Analysis of microstructural, mechanical and thermal behaviourof thixocast LM25-10wt% SiC composite at different processingtemperatures

ANOOP KUMAR TULI1, PRADEEP SINGH2,* , S DAS3, D P MONDAL3 and J P SHAKYA2

1S.V. Polytechnic College, Shyamla Hills, Bhopal 462002, India2Department of Mechanical Engineering, Samrat Ashok Technological Institute, Vidisha 464001, India3CSIR-Advanced Materials and Processes Research Institute (AMPRI), Bhopal 462026, India

e-mail: erpradeep3408@gmail.com

MS received 15 March 2021; revised 30 June 2021; accepted 13 September 2021

Abstract. In this study, composite was made by the addition of 10 wt% SiC particles in the LM25 matrix

through gravity casting and thixocasting processes. Thixocasting temperature was varied to investigate its effect

on the microstructure, mechanical and thermal properties. Properties of thixocast samples were also compared

with gravity cast ones. XRD analysis was done to analyse the phases present in the samples. Optical microscope

and scanning electron microscope were used to record the microstructures of gravity cast as well as thixocast

samples. Tensile, compressive and flexural tests were performed on universal testing machine with the prepared

composite samples according to the ASTM standards. Thermal diffusivity tests of gravity cast and thixocast

samples were done on laser flash diffusivity instrument. Results reveal that thixocast samples are sounder than

the gravity cast samples. The microstructure of a-Al phase transformed from dendritic to globular shape due to

the thixocasting. Also, volume fraction of eutectic phase in the sample increased with the increment of thixocast

temperature. Tensile strength, compressive strength and flexural strength were found higher in the case of

thixocast samples as compared to gravity cast. Thermal diffusivity considerably increased due to thixocasting,

while its value slightly reduced with increment of thixocast temperature.

Keywords. Thixocasting; LM25 alloy; mechanical properties; thermal properties; composite.

1. Introduction

The trend of light-weight materials in the automobile and

aerospace industry is increasing day by day due to fuel

economy concern. Al alloys have higher specific strength,

good weldability, lower melting point and noble thermal

conductivity [1–3]. These are the most appealing properties

which are well suited for the synthesis of various compo-

nents in automotive industries. Vehicle frame, cylinder

heads, engine blocks, piston, steering and brake compo-

nents, rotors etc. are being manufactured by the use of Al

alloys [4–6]. Due to lower melting point and good forma-

bility of the Al alloys, number of components are processed

through casting or forging techniques. Components made of

forging techniques have better mechanical properties than

the parts prepared through casting process [7, 8]. However,

higher cost and limited shape and size of the components

restrict the forging process for the wider applications.

Casting technique provides easiness in manufacturing of

different shape and size components, but mechanical

properties are inadequate as compared to the forged com-

ponents. This is due to the various defects like dendritic

grains, voids, inclusions, inappropriate dimensions associ-

ated in the cast parts [9].

Defects in the products synthesized through casting

techniques can be minimized by the adoption of semi-

solid casting process. Semisolidcasting process is of

growing industrial significance as it offers several

advantages over the conventional casting owing to lower

porosity and forming temperatures, improved flow prop-

erties, reduced process force, near net-shape forming,

better mechanical properties. In this process, metal is

heated at semi solidus temperature to obtain the globular

solid phase in the certain fraction of liquid phase

[10, 11]. Movement of globular solid particles becomes

easy in the liquid phase which in turn minimize the pores

and voids in the product. Because of the semi-solid state

and globular solid particles, dendritic structure of the

material can be minimized which is beneficial to improve

the mechanical properties. Through this technique,

porosity free complex parts with tight tolerance and

excellent mechanical properties can be obtained.*For correspondence

Sådhanå (2021) 46:219 � Indian Academy of Sciences

https://doi.org/10.1007/s12046-021-01757-3Sadhana(0123456789().,-volV)FT3](0123456789().,-volV)

