Solidification Behaviour of Ti–Cu–Fe–Co–Ni High Entropy Alloys

The evolution of microstructure and phase formation in equiatomic Ti20Fe20Ni20Co20Cu20 high entropy alloy synthesized by conventional arc melting followed with suction casting and ball milling with spark plasma sintering route is distinctly different. The cast microstructure exhibits one body centre cubic and two face centre cubic high entropy phases based on titanium, cobalt and copper respectively along with an eutectic containing Ti2Ni type Laves phase. On the contrary, spinodal decomposed microstructure consisting of cobalt and copper solid solution is obtained in the sintered sample. However, long term annealing of cast sample at 950 oC reveals an eutectoid transformation with different phases than the cast sample. The aforementioned observations are discussed using CALPHAD thermodynamical approach and available literature

The evolution of microstructure and phase formation in equiatomic Ti20Fe20Ni20Co20Cu20 high entropy alloy synthesised by conventional arc melting followed with suction casting and ball milling with spark plasma sintering route is distinctly different. The cast microstructure exhibits one body centre cubic and two face centre cubic high entropy phases based on titanium, cobalt and copper respectively along with a eutectic containing Ti2Ni type Laves phase. On the contrary, spinodal decomposed microstructure consisting of cobalt and copper solid solution is obtained in the sintered sample. However, long term annealing of cast sample at 9508C reveals a eutectoid transformation with different phases than the cast sample. The aforementioned observations are discussed using CALPHAD thermodynamical approach and available literature.

Transactions of the Indian Institute of Metals, 2018

The current study is aimed to understand the microstructure evolution in Co 20 Fe 20 Mn 20 Ni 20 Ti 20 and Co 25 Fe 25 Mn 5 Ni 25 Ti 20 high entropy alloys (HEAs). The complete X-ray diffraction and scanning electron microscopy coupled with energy dispersive spectroscopy analyses are carried out to recognize the phases in addition to understand the series of phase development in the presently studied HEAs. For Co-Fe-Mn-Ni-Ti HEAs, at first BCC (b) phase primary dendritic phase is created from the liquid, followed by peritectic reaction to form FCC (a) phase (i.e. BCC (b) ? L ? FCC (a)). Then the left over liquid undergoes eutectic reaction to form FCC (a) and Ti 2 (Ni, Co) phases (i.e. L ? FCC (a) ? Ti 2 (Ni, Co)). Furthermore, the pseudo quasi-peritectic reactions i.e. L ? BCC (b) ? FCC (a) ?Ti 2 (Ni, Co) has been proposed based on the microstructure evolution study in case of Co-Fe-Mn-Ni-Ti HEAs. It is important to note that Co-Fe-Mn-Ni-Ti HEAs show eutectics between FCC (a) and Ti 2 (Ni, Co) phases. The multicomponent Co 25 Fe 25 Mn 5 Ni 25 Ti 20 eutectic HEA shows the retention of high strength at elevated temperature.

Metallurgical and Materials Transactions A, 2014

In the present study, a Co1.5CrFeNi1.5Ti0.5 high-entropy alloy has been investigated for its high-temperature microstructural stability. This material is shown to possess mainly a facecentered cubic (FCC) structure; the g phase is present at the interdendritic region in the as-cast condition, and it is stable between 1073 K and 1273 K (800°C and 1000°C); c¢ particles are found throughout the microstructures below 1073 K (800°C). Segregation analysis has been conducted on a single crystal sample fabricated by a directional solidification process with a single crystal seed. Results show that Co, Cr, and Fe partition toward the dendritic region, while Ni and Ti partition toward the interdendritic areas. Scheil analysis indicates that the solid-liquid partitioning ratio of each element is very similar to those in typical single crystal superalloys.

