Reaction-sintering of intermetallic alloys of the Ni–Al–Mo system

Reaction-sintering of intermetallic alloys of the Ni±Al±Mo system

S. Gialanellaa,*, L. Lutterottia, A. Molinaria, J. Kaziorb, T. Pieczonkac

aDipartimento di Ingegneria dei Materiali, UniversitaÁ di Trento, Mesiano di Trento, 38050 Trento I, ItalybCracow University of Technology, Cracow, Poland

cAcademy of Mining and Metallurgy, Cracow, Poland

Received 20 June 1999; accepted 18 October 1999

Abstract

A powder metallurgy route has been used for producing binary and ternary alloys of the Ni±Al±Mo system. Elemental powder

mixtures were compacted and, then, sintered in a dilatometer. In this way the dimensional changes involved with thermally inducedtransformations could be followed during continuous heating runs up to the sintering temperatures. Sintering was assisted by theformation of a liquid phase, promoted by the heat output coming from the intermetallic phase formation reactions. The amount of

liquid phase and the e�ciency of sintering was highly dependent on the heating rate. A threshold value for optimal densi®cationwas identi®ed for some compositions. The e�ect of other processing parameters, such as pre-sintering compaction pressure andsintering atmosphere has been considered too. The characterisation of the ®nal products was mainly based on X-ray di�raction

analyses. The microstructural parameters and the phase composition of the sintered materials were evaluated. On the basis of theseresults it is possible to draw some conclusions concerning the main phenomena occurring during the sintering process. # 2000Elsevier Science Ltd. All rights reserved.

Keywords: A. Nickel aluminides, based on Ni3Al and NiAl; C. Reaction synthesis; F. Di�raction

1. Introduction

Several process routes have been adopted for theproduction of intermetallic-based alloys [1]. For thisclass of materials, powder metallurgy (PM) methods areparticularly interesting for most of the reasons why theyare so for ceramics. Indeed, near-net-shape componentscan be directly manufactured, thus avoiding thosemachining and/or mechanical working steps which areusually di�cult to carry out owing to the intrinsic brit-tleness of many intermetallic compounds. Several PMtechniques have been pro®tably applied to the synthesisof intermetallic materials [2,3] and novel approaches arebeing developed to satisfy speci®c requirements and toimprove the quality of the ®nal products [4]. Forinstance, self-propagating high-temperature synthesis(SHS) methods are receiving increasing attention for thesynthesis of intermetallic materials. These processes arebased on the propagation of a reaction front through acompact of elemental powders, that react to form new

phases, while densi®cation and sintering occur. Thesethermally activated transformations, are sustained bythe exothermic heat of formation of the relevant inter-metallic phases. In the present study a particular SHSprocess has been considered, i.e. reactive sintering (RS).Elemental powders, pressed together to form ``green''compacts, are subsequently placed in a furnace andheated up to trigger the formation reactions. Theenthalpy output, associated with the solid state reac-tions occurring at the interface of powder grains mayinduce the formation of a liquid phase, which startsspreading along grain boundaries thanks to capillaryforce. Once the liquid phase is formed, the sinteringkinetics is very much enhanced and rapidly accom-plished.Several intermetallics have been reactively synthe-

sized. For example Ni, Fe and Ti aluminides [5±12] andrefractory silicides [13±14]. One of the most interestingfeatures of reaction-sintering is the comparatively lowtemperature at which the process can be carried out,also in the case of high-melting materials, with sig-ni®cant energy savings. Indeed, temperatures close tothe lowest eutectic in the system are interesting for thiskind of process. However, RS products often su�er

0966-9795/00/$ - see front matter # 2000 Elsevier Science Ltd. All rights reserved.

PI I : S0966-9795(99 )00108-9

Intermetallics 8 (2000) 279±286

* Corresponding author. Tel.: +39-0461-882420; fax: +39-0461-

881977.

E-mail address: [email protected] (S. Gialanella).

from porosity, inhomogeneus shrinkage, impurity seg-regation, and other aspects related to the processingparameters.It was formerly studied how such parameters as

``green'' compaction pressure, heating rate, sinteringatmosphere may in¯uence sintering, which was carriedout for this purpose in a dilatometer, of Ni±Al±Moalloys [15]. In the present study, the e�ciency of sinter-ing as a function of the processing parameters has beenconsidered. A microstructural investigation of sinteredspecimens was carried out and the concentrations of thephases present in the bulk products were evaluatedthrough an X-ray di�raction (XRD) approach. On thebasis of the results obtained so far it was possible toidentify the best sintering conditions and possibleimprovements to the process.