Semi-solid metal processing technique can be catego-

rized in two different techniques; known as rheocasting and

thixocasting. In rheocasting technique, a metal alloy is

heated at a semi-solidus temperature to achieve slurry of

liquid and globular solid phases. The weight fraction of

liquid and solid phase in the slurry depends on the pro-

cessing temperature in the semisolid region as shown in

figure 1a. With increment of processing temperature in the

semi-solidus region, the amount of liquid phase present in

the slurry increases (Lever Rule). In rheocasting process,

slurry is directly poured in the mould and allowed to

solidify to obtain the final product. On the other hand, in

thixocasting process, slurry is solidified in the form of ingot

and then ingots are cut into small pieces, melted again in

the semi solidus region to find the desired viscosity, stirred,

filled in the predefined mould cavity and allowed to cool

[12]. Thixocast products have extremely high quality in the

form of close tolerance, voids free and high mechanical

properties. However, the manufacturing cost of the thixo-

cast products is higher than the rheocast products.

Thixocasting process is more suitable for the larger

products made of Al alloys. In the conventional casting

process, more heat is required to extract from the

conventional casting as compared to thixocasting process.

Therefore, solidification time is higher in cast product

which is responsible for the dendritic grain structure, hence

reduces the mechanical properties. In thixocasting, the heat

energy associated with the slurry is much lower than its

melt, causing rapid solidification as compared to the con-

ventional casting. Consequently, globular microstructure of

the grains retained without entrapment of gases and

improved the properties of the final product.

Mechanical properties of Al alloys can be improved by

the addition of ceramic materials which have excellent

strength and high elastic modulus. Hard ceramic materials

are reinforced in the form of particulates or whiskers in

highly tough and ductile matrix of Al alloys to make Al

matrix composite (AMC). Carbon nano tube (CNT), alu-

mina (Al2O3), silicon carbide (SiC), titanium boride (TiB2),

graphite (Gr) are the most commonly used ceramic mate-

rials which are reinforced in the Al alloy matrix [13–17]. In

the present time, researchers are trying to develop ceramic

reinforced Al alloy composites which associate with high

strength along with toughness for the aerospace and auto-

mobile applications.

The aim of the present study is the synthesis of LM25

alloy composite reinforced with silicon carbide (SiC) par-

ticulates through thixocasting method. Thixocasting tem-

perature was optimized to find the best mechanical

properties of the synthesized composite. Different thixo-

casting temperatures were selected from the Al-Si phase

diagram which are located in the semi-solid region as

shown in figure 1b [18].

2. Experimental procedure

2.1 Materials and methods

LM25 alloy was used as the base material. The chemical

composition of LM25 alloy was determined by the optical

emission spectrometer (model: SPECTRO Max LMF05,

Germany) and presented in table 1. From table 1, it is

confirmed that silicon (Si) is the main alloying element in

the LM25 alloy with weight percentage 7.46. The average

size of SiC particles was analysed by the laser scattering

particle size analyser (model: Horiba LA-950) and it was

measured as 25 lm. LM25-10SiC composite was made by

the addition of 10 wt% preheated SiC particles in the LM25

alloy melt. The synthesis procedure of LM25-10SiC com-

posite is represented by the schematic diagram as shown in

figure 2. Initially, LM25 alloy was melted in the electric

Figure 1. (a) Represents the schematic phase diagram showing

semisolidous or thixocast temerature, (b) shows the split Al-Si

phase digram.

Table 1. Chemical Composition (wt%) of LM25 Alloy.

