Low temperature compressive creep in electrodeposited nanocrystalline nickel

Steady state creep was not observed during large strain compressive creep in electrodeposited nanocrystalline-Ni. An additional exothermic peak during differential scanning calorimetry of deformed samples is attributed to recovery associated with geometrically necessary dislocations. The lack of change in texture suggests that grain boundary sliding and rotation occurred during creep

Effect of molybdenum and niobium on the phase formation and hardness of nanocrystalline CoCrFeNi high entropy alloys

In the present study, influence of molybdenum and niobium additions on phase formation during mechanical alloying and spark plasma sintering of CoCrFeNi high entropy alloy was studied. Major FCC and minor BCC phase were observed after mechanical alloying of CoCrFeNi. However, major FCC and sigma phase were observed after spark plasma sintering. A maximum relative density of 95% was obtained with the hardness of 570 HV in CoCrFeNi HEA. The phase formation behavior was not significantly affected by the addition of molybdenum or niobium. However, addition of Mo to CoCrFeNi increased the hardness from 570 HV to 620 HV and the hardness increased to 710 HV with combined addition of molybdenum and niobium. After sintering, major FCC phase with crystallite size of 60–70 nm was observed in all the compositions. Further, the microstructure and hardness retention was observed in CoCrFeNiMo_0.2 with annealing temperature up to 800°C

High temperature deformation behaviour of a Mg-0.8Al alloy

There is considerable interest currently in developing magnesium based alloys as replacements for aluminum alloys in automobile applications, due to their high specific strength as compared to aluminum alloys. However, the poor formability of magnesium alloys has restricted their applications; superplasticity can be utilized to form components with complex shapes. In the present study, the compressive deformation characteristics of a Mg-0.8 wt% Al alloy with an initial grain size of 19 ± 1.0 μmm have been studied in the temperature range of 623-673 K and at strain rates ranging from 10<SUP>-7</SUP> to 10<SUP>-3</SUP> s<SUP>-1</SUP>. The stress exponent was observed to decrease with a decrease in stress. The results are analyzed in terms of the existing theoretical models for high temperature deformation. Furthermore, the potential for superplasticity in this alloy is explored, based on the mechanical and microstructural characteristics of the alloy

The deformation characteristics of nanocrystalline metals

A reduction in the grain size of a metal offers a means for enhancing the strength by retarding dislocation activity, and at the same time enhancing ductility by processes involving grain boundaries. This overview critically examines various issues relating to strengthening and ductility in nanometals by considering both limitations to strengthening and processes contributing to weakening. It highlights the continuing interesting areas for further research

Low temperature compressive creep in electrodeposited nanocrystalline nickel

Steady state creep was not observed during large strain compressive creep in electrodeposited nanocrystalline-Ni. An additional exothermic peak during differential scanning calorimetry of deformed samples is attributed to recovery associated with geometrically necessary dislocations. The lack of change in texture suggests that grain boundary sliding and rotation occurred during creep

Hot working of an as-Cast Mg-2%Al alloy

Magnesium based alloys are being considered currently as replacements for aluminum alloy components in automobile industry, due to their high specific strength as compared to aluminum alloys. However, the utilization of Mg alloys is restricted due to their poor formability, and consequently these have been used mostly in the as cast condition. In order to increase the utilization of the wrought forms, it is necessary to develop optimum deformation processing conditions for producing defect free wrought products economically from the cast alloys. In the present study, the compressive deformation characteristics of as cast Mg-2wt% Al alloy with an equiaxed initial grain size of 150 +/- 10 mum have been studied in the temperature range of 573-723 K and at strain rates ranging from 10(-2) to 10 s(-1). The results are analyzed in terms of the existing theoretical hot working models together with the microstructural characteristics of the alloy. Furthermore, optimum. conditions for subsequent deformation processing have been identified based on a combination of mechanical data together with the deformed microstructures

Phase formation in mechanically alloyed Al_xCoCrCuFeNi (x = 0.45, 1, 2.5, 5 mol) high entropy alloys