2. Experimental

Pure elemental powders, with particle size rangingfrom 20 to 63 mm, were used for the sintering experi-ments. Binary and ternary compositions were investi-gated. Two binary Ni±Al powder mixtures withcompositions Ni±25Al at% (Ni3Al) and Ni±50Al at%(NiAl) were prepared. In addition, three ternary com-positions were considered. `À'' composition (Ni±42Al±5Mo at%) falls in the two phase region, where the b±NiAland a±Mo mixture is stable. For the other two composi-tions, ``B'' (Ni±36Al±5Mo at%) and ``C'' (Ni±30Al±5Moat%), the g0-Ni3Al phase is present too.Powder mixing was carried out in a turbula mixer for

60 min. Powder mixtures were then uniaxially pressedinto 4 � 4 � 15 mm3 ``green'' compacts using a pressureof 300 or 400 MPa. No lubricant was used. ``Green''densities were near 75% of theoretical values. Sinteringwas performed in a horizontal NETZSCH 402E dilat-ometer under vacuum better than 10ÿ3 Pa. Alter-natively, a hydrogen ¯ux was used. Specimens wereheated at di�erent rates, which, depending on the com-position, varied from 2 up to 30�C/min. The holdingtime at the maximum sintering temperature of 750�Cwas 15 min in all cases.After sintering, the bulk pieces were analysed for their

phase composition by X-ray di�ractometry (XRD).Di�raction measurements were carried out on alloysamples, which were metallographically polished toremove a thin outer oxide layer, which was inevitablypresent after the sintering treatments. Di�raction pat-terns were acquired with a wide angle di�ractometerusing a CuKa radiation monochromated on the dif-fracted beam. All di�raction data were analysed using afull pattern-®tting procedure based on the Rietveldmethod [16]. The microstructural parameters of thephases present in the ®nal products were evaluatedalong with their concentrations.

Optical microscopy observations were carried out onmetallographically polished and etched (33% HNO3±33% CH3COOH±33% dist. H2O±1% HF) samples.

3. Results

3.1. Binary compositions: Ni3Al material

In Fig. 1 a typical dilatometric curve and the corre-sponding temperature cycle for a powder mixture havingNi3Al composition are shown. The powders, compactedat 300 MPa pressure, were heated up with a 10�C/minrate. A sudden expansion was observed at 494�C. Atthis same temperature an exothermic peak was recordedin the temperature curve. The volume change is not fullyrecovered upon cooling down the sample, which in factdisplays a residual elongation of 1.8%. The situationwas improved when higher heating rates were adopted,as clearly shown by the diagram in Fig. 2(a), in whichthe densi®cation behaviour for various heating rates isillustrated. At 20�C/min a contraction of 1.5% wasobtained. From XRD data analysis it was possible toevaluate the phase compositions of the reactively sin-tered materials. Fig. 2(b) displays the evolution of theconcentrations (wt%) of the phases which were identi-®ed in the di�raction patterns of the ®nal products. Thehighest amount of Ni3Al, i.e. the expected intermetallicphase, was obtained at the maximum heating rate. Dif-fraction peaks from NiAl were also detected in the dif-fraction patterns. The concentration of this phase goesdown with increasing heating rate (Fig. 2(b)). Someunreacted nickel was also present, possibly as a Ni±Alsolid solution. When heating the powder compacts atthe slowest rate, i.e. 2�C/min, a small amount of Al3Ni2was observed (Fig. 3). The optical micrographs in Fig. 4show the e�ect of di�erent heating rates on the resulting

Fig. 1. Dilatometric and temperature curves for the Ni3Al specimen

sintered at 10�C/min under vacuum.

280 S. Gialanella et al. / Intermetallics 8 (2000) 279±286

microstructure of the materials. In Fig. 4(a) the mor-phology of the Ni3Al material sintered at 10�C/min dis-plays a di�use porosity: large pores scattered amongcrystallised grains, and regions with many round shapedpores clustered together can be seen. Porosity is compara-tively lower in the material sintered at 20�C/min (Fig.4(b)).For the Ni3Al composition other sintering conditions

were explored. In particular, with a reducing hydrogen

atmosphere a general suppression of the intermetallicphase formation reactions was observed. In the pro-ducts the majority phase was nickel (Table 1).A very positive e�ect when using a higher pre-sinter-

ing compaction pressure was observed. A sample com-pacted at 400 MPa, rather than 300 MPa and thensintered at 10�C/min produced a higher yield of Ni3Althan that obtained under other conditions. This datumis quoted in Table 1, where a summary of the sinteringconditions and the corresponding phase concentrationsfound at the end of the process for the Ni3Al powdermixture is given. In this table the exothermic peak tem-peratures are also listed.