Si Mg Cu Mn Zn Pb Sn Fe Ni Ti Al

7.5 0.2 0.1 0.3 0.1 0.1 0.05 0.5 0.1 0.2Remainder

219 Page 2 of 12 Sådhanå (2021) 46:219

resistance furnace at temperature range of 700–720�C and

then 10 wt% preheated SiC particles (SiC particles were

preheated at 600–700�C for 6 hours to avoid splashing and

cracking in the SiC particles due to temperature difference

during adding in the LM25 alloy melt) were added in the

LM25 alloy melt. Mixture was simultaneously stirred using

mechanical stirrer up to 3 minutes for the proper distribu-

tion of SiC particles in the melt. Coveral 11 was used as

cover flux and dry nitrogen gas as degasser. Small amount

of Al – TiB2 master alloy was added in composite melt

prior to pouring into the die for casting to achieve relatively

globular dendritic structure and grain refinement of matrix

material. The liquid mixture was poured in the cast iron

finger-shaped mould cavity and allowed to solidify.

For the thixocasting, finger-shaped cylindrical billets

were cut in to the small pieces. These pieces were again

partially melted in a cylindrical die of inner diameter 40

mm and height 115 mm. For partial melting, the selected

temperatures of the pieces were kept in semi-solid region

according to the Al-Si phase diagram [18]. To investigate

the effect of processing temperature, pieces were heated at

three different semi solidus temperatures (590 �C, 600 �Cand 610 �C). Finally, semi-solid pieces were pressurised

within the same cylindrical die by using a 400 ton pressure

die casting machine. After solidification, thixocast samples

were removed from the die. The samples thixocast at 590

�C, 600 �C and 610 �C are assigned as Thixo 590, Thixo

600 and Thixo 610, while gravity cast LM25-SiC sample as

As Cast.

2.2 Characterization techniques

For the microstructural analysis, thixocast samples of each

group were radially cut into small pieces and cold mounted

using resin and hardener for the handling purpose during

polishing. Mounted samples were polished by the use of

sandpapers of 100 to 1600 grits. After sandpaper polishing,

samples were mirror polished using cloth on disc polisher.

Microstructure of mirror polished samples were recorded

Figure 2. Schematic diagram of synthesis of thixocast samples.

Figure 3. Universal testing machine (UTM) with their respective test samples of tensile (a), compressive (b) and flexural (c).

Sådhanå (2021) 46:219 Page 3 of 12 219

by the use of Scanning Electron Microscope (model: JEOL

5600). XRD analysis was performed using XRD diffraction

machine (Rigaku Japan: miniflex ii) of CuKa radiation with

scan rate 0.2�/s. Different elements and phases present in

the samples were determined by matching the d value of

each peak with the standard JCPDS file. Density was cal-

culated by the measurement of weight and volume of the

each sample.

The tensile test of gravity cast and thixocast samples was

carried out at a strain rate of 0.01/s on standard tensile

specimens (as per ASTM standard B557) [19]. During the

test, the samples were pulled until failure. The stress and

strain were recorded in the system interfaced with UTM

(Instron: Model 8801). The UTM machine and standard

tensile specimen is shown in figure 3a. The compression

test of gravity cast and thixocast samples was carried out at

a strain rate of 0.01/s on cylindrical samples of 12 mm

diameter and 20 mm length on the same UTM machine as

displayed in figure 3b. During the compression test,

surfaces between the anvil and specimen were lubricated

with MoS2 to avoid friction and proper deformation of the

contact surfaces. To investigate the flexural behaviour of

the composite, three point bending test was performed on

the same UTM with cross head speed 1 mm/min or 0.004/s

strain rate. The UTM with bending specimen is shown in

figure 3c. The specimens used for bending test were pre-

pared in the rectangular die during thixocasting to obtain

rectangular bar shape. The span length of the rectangular

specimen was 70 mm, while breadth and thickness were 55

mm and 4.2 mm, respectively [20].

Thermal diffusivity (aT) of the as cast and thixocast

samples were determined by a vertical laser flash diffusivity

instrument (model: LFA 467 Hyper flash) at different

temperatures (room temperature and 50 �C to 300 �C with

an alternate temperature of 50 �C). Disc specimens with

diameter 12 mm and thickness 2 ± 0.5 mm were prepared

for this test. The experiment was done in the vacuum shield

chamber of the instrument with graphite heater. Specimens

were contained in a graphite coated sapphire cup while

laser passes through the transparent top striking the upper

surface, resulting in temperature rise in the rear face

monitored by infrared detector.

