
percent of any alloying element. Chill-suction cast samples show equiaxed cells
in the samples with dispersions of particles inside and some segregated particles
at the cell boundaries. To achieve a further increase in the number density of
precipitates we processed the suction cast alloys with additional heat treatment
at 375 and 450°C. All the suction cast alloys with varying Ni content and keeping
the Sc and Zr constant at 0.10 and 0.15at% respectively exhibit formation of
Al9Ni2 phase. The alloy Al-0.20at%Ni-0.10at%Sc-0.15at%Zr also contain stable
phase of Al3Ni with an eutectic morphology. The DSC experiments in the dynamic
mode with heating rate of 10°C min-1 exhibit two distinct exothermic peaks due
to precipitates from solution at 375 and 450°C. The TEM analysis using STEMEDX
has further confirmed the existence of nano-sized particles 30-50 nm of both phases of Al3Sc and Al3 (Sc, Zr). The tensile yield strength of the as cast alloy show 200MPa while after precipitation treatment, we observe improved yield strength 350-450MPa. Thermal stability of the alloys were tested after peak aged condition and exposed to 200°C for 250h. The results show that the yield strength is unaffected implying the coarsening resistance of the precipitate
particles.
Overall the thesis establishes that with minimum alloying additions, it is possible
to design alloys that are expected to perform for high temperature applications
by the formation of set of dispersions of Al9Ni2 (monoclinic) and precipitates of
ordered cubic phases of (L12) structure of Al3Zr, Al3Sc and Al3 (Sc, Zr) with
required number density of particles. | en_US |
