Conclusions

Reactive sintering (RHP and RHIP) have been utilized to produce a variety of ceramics, intermetallics, and composite materials. Table 2 summarizes the processing conditions and tensile properties of select materials produced by reactive powder processing. This is not intended to be an all-inclusive list, but representative of materials produced by these methods.

Table 2 Conditions and tensile properties of select compounds, alloys, and composites fabricated by RS, RHP, and RHIP

Materials

Process

Microstructure

Room-temperature tensile properties

Ref

Yield strength

Ultimate tensile strength

Elongation to fracture

MPa

(f7lITS),MPa

(if), %

Nickel aluminides

Ni3Al

Ni + 3Al: RS: 700 °C, vacuum

Ni3Al 97% dense

270

270

1

18

NijAl + B

Ni + 3Al + B doped: RS: 700 °C, vacuum

Ni3Al 97% dense

353

682

12

18

Ni + 3Al + B doped: RHIP:

Ni3Al, Ni5Al3 100%

265

727

10

18

800 °C, 0.5 h, 104 MPa

dense

Ni + 3Al + B doped: RHIP:

Ni3Al, Ni5Al3 100%

494

677

2

18

1100 °C, 1 h, 170 MPa

dense

Ni + 3Al + B doped: RHIP:

Ni3Al 100% dense

591

827

5

18

1100 °C, 1 h, 170 MPa + heat

treatment

Ni3Al

Ni + 3Al; RS: 620 °C, 15 min

Ni3Al: 98% dense

21

(heat rate 60 K/min)

Ni + 3Al; CS + HP: 550 °C, 25

Ni3Al: 98% dense

37

MPa initiated reaction with a

Ti + B at 550 °C

NijAl + B

Ni + 3Al + B doped; RHP: 1250 °C

Ni3Al 98.6% dense

401

401 0.3

34

NijAl + B + 3.56

Ni + 3Al + B doped + Al2O3;

Ni3Al, Al2O3 99%

521

625 4.7

34

vol% Al2O3

RHP: 1250 °C

dense

particles

Ni3Al + B + 6.88

Ni + 3Al + B doped + Al2O3;

Ni3Al, Al2O3 98.7%

486

486 0.9

34

vol% Al2O3

RHP: 1250 °C

dense

particles

Ni3Al + 20 vol%

Ni + 3Al + Y2O3: RHIP: 800

Ni3Al, Y2O3 100%

464 ...

18

Y2O3 particles

°C, 1 h, 170 MPa

dense

NiAl

Ni + Al + 25 wt% NiAl: RS: 700 °C, 15 min vacuum

NiAl 100 dense

25

Ni + Al + 20 wt% NiAl: RS +

NiAl 100% dense

135

154 (at 800 °C) 14

25

HIP: 1200 °C, 172 MPa, 1 h

NiAl + 20 vol%

Ni + Al + 20 wt% NiAl +

NiAl, TiB2 100%

344

344 (at 800 °C) 0

25

TiB2 particles

TiB2: RS + HIP: 1200 °C, 172 MPa, 1 h

dense

NiAl + 10 vol%

Ni + Al + Al2O3 RS + HIP:

NiAl, Al2O3 100%

142

163 (at 800 °C) 3

38

Al2O3 aligned sort

1200 °C, 172 MPa, 1 h

dense

fibers

Iron aluminides

FejAl

Fe + 3Al; RHIP: 1000 °C, 140 MPa

Fe3Al 100% dense

759

759 0

39

FejAl + 5% Cr

Fe + 3Al + 5Cr: RHIP + 24 h/1100 °C/ + HIP 1100 °C/207 MPa/ + 2 h/750 °C/oil quench

Fe3Al 100% dense

857

1095 7

39

Fe3Al

Fe + 3Al; RHP 1250 °C

Fe3Al 98.2% dense

404

521 0.3

34

Titanium aluminides

TiAl

Ti + Al: extruded (R = 350) + RS: 1350 °C, 6 h

TiAl 98% dense

33

Ti-50Al

Ti + Al; extruded (R = 14) + RHIP: 1100 °C, 2 h, 150 MPa

TiAl; Ti3Al 100% dense

450

750 (at 700 °C) . . .

40

Ti-49Al

Ti + Al; extruded (R = 14) + RHIP: 1000 °C, 3 h, 200 MPa + HT: 1250 °C, 3 h

TiAl; Ti3Al 100% dense

380

710 . . .

40

Ti + Al; extruded (R = 14) +

TiAl; Ti3Al 100%

300

750 (at 700 °C) . . .