Journal of Materials Engineering and Performance, 2020

There has been great attention on high-entropy alloys (HEAs) over the past decade. Unlike conventional alloy systems, HEAs commonly include at least five principal elements with equiatomic or near-equiatomic ratio. HEAs with their superior mechanical, magnetic, and thermal properties are promising materials for critical engineering applications. Medium-entropy alloys (MEAs), which consist of less than five principal elements, have very similar structural features with HEAs such as robust thermodynamic stability and exceptional mechanical performance. The insights of MEAs have not been fully revealed yet. In the present study, novel MEAs (Cu 20 Ni 20 Al 30 Ti 30 , Cu 25 Ni 25 Al 25 Ti 25 , Cu 34 Ni 22 Al 22 Ti 22 , and Cu 35 Ni 25 Al 20 Ti 20 ) have been designed using thermo-physical calculations and Thermo-Calc software. These MEAs were then produced using copper heart arc melting and suction cast into cylindrical rods with 3 mm diameters. X-ray diffraction (XRD), optical microscop...

Proceedings of Solidification and Gravity VII, 2018

The present study is aimed at understanding the sequence of phase evolution during solidification in Co 25 Fe 25 Ni 25 Ti 20 V 5 eutectic high entropy alloys (EHEAs), synthesized byvacuum arc melting cum suction casting technique. The detailed X-ray diffraction (XRD) andelectron microscopic (SEM and TEM) coupled with energy dispersive spectroscopic (EDS)analyses reveal the presence of BCC (β), FCC_1 (α1), FCC_2 (α2) and Ni 3 Ti phases. For Co-Fe-Ni-Ti-V HEA, at first Ni 3 Ti (DO24) primary dendritic phase is formed from theliquid, followed by peritectic reaction to form FCC_1 (α1) phase (i.e. Ni 3 Ti + L⟶FCC_1(α1)). Then the remaining liquid undergoes eutectic reaction to form FCC_1 (α1) and FCC_2 (α2)phases (i.e. coarse eutectic: L⟶FCC_1 (α1) + FCC_2 (α2)). Finally, the remaining liquidundergoes eutectic reaction to form BCC (β), and FCC_2 (α2) (i.e. fineeutectic: L⟶BCC (β) + FCC_2 (α2). Therefore, based upon sequence of microstructuralevolution, two pseudo-quasiperitectic reactions i.e. L+ Ni 3 Ti → FCC_1 (α1) + FCC_2 (α2) and L+ FCC_1 (α1) ⟶BCC (β) + FCC_2 (α2) have been proposed for the investigated EHEA. It isalso found that Co 25 Fe 25 Ni 25 Ti 20 V 5 EHEAs retains high strength at elevatedtemperature.

Journal of Alloys and Compounds, 2020

High entropy alloys has been proposed as novel binder phases in cemented carbides and cermets. Many aspects related to the stability of these alloys during the liquid phase sintering process are still unclear and were addressed in this work. Consolidated Ti(C,N)-based cermets using four different (Co,Fe,Ni)-based high entropy alloys as the binder phase were obtained. The chosen alloys -CoCrCuFeNi, CoCrFeNiV, CoCrFeMnNi and CoFeMnNiV -were previously synthesized through mechanical alloying and a single alloyed solid solution phase with fcc structure and nanometric character was always obtained. The powdered alloys and the consolidated cermets were analyzed by X-ray diffraction, scanning electron microscopy, X-ray energy dispersive spectrometry and transmission electron microscopy. Differential thermal analysis was employed to determine the melting point of the four high entropy alloys that ranged between 1310 ºC and 1375 ºC. Although a high temperature of 1575 ºC was required to obtain the highest cermet densification by pressureless sintering, porosity still remained in most of the cermets. Best densification was achieved when CoCrFeNiV was used as the binder phase. During liquid phase sintering, different compositional changes were observed in the ceramic and binder phases. A core-rim microstructure was observed in cermets containing V in the alloys (CoCrFeNiV and CoFeMnNiV), since this element was incorporated to the carbonitride structure during sintering. A slight Cr segregation was detected in cermets containing Cr, leading to CrTi-rich alloys in small binder regions. However, a great Cu segregation was produced when CoCrCuFeNi was used, and the formation of two different fcc alloys -a Cu-rich and a Cu-depleted-was observed. Finally, a loss of Mn was also evidenced in CoCrFeMnNi and CoFeMnNiV, probably due to its sublimation at the sintering temperature.