3. Results and discussion

3.1 Density analysis

The actual density (qaÞ of machined as cast as well as

thixocast billets was calculated by Archimedes principle

according to the ASTM B862-17 [21]. The theoretical

density was also measured to investigate porosity in the

samples. It was measured by the equation of rule of mixture

as given below;

1

qT¼ mLM25

qLM25

þ mSiC

qSiCð1Þ

where qT is the theoretical density of the composite,

mLM25 and mSiC are the mass fractions of LM25 alloy and

SiC particulates used to prepare composite, qLM25 and qSiCare densities of LM25 and SiC particulates. From equation

(1), theoretical density of LM25-10SiC composite was

obtained as 2.73 g/cm3.

Porosity present in the samples was determined by the

following equation:

Porosity %ð Þ ¼ 1� qaqT

� �� 100 ð2Þ

Density variation (actual) of as cast and thixocast sam-

ples is shown in figure 4a. From the figure, it can be

depicted that due to the thixocasting, improvement in

density takes place. For the sample Thixo 590, density is

about 1.5% higher than the As Cast sample. In the group of

thixocast samples, the Thixo 590 sample has slightly higher

Figure 4. Variation of density (a) and porosity (b) for the As

Cast and thixocast samples.

219 Page 4 of 12 Sådhanå (2021) 46:219

density than Thixo 600 and Thixo 610 samples. Porosity

present in the different samples are shown in figure 4b. This

figure indicates that As Cast sample has about 63% higher

porosity than the thixocast samples. This indicates that the

soundness of the product prepared by the thixocasting

method is better than the gravity cast products.

3.2 XRD analysis

Figure 5 represents the Xray diffraction pattern for the

LM25 as cast and Thixo 590 samples. In LM25 alloy, Si is

abudantly present with 7.5 wt%. Therefore, peaks of Si can

be easily detected. The Al-Si phase diagram suggests that

the solubility of Si in a-Al is very low [22]. At 577�C, only1.65 wt% Si can be totally dissolved in the a-Al. The sol-

ubility limit also reduces with the reduction of temperature.

Also, any intermetallic or precipitate of Al and Si can not

be formed. Therefore, these metals present as the a-Al andthe eutectic Si particles. Mg2Si phase is also detected with

the base materials. Diffracction pattern of Thixo 590 sam-

ple is also represented in figure 5. The diffraction pattern of

Thixo 590 contains the peaks of Al, Si, Mg2Si and the SiC.

Actually, Al, Si and Mg2Si particles present due to the

LM25 alloy and SiC peaks were present due to the addition

of SiC particles as a reinforcement in the LM25 base alloy.

The observed peaks of the thixocast samples are similar to

the as cast samples made by the use of same alloy. It

indicates that due to thixocasting, the phase formation does

not change. Additionally, the increment of thixocast tem-

perature does not alter the formed phases.

Figure 5. XRD analysis for the as cast LM25 and thixocast composite sample at 590 �C.

Figure 6. Surface microstructures of the different samples. (a),

(b), (c) and (d) represent the microstructure of as cast and

thixocast samples processed at 590 �C, 600 �C and 610 �C,respectively.

Sådhanå (2021) 46:219 Page 5 of 12 219

3.3 Microstructural, grain size and phase analysis

Cross sectional microstructures of As Cast and Thixo 590,

Thixo 600 and Thixo 610 samples are shown in figure 6.

Figure 6a represents the surface microstructure of the As

Cast sample in which dendritic-shaped a-Al phase was

observed. Eutectic Si phase is uniformly distributed in the

microstructure. Microstructure for the Thixo 590 sample is

shown in figure 6b. As can be seen in this figure that

dendritic a-Al transforms in the globular shape due to

thixocasting. Eutectic Si phase (black spots) is also present

in the microstructure. From the inset microstructure, it can

be clearly seen that SiC particles embedded in the a-Al aswell as eutectic Si phase. With the increment of thixocast

temperature from 590 �C to 600 �C, level of sphericity of

globular a-Al increases (figure 6c). However, for Thixo 610sample, sphericity of the a-Al phase reduces and globular

shape changes in the dendritic type of structure (figure 6d).