Alloying behavior and phase transformations in AlxCoCrCuFeNi (x = 0.45, 1, 2.5, 5 mol) multi-component high entropy alloys that are synthesized by mechanical alloying were studied. Two FCC phases along with a BCC phase were formed in Al0.45CoCrCuFeNi and AlCoCrCuFeNi, while a single B2 phase was observed in higher Al containing alloys Al2.5CoCrCuFeNi and Al5CoCrCuFeNi. DSC analysis indicates that BCC phase present in the alloys could be Fe–Cr type solid solution. A detailed analysis suggests that two melting peaks observed during DSC in lower Al containing alloys can be attributed to that of Cu–Ni and Fe–Ni FCC solid solutions. The BCC phase disappears in Al0.45CoCrCuFeNi and AlCoCrCuFeNi at high temperatures during DSC. However, Al5CoCrCuFeNi retains its B2 structure despite of heating in DSC. Further, phases present in these alloys retain nanocrystallinity even after exposure to high temperatures. A critical analysis is presented to illustrate that solid solution formation criteria proposed for high entropy alloys in the literature are unable to explain the phase formation in the present study of alloys. Besides, these criteria seem to be applicable to high entropy alloys only under very specific conditions

Alloying behavior in multi-component AlCoCrCuFe and NiCoCrCuFe high entropy alloys

Multi-component High Entropy Alloys (HEAs) are observed to form simple solid solutions in contrary to general perception that complex compounds may form in such multi-component equi-atomic alloys. In the present study, alloying behavior was investigated using XRD in AlCoCrCuFe and NiCoCrCuFe equi-atomic high entropy alloys synthesized by Mechanical Alloying (MA) and Spark Plasma Sintering (SPS). Simple FCC and BCC phases evolved after MA, while Cu-rich FCC and sigma (&#963;) phases evolved along with FCC and BCC phases after SPS. Further, NiCoCuFe, NiCoCrFe and NiCoFe equi-atomic alloys were investigated to confirm the formation of Cu-rich FCC, and &#963; phases. The hardness was observed to be 770 &#177; 10 HV for AlCoCrCuFe and 400 &#177; 10 HV for NiCoCrCuFe. Phase evolution after MA and SPS indicate that configurational entropy is not sufficient enough to suppress the formation of Cu-rich FCC and &#963; phases and enthalpy of mixing appears to play an important role in determining the phase formation in high entropy alloys after sintering

Alloying, thermal stability and strengthening in spark plasma sintered Al_xCoCrCuFeNi high entropy alloys

AlxCoCrCuFeNi (x = 0.45, 1, 2.5 and 5 mol) multi-component high entropy alloys synthesized by mechanical alloying were spark plasma sintered to produce high dense compacts. X-ray diffraction and scanning electron microscopy studies reveal that these sintered alloys exhibit varying microstructures from single phase to three phases depending on Al content. The thermal stability studies carried out in the temperature range of 400–600°C for duration of 2–10 h in Ar atmosphere suggest that these alloys exhibit excellent thermal stability in terms of phases and crystallite size. Highest specific hardness of 160 (HV/g cm−3) is achieved in the sintered Al5CoCrCuFeNi alloy and there is no significant change in the hardness after heat treatment of Al0.45CoCrCuFeNi and AlCoCrCuFeNi alloys. Hall–Petch analysis based on hardness measurements carried out on sintered samples reveals that solid solution strengthening seems to increase with increase in Al content

Characterization of oxide dispersed AlCoCrFe high entropy alloy synthesized by mechanical alloying and spark plasma sintering

The present study deals with phase evolution of oxide dispersed AlCoCrFe high entropy alloy during mechanical alloying and spark plasma sintering. Mechanical alloying of AlCoCrFe resulted in a single BCC phase. However, ordering of BCC phase with evolution of chromium carbide and sigma phase were observed after spark plasma sintering. High hardness of 1,050 &#177; 20 HV1 and 1,070 &#177; 20 HV1 was observed for AlCoCrFe high entropy alloy without and with oxide dispersion, respectively. Significant contribution from solid solution strengthening effect in high entropy alloys appears to have overwhelmed the effect of oxide dispersion on hardness