40

RHIP: 1000 °C, 3 h, 200 MPa

dense

+ HT: 1250 °C, 3 h

Ti-48Al-2Cr

Ti + Al + Cr; extrude + RHIP: 1100 °C, 3 h, 125 MPa

TiAl; Ti3Al 100% dense

700

1500 0.2

41

Ti + Al + Cr; extrude + RHIP:

TiAl; Ti3Al 100%

650

1550 (at 500 °C) 0.25

41

1100 °C, 15 min, 125 MPa

dense

TiAl-Ti5Si3

Ti + Al + Si: RHP: 1300 °C, 15 min, 6.5 MPa

TiAl; Ti3Al; Ti5Si3

42

TiAl-TiB2

Ti + Al + B: RHP: 1300 °C, 15 min, 6.5 MPa

TiAl; Ti3Al; TiB2

42

TiAl-TiC

Ti + Al + C: RHP: 1300 °C, 15 min, 6.5 MPa

TiAl; Ti3Al; TiC

42

(Al,Cr)3Ti

Al + Ti + Cr: RHIP: 1250 °C, 2 h, 173 MPa

Al66Cr9Ti25 100% dense

470

1360 (in compression) 14

43

Other intermetallics

MoSi2

Mo + 2Si: RHP: 1600 °C, 45 min, 36 MPa

MoSi2 89% dense

44

Mo(Si,Al)2

33Mo + 46.7Si + 20Al: RHP: 850 °C, 15 min, 30 MPa

Mo(Si,Al)2, Mo, Mo3Si, 90% dense

27

NbAl3

Nb + 3Al: RHIP: 1200 °C, 4 h, 173 MPa

NbAl3, NbAl2 >98% dense

22

NbAl3

Nb + 3Al: RS: 1200 °C, 1 h

NbAl3 95% dense

22

Ceramics and cermets

TiC

Ti + C: combustion synthesized

TiC 50% dense

11

Ti + C: RHP: 1600 ° C, 27.6

TiC 95% dense

11

MPa

SiC

Si + C: high-pressure combustion synthesis 3 GPa

JSiC 90% dense . . .

45

TiB2

Ti + 2B: high-pressure combustion synthesis 3 GPa

TiB2 95% dense . . .

46

TiB2/SiC-Ni

TiH2 + Si + B4C + (2 wt%)Ni:

TiB2, SiC, Ni 99% . . .

rc(SENB) = 6.6 . . .

47

RHP: 2000 °C, 1 h, 30 MPa

dense

MPa (JF (three-point bend) = 496 MPa

TiB2/SiC

TiH2 + Si + B4C: RHP: 2000

TiB2, SiC, Ni 99% . . .

rc(SENB) = 8.7 . . .

47

°C, 1 h, 30 MPa

dense

MPa v/^ (Jp (three-point bend) = 330 MPa

TiB2-Al

TiH2 + AlB2: RHP: 1500 °C, 34 MPa

TiB2-Al . . .

48

TiC-NiAl

Ti + C + Ni + Al: RHP: 21 MPa

TiC, NiAl 94-100% . . . dense

30

CS, combustion synthesis; Kc, critical stress intensity; iJF, fracture stress; SENB, single-edge notched bend test

CS, combustion synthesis; Kc, critical stress intensity; iJF, fracture stress; SENB, single-edge notched bend test

The microstructure that develops during reactive sintering is influenced by a variety of process parameters, including the green structure, the sintering conditions, and product composition. An interconnected configuration of the reactant powders minimizes pore formation during reactive sintering. The distribution of the reactants in the green structure can be adjusted by altering the powder particle characteristics (e.g., particle size ratio) or the green processing method (e.g., extrusion, rolling versus compaction). The parameters of the sintering cycle that influence reactive sintering, include heating rate, sintering atmosphere, and application of pressure. Heating rates and sintering atmospheres that limit interdiffusion and intermediate compound formation prior to the reaction between the powders are preferred. Application of pressure during combustion synthesis results in dense products, but the microstructure of these products may be inhomogeneous, particularly if large amounts of transient liquid form during processing. Both the densification and inhomogeneity can be attributed to the applied pressure forcing the transient liquid into pores between solid powders. Finally, composition of the product can affect the reaction, by the addition of alloying elements and diluent phases (inert, high melting, reinforcement phases). These can affect the amount, duration, and distribution of the transient liquid phase and can also affect the diffusion rates between the reacting elements. With the judicial selection of all these parameters, dense and useful components can be produced by reactive sintering.

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