In the previous work, it was found that with the increment

of semisolid temperature, the fineness and globular shape of

a-Al decline and size with dendritic shape of the grain

increases [23]. Similar kind of behaviour of a-Al phase wasobserved in the present work.

Intercept method (ASTEM E112-13) was used to analyse

the average grain size of the each sample [24]. In this

method, number of randomly oriented straight lines was

drawn on the micrograph and then actual length of the line

is divided by the number of intercepts cut by the line on the

grain boundary. Aspect ratio of the grains was determined

by the average ratio of major and minor dimensions of 500

grains. The obtained average grain size and aspect ratio for

the as cast and thixocast samples are shown in figure 7a, b,

respectively. It can be seen that average grain size of As

Cast is about 150 lm. After thixocasting, the average

grain size reduces significantly with attaining minimum

value about 50 lm for Thixo 590 sample (figure 7a). The

aspect ratio of the As Cast sample is much higher than the

other thixocast sample. It is due to the dendritic shape of

the grains. In the group of thixocast samples, aspect ratio

is minimum for the Thixo 590, while it increases slightly

with the increment of thixocasting temperature from 590

�C to 600 �C and 610 �C (figure 7b). In the thixocasting

process, recrystallization and reorientation take place and

formation of new grains occurs. The grain growth depends

on the viscosity of the formed slurry at a certain thixo-

casting temperature. For higher viscosity, the outer pres-

sure on the surface of newly formed grain will be more

which depresses the grain growth after attaining a definite

size. With increment of thixocasting temperature, viscos-

ity of the slurry reduces [25] and therefore, pressure

applied by the viscous fluid on the new born grain reduces,

causing increment in grain size and aspect ratio.

Volume fractions of formed eutectic phase in the sam-

ples were obtained by the evaluation of area fraction of

eutectic phase in the recorded micrograph of each sample.

The area fraction of eutectic phase was calculated by the

use of Image J software through image threshold method

as shown in figure 8. Figure 8a represents the original

micrograph, while figure 8b shows the threshold micro-

graph. Threshold eutectic phase is shown by the red col-

our, while white colour shows the a-Al phase. The area

fraction of red colour in the image is about 30%. This is

how eutectic phase was measured for as cast and thixocast

samples and shown by a histogram in figure 8c. From this

figure, it can be determined that volume fraction of

eutectic phase is higher for the thixocast samples. With the

increment of thixocast temperature, the content of eutectic

phase increases, while a-Al phase decreases (figure 8c, d).Amount of eutectic phase present in the thixocast sample

depends on the extent of liquid metal in the slurry during

the semisolid processing. As suggested by the Al-Si phase

diagram, amount of liquid in the slurry increases with the

increment of thixocast temperature, therefore volume

fraction of eutectic phase also increases in the final

product.

Figure 7. Variation of grain size and aspect ratio obtained by the

microstructures of different samples. (a) represents the variation of

grain size, while (b) shows the change in aspect ratio.

