Effect of Heat Treatment on Mechanical Properties …
223
casting. For aluminum alloys, the TiB 2 is also used as a grain refiner and presence
of the TiB 2 particles helps in controlling the development of fine microscopic voids
in the microstructure due to the occurrence of shrinkage during solidification. Thus,
the stress concentration points are noticeably reduced due to a decreased level of
microscopic porosity, which is conductive for restricting crack propagation during
deformation.
By considering five different points, the average value for bulk hardness of both
the aluminum alloy and aluminum alloy composites was determined and the test
results are summarized in Table 2. Hardness of the aluminum alloy composite is
due to a fine dispersion of the reinforcing TiB 2 particulates through the aluminum
alloy metal matrix. In the current study, the Al-4.5%Cu-6 weight pct. TiB 2 shows a
greater hardness value when compared to the composite that contains 3 weight pct.
of the TiB 2 reinforcing particulates. Hardness of the composite is also influenced
by the nature of matrix-reinforcement interfaces. In the as-cast condition, hardness
value of the alloy was 43 HV and it gradually increases to 83 HV for the peak-aged
condition. In comparison, hardness of the aluminum alloy composite containing 6
weight percent TiB 2 particulates was 62 HV. The influence of ageing on hardness,
as shown in Table 2, is mainly due to the higher holding temperature for both the
aluminum alloy and aluminum alloy-based composites that results in a noticeable
increase in the dislocation density due to intrinsic differences in the coefficient of
thermal expansion between the soft aluminum (22.6 × 10
−7 K
−1 ) alloy metal matrix
and the hard, brittle, and elastically deforming TiB 2 particulates (6 × 10
−7 K
−1 ).
The observed increase in dislocation density occurs at the matrix-particle interfaces
during quenching. This often results and/or contributes to an observable improvement
in hardness of the composite material.
Scanning Electron Microscopy
To study the dispersion of the particulate reinforcements through the aluminum alloy
metal matrix, both the unreinforced aluminum alloy and reinforced aluminum alloy
test specimens were mechanically ground and then fine polished to get near mirrorlike surface finish. The samples were prepared for microscopic examination very
much in conformance with procedures detailed for metallographic preparation of
samples for purpose of microscopic observation. The polished samples were then
etched using the Keller’s reagent [a solution mixture of 1.5% hydrochloric acid
(HCl), 95% distilled water, 2.5% nitric acid (HNO 3 ), and 1% hydrofluoric acid
(HF)]. The etched samples were then observed in a field emission-scanning electron
microscope and energy dispersive spectroscopy (EDS) processes were conducted
on these samples[Model: FEI Quanta 200 FE-SEM]. In the present investigation,
it was found that for the aluminum alloy reinforced with TiB 2- particulates, the
reinforcing TiB2 particles were distributed uniformly through the microstructure.
At some locations, segregation of the reinforcing TiB 2 particles occurred when the
weight fraction of the TiB 2 particulates increases as is shown in Fig. 2 b and c.
223
casting. For aluminum alloys, the TiB 2 is also used as a grain refiner and presence
of the TiB 2 particles helps in controlling the development of fine microscopic voids
in the microstructure due to the occurrence of shrinkage during solidification. Thus,
the stress concentration points are noticeably reduced due to a decreased level of
microscopic porosity, which is conductive for restricting crack propagation during
deformation.
By considering five different points, the average value for bulk hardness of both
the aluminum alloy and aluminum alloy composites was determined and the test
results are summarized in Table 2. Hardness of the aluminum alloy composite is
due to a fine dispersion of the reinforcing TiB 2 particulates through the aluminum
alloy metal matrix. In the current study, the Al-4.5%Cu-6 weight pct. TiB 2 shows a
greater hardness value when compared to the composite that contains 3 weight pct.
of the TiB 2 reinforcing particulates. Hardness of the composite is also influenced
by the nature of matrix-reinforcement interfaces. In the as-cast condition, hardness
value of the alloy was 43 HV and it gradually increases to 83 HV for the peak-aged
condition. In comparison, hardness of the aluminum alloy composite containing 6
weight percent TiB 2 particulates was 62 HV. The influence of ageing on hardness,
as shown in Table 2, is mainly due to the higher holding temperature for both the
aluminum alloy and aluminum alloy-based composites that results in a noticeable
increase in the dislocation density due to intrinsic differences in the coefficient of
thermal expansion between the soft aluminum (22.6 × 10
−7 K
−1 ) alloy metal matrix
and the hard, brittle, and elastically deforming TiB 2 particulates (6 × 10
−7 K
−1 ).
The observed increase in dislocation density occurs at the matrix-particle interfaces
during quenching. This often results and/or contributes to an observable improvement
in hardness of the composite material.
Scanning Electron Microscopy
To study the dispersion of the particulate reinforcements through the aluminum alloy
metal matrix, both the unreinforced aluminum alloy and reinforced aluminum alloy
test specimens were mechanically ground and then fine polished to get near mirrorlike surface finish. The samples were prepared for microscopic examination very
much in conformance with procedures detailed for metallographic preparation of
samples for purpose of microscopic observation. The polished samples were then
etched using the Keller’s reagent [a solution mixture of 1.5% hydrochloric acid
(HCl), 95% distilled water, 2.5% nitric acid (HNO 3 ), and 1% hydrofluoric acid
(HF)]. The etched samples were then observed in a field emission-scanning electron
microscope and energy dispersive spectroscopy (EDS) processes were conducted
on these samples[Model: FEI Quanta 200 FE-SEM]. In the present investigation,
it was found that for the aluminum alloy reinforced with TiB 2- particulates, the
reinforcing TiB2 particles were distributed uniformly through the microstructure.
At some locations, segregation of the reinforcing TiB 2 particles occurred when the
weight fraction of the TiB 2 particulates increases as is shown in Fig. 2 b and c.