Materials Science and Engineering: A, 2005

It is now well established that dilute Cu-Ti alloys with titanium in the range of 2.5-5 wt.% decompose by a spinodal mechanism and, also, that the decomposition begins during the process of quenching itself from the temperature at which the alloy is solution treated. To study the initial stages of the decomposition reaction, the necessary suppression of the decomposition was attempted and achieved either by addition of a small quantity of a third element, namely, cobalt to the binary Cu-Ti alloy or by melt-spinning the binary alloy. Both the approaches were found to be successful. The results obtained through the second approach were recently published. The present work reports the results obtained through the former approach. In this work, the decomposition reaction in the ternary alloy was studied at temperatures of 673 and 723 K. The important findings relate to the effect of cobalt on the precipitation process and the properties of the alloy. During aging, the metastable Cu 4 Ti (D1 a) was seen to evolve in this ternary alloy without the initial 1 1/2 0 ordering in the titanium enriched regions and the formation of the special point N 3 M phase that were seen to precede the formation of the metastable Cu 4 Ti (D1 a) in the melt spun alloy. Also, the ternary alloy was found, in the aged condition, to be extremely strong and ductile. Moreover, unlike the Cu-Ti binary alloy, the ternary alloy was found to be resistant to over-aging.

Materials Science and Engineering: A, 1999

Alloys of composition Cu-(11.8-13.5)%Al-(3.2-4)%Ni-(2-3)%Mn and 0-1%Ti (wt.%) were cast using the melt spinning method in He atmosphere. Ribbons obtained in this process showed grains from 0.5 to 30 mm depending on the type of alloy and wheel speed. Bulk alloys and most of the ribbons contained mixed 18R and 2H type martensite at room temperature (RT). Some ribbons, crystallizing at the highest cooling rate, retained also b phase due to a drop of M s below RT. The M s temperatures in ribbons were strongly lowered with increasing wheel speed controlling the solidification rate. This drop of M s shows a linear relationship with d − 1/2 , where d is grain size. The strongest decrease of M s and smallest grains were found in the ribbons containing titanium due to its grain refinement effect. The cubic Ti rich precipitates, present in both Cu-Al-Ni-Ti and Cu-Al-Ni-Mn-Ti bulk, were dispersed in ribbons cast with intermediate cooling rates of up to 26 m s − 1 , but suppressed for higher cooling rates. The transformation hysteresis loop was much broader in ribbons due to presence of coherent Ti rich precipitates and differences in grain size which is particularly important in the ultra small grain size range.

(Fe, Co, Ni) rich dendrites nucleate primarily in CoCrFeMoNi and CoCrCu 0.1 FeMoNi alloys, followed by peritetic and eutectic reactions. The quasi-peritectic reaction occurs between the primary Mo-rich dendrites and liquids in the CoCrCu 0.3 FeMoNi melts, and transfers to a eutectic coupled-growth at the edge of the quasi-peritectic structure. Subsequently, eutectic reaction happens in the remnant liquids. Liquid-phase separations have occurred in CoCrCu x FeMoNi alloys when x Z0.5. Meanwhile, some na-noscale precipitates are obtained in the Cu-rich region. Two crystal structures, FCC and BCC, are identified in CoCrCu x FeMoNi high entropy alloys. Amazingly, a pretty high plastic strain (51.6%) is achieved in CoCrCu 0.1 FeMoNi alloy when the compressive strength reaches to 3012 Mpa. With the increase of Cu content, atomic size difference (ΔR) and electro-negativity difference (ΔX) decrease while valence electron concentration (VEC), mixing enthalpy (ΔH) and mixing entropy (ΔS) increase. Consequently, the valence electron concentration (VEC) values range for the formation of mixture of FCC and BCC structures can be enlarged to 6.87–8.35 based on the study of this paper. It is the positive enthalpies of mixing that causes the liquid-phase separation in CoCrCu x FeMoNi high entropy alloys.