219 Page 6 of 12 Sådhanå (2021) 46:219

3.4 Analysis of tensile test and fractography

The tensile stress-strain curve of as cast and thixocast

samples is shown in figure 9. The ultimate tensile strength

(UTS), yield stress at 0.2% strain and percentage elongation

of each sample is also given in table 2. It can be observed

that thixocast samples have comparatively higher UTS than

the gravity cast or as cast sample. UTS of gravity cast

sample is about 27% to 37% lower than the thixocast

samples. Also, the elongation improves due to the thixo-

casting. For the As cast, Thixo 590, Thixo 600 and Thixo

610 samples, the elongation at the point of ultimate tensile

strength was measured as 0.7 %, 1%, 1.2% and 1.2%,

respectively. The reason of increment of elongation due to

thixocasting may be the good bonding among the rein-

forced SiC particles with the eutectic silicon phase and

globular a-Al phase. The increment in ultimate tensile

strength of the thixocast samples is due to the evolution of

primary a-Al from dendritic to globular shape. The den-

dritic microstructure of a-Al in as cast sample influences

the soundness of the product. During solidification, for-

mation of dendritic structure may cause the generation of

micro-porosity along the interface of pre-solidified phase

and liquid phase. Also, dissolved gases evaporates from the

casting during the solidification which create micro-

porosity. In the thixocast samples, these micro-porosities

minimized in the influence of globular microstructure of a-Al and application of pressure. Presence of micro-porosity

Figure 8. Original microstructure (a) and threshold microstructure through image J software to investigate the volume fraction of

different phases (b). (c) Represents the variation of volume fraction of eutectic phase, while (d) shows the variation of volume fraction of

a-Al phase for different samples.

Sådhanå (2021) 46:219 Page 7 of 12 219

in the material considerably affect the mechanical proper-

ties [26, 27]. In the group of thixocast samples, it may be

observed that tensile strength influences by the thixocasting

temperature. For Thixo 590 and Thixo 600 samples the

variation of UTS is minute. But for Thixo 610 sample, it is

about 6 to 7 % lower. For the uniform distribution of SiC

particles, an optimized viscosity of the slurry is required for

the proper movement of the mixture due to mechanical

stirring during the time of thixocasting. At the higher

thixocasting temperature the viscosity of the slurry

decreases, therefore the chance of agglomeration of SiC

particles may increase. Hence, strength reduces with

increment of processing temperature. It is demonstrated

that it is better to conduct thixocasting of composite at

lower temperature (590�C).

The tensile fractography of as cast and thixocast samples

is shown in figure 10. Figure 10a1, a2 are the fractograph for

As Cast sample at lower and higher magnifications

respectively. In figure 10a1, the presence of voids can be

clearly seen. These voids may arise due to the dendritic

structure of a-Al and evolution of dissolved gases from the

casting during solidification. The agglomerated SiC parti-

cles (circled in figure 10a1) were also detected in the

fractography and the distribution of SiC particles in the

matrix is uneven. It is due to the difference in density of

SiC particles and matrix material (LM 25). There is more

chance of drainage of heavy SiC particles along the wall of

the container because of the vortex formed in the influence

of mechanical stirring of the melt during casting. It causes

the SiC particles agglomeration and nonuniform distribu-

tion. Probably, this is also a major reason of weakening of

as cast sample. In Figure 10a2, thick plates with ridges can

be easily detected. Some dimples were also found in the as

cast sample. This reveals that behaviour of the tensile

failure of the as cast sample is the combination of ductile

and the brittle nature. The reason of ductile failure is the

presence of soft a-Al phase, while brittle failure might

takes place at the interface of SiC particles and the matrix.

Figure 10b1, b2 represent the fractography of Thixo 590

sample. In figure 10b1, it can be observed that SiC particles

uniformly distributed in the matrix. Also, fine plates with

ridges and more number of tiny dimples present in the

Thixo 590 as compared to as cast sample (figure 10b2). The

fineness of the plates is caused due to the shearing among

the fine and globular a-Al and SiC particles, while more

number of dimples narrates the ductile nature of the

thixocast material. Figure 10c1, c2 are the fractograph of

Thixo 600 sample. In this sample, fractured particles are

bigger in size than the Thixo 590 (figure 10c1). Also,

cleavage steps are found with thick as well as fine ridges

Figure 9. Tensile stress-strain diagram for the gravity cast and

thixocast samples.

Table 2. Tensile, compressive and flexural properties of the gravity cast (As Cast) and thixocast samples.

Sample name Engineering stress at yield (ryt) (MPa) Engineering strain at yield (et) (%) Ultimate strength (rut) (MPa))

Tensile propertyAs Cast 100 ± 5 4.87 ± 0.24 123 ± 6.15

Thixo 590 145 ± 7 5.90 ± 0.29 170 ± 8.50

Thixo 600 130 ± 7 5.08 ± 0.25 152 ± 7.60

Thixo 610 125 ± 6 5.08 ± 0.25 142 ± 7.10

Compressive propertyAs Cast 243 ± 13 3.5 ± 0.17 347 ± 17.35

Thixo 590 337 ± 18 5.4 ± 0.27 473 ± 23.65

Thixo 600 333 ± 18 4.4 ± 0.22 467 ± 23.35

Thixo 610 314 ± 16 4.3 ± 0.21 461 ± 23.05

Flexural propertyAs Cast 219 ± 10.95 2.12 ± 0.10 79 ± 4

Thixo 590 338 ± 16.90 2.35 ± 0.11 121 ± 6

Thixo 600 303 ± 15.50 3.84 ± 0.19 120 ± 6

Thixo 610 301 ± 15.05 2.68 ± 0.13 112 ± 5

219 Page 8 of 12 Sådhanå (2021) 46:219

(figure 10c2). The fractographs of Thixo 610 are near

similar to the fractographs of as cast sample, but the ridges

are fine and dimples are more in numbers. It is due to the

tending of a-Al phase towards the dendritic shape.

3.5 Compression test Analysis

Figure 11 shows that the compressive strength for the

thixocast composite samples improved considerably as

compared to that of gravity cast samples. The value of yield

stress, ultimate compressive stress (UCS) and elongation

are given in table 2. It was noted that the strength of

thixocast samples varies with the thixocasting temperature.

At higher temperature a-Al become coarser and Si becomes

finer. But at lower temperature a-Al become finer and Si

needles become coarser. At higher temperature more SiC

pushed by the dendrites which may get either embedded in

a-Al matrix or segregated to the inter-dendritic region. The

improvement in strength and strain at failure is due to

microstructural modifications, better bonding of SiC within

the matrix and uniform distribution of SiC particles.

Coarser dendrite and Si needles with lower amount of a-Alcauses decrease in strength when processing temperature is

increased. Also, the strength noted to be decreased with an

increase in thixocasting temperature.

3.6 Analysis of three point bending test

The flexural stress-strain curves for the different thixocast

and as cast composite samples made of LM25 and 10 wt%

SiC are shown in figure 12. The trend of the stress-strain

curve is similar as discussed above in the tensile and

compressive stress-strain diagram. Flexural properties are

also recorded in table 2. The flexural properties for the

thixocast composite samples found to be improved

Figure 10. Microstructures of fractured surfaces of the various

samples subjected to the tensile test.

Figure 11. Compressive stress-strain diagram for the gravity cast

and thixocast samples.

Figure 12. Flexural stress-strain diagram for the gravity cast and

thixocast samples.

Sådhanå (2021) 46:219 Page 9 of 12 219

considerably as compared to that of gravity cast samples. It

is caused due to the change in microstructure from the

dendritic to the globular in shape. The uniform distribution

of SiC particles in the eutectic phase also improves the

bonding among the particles. Also, the flexural strength of

thixocast samples found to be changed with the change in

thixocasting temperature. The improvement in flexural

strength of thixocast composite is due to microstructural

modifications and better bonding between SiC and the

matrix. Thixocasting results in uniform distribution of SiC

particles which also would cause better flexural properties.

There is a decrease in flexural strength with an increase in

thixocasting temperature due to coarser dendrite, coarser Si

needles and lower amount of a-Al.

The fracture surface of LM25-10SiC gravity cast composite

showed fracture along the agglomerated SiC particles,

because these are the weaker regions (figure 13a). The parti-

cles also get sheared during deformation. The matrix contains

dendrites of a-Al and eutectic silicon. As a result, finer den-

drites get sheared and ridges formed around dendrites. These

lead to flower type fracture of matrix (figure 13b). The ridges

and shearing of matrix demonstrate brittle type fracture. The

same composite when thixocast, the particles also moved and

the dendrites get finer. Due to this, materials developed some

kind of ductility. Fracture surface showed some dimples.

However, at the same time, particle shearing and debonding

took place (figure 13c). In addition to this, shearing of den-

drites and SiC particles took place (figure 13d). This

Figure 13. Microstructures of fractured surfaces of the various samples subjected to the flexural test.

219 Page 10 of 12 Sådhanå (2021) 46:219

figure also showed debonding of particles. When thixocast at

higher temperatures, because of more amount of liquid phase,

the particle may move and agglomerate. The fracture may

occur through these agglomerated particles through bonding

of particles in the inter-dendritic region (figure 13e). Here

also, micro-cracks are formed, dendrites get sheared and inter-

dendritic regions get fractured through formation of ridges

(figure 13f). All these characteristics dictate that the fracture

occurs mainly through brittle mode. However, thixocast

sample (when thixocast at lower temperature) exhibits some

extent of ductility. The ductility of composite reduces when

thixocast at higher temperature.

3.7 Analysis of thermal diffusivity

Measured thermal diffusivity of as cast and thixocast samples

at different temperatures is shown in figure 14. With incre-

ment of temperature, thermal diffusivity reduces for all the

samples. The reduction of thermal diffusivity is almost linear.

In the previous works done by researchers, it was mentioned

that thermal diffusivity reduces with temperature [28]. The

figure shows that thermal diffusivity of the thixocast sample

is higher than the as cast sample. It is about 67 mm2/s for the

Thixo 590 which is 18% higher than the As Cast sample at

room temperature. It is due to the less porosity and globular

a-Al with fine eutectic silicon phase present in the thixocast

sample [29]. Thermal diffusivity of Thixo 590 is also higher

than Thixo 600 and Thixo 610. It indicates that higher

thixocasting temperature is unfavourable for the thermal

diffusivity. Payandesh et al stated that higher volume fraction

of solid phase in the material is beneficial to improve its

thermal behaviour [30]. Also, the dendritic nature of a-Alphase increases, while eutectic silicon phase becomes thick

with increment of thixocast temperature which may cause

reduction in the thermal diffusivity.

4. Conclusions

LM25-10wt% SiC composites were prepared through grav-

ity cast (as cast) and thixocast method at different thixo-

casting temperature. Microstructural evolution, mechanical

properties and thermal behaviour were compared for the as

cast and thixocast samples. Also, the effect of thixocasting

temperature on these entities were analysed. According to

the results and discussion, following conclusions are made.

• Density of the thixocast samples is considerably higher

than the density of gravity cast samples. With incre-

ment of thixocasting temperature, slight reduction in

the density of thixocast sample was observed.

• Gravity cast sample contains dendritic a-Al phase,

while due to thixocasting, dendritic a-Al phase trans-

forms in globular shape. The volume fraction of

eutectic silicon phase in the thixocast samples

increases with the thixocasting temperature.

• Thixocast composite (LM25-10SiC) samples were

found with significant improvement in mechanical

properties (tensile, compressive and flexural strengths)

as compared to the gravity cast sample due to fine and

globular microstructure. However, mechanical proper-

ties slightly degrade with the increment of thixocasting

temperature from 590 �C to 610 �C.• Tensile fracture analysis reveals that decohesion of SiC

particles takes place for the gravity cast sample. But in

thixocast sample, particle shearing is the major mech-

anism of fracture. The SiC particles distribution and

their interface bonding with matrix are much better

when thixocast at 590�C. This is also true for the

flexural behaviour of the material.

• Thermal diffusivity of thixocast samples is considerably

higher than the gravity cast sample. With increment of

thixocasting temperature, thermal diffusivity slightly

reduces. Also, the value of thermal diffusivity decreases

linearly with the increment of operating temperature.

Acknowledgements

The authors are thankful to the Director, CSIR-AMPRI,

Bhopal for providing necessary equipment facility to

perform this work.

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