3.2. Binary compositions: NiAl material

The sintering behaviour of the NiAl powder mixturespresents some di�erences as compared to the Ni3Almaterials. Fig. 5 shows the dilatometric and the corre-sponding temperature curve for the sample sintered at10�C/min under vacuum (which was the only atmo-sphere considered for the present composition) andpressed before sintering at 300 MPa. A sharp dimen-sional drop occurs at 484�C, corresponding to a strongexothermic peak in the temperature curve. A maximumrate of 10�C/min was adopted for the NiAl composi-tion, in order to prevent explosive reactions, actuallyobserved during a few attempts carried out at higherrates. At 10�C/min the formation reaction proved to befully accomplished and NiAl was the only phase detec-ted in the di�raction pattern, as shown in Fig. 6. Thedi�raction patterns for the specimens sintered at 2, 3and 4�C/min are very much the same as that in Fig. 6.However, a signi®cant di�erence was observed in thedilatometry behaviour of the specimen for which thelowest heating rate was adopted. In this case no exo-thermic peak was recorded, nor shrinkage associated withthe formation-sintering reactions. Microscopy obser-vations conducted on the sample sintered at 4�C/mindisplayed a fairly uniform microstructure with residualporosity mainly localised along grain boundaries(Fig. 7).In view of the improvements obtained with the Ni3Al

materials upon increasing the pre-sintering compactionpressure from 300 MPa to 400 MPa, the same was donefor the NiAl samples. This time the increased pressurereduced, rather than increasing, the reaction yield, inthat other phases than NiAl formed. This is shown inthe di�raction pattern of the specimen sintered at2�C/min (Fig. 8), where also the re¯ections of Ni3Al andNi2Al3 were detected. Moreover the exothermic peakcorresponding to the liquid phase assisted formation ofintermetallics is very close to the lowest eutectic tem-perature of the Ni±Al system, i.e. 640�C. In Table 2 themain aspects concerning the sintering experiments withNiAl specimens are listed.

Fig. 3. XRD pattern of the Ni3Al sample heated at 2�C/min. The

experimental (dots) and simulated (line) patterns are displayed.

Fig. 2. Diagrams showing the e�ect of the heating rate on the (a) ®nal

densi®cation and (b) phase compositions of the Ni3Al specimens. The

actual values of sample expansion are indicated in the graph.

S. Gialanella et al. / Intermetallics 8 (2000) 279±286 281

3.3. Ternary compositions: Ni±Al±Mo

Ternary Ni±Al±Mo powder mixtures were consideredfollowing up a former investigation, carried out onsimilar compositions [15]. On that occasion the dimen-sional changes involved with dilatometric heating andcooling runs were studied. On the basis of those ®ndingsand of the indications coming from the present experi-ments on binary compositions, dilatometric runs onternary powder compacts were carried out undervacuum, using in all cases a compaction pressure of 300MPa. In Table 3 the relevant paramenters of this seriesof tests and the results of subsequent sample analysisare displayed.Powder mixture having `À'' composition displayed

a very critical dependence on heating rate. Less than1�C/min was su�cient to swap from a permanentexpansion to a contraction, as shown by the dilato-metric curves for 2 and 2.5�C/min (Fig. 9). A bettersintering behaviour at the higher heating rate is alsocon®rmed by the phase compositions of the bulk speci-mens, as indicated in Table 3. The occurrence of aminor amount of Ni3Al can be noticed in addition tothe target phases: NiAl and Mo. Additional experi-ments, carried out at higher heating rates, up to a max-imum of 30�C/min, con®rmed a positive response of thepowders to the sintering process up to 10�C/min,

whereas at the highest rates the bulk materials tend toloose the original shape of the ``green'' compacts.For the ``B'' composition a similar transition in the

densi®cation behaviour, as that observed for `À'' com-position, occurred when increasing the heating ratefrom 7.5 to 10�C/min. The expected phases were allpresent in the ®nal products, as displayed by the XRDpatterns in Fig. 10. Little amounts of nickel and, to alower extent in the 10�C/min sample, of AlMo3 were

Table 1

Sintering parameters and resulting phases for the samples having Ni3Al composition

Working atmosphere Heating rate

(�C/min)

Compaction

pressure

(MPa)

Exothermic

peak (�C)Phases (wt%)

Ni NiAl Ni3Al Ni2Al3

Hydrogen 2 300 NO 68 13 9 10

Hydrogen 10 300 631 68 15 11 6

Vacuum 2 300 NO 39 22 35 4

Vacuum 10 300 494 29 6 65 0

Vacuum 15 300 504 34 4 62 0

Vacuum 20 300 504 18 2 80 0

Vacuum 10 400 495 2 11 87 0

Fig. 4. Optical micrographs of the Ni3Al specimen sintered under vacuum at (a) 10�C/min and (b) 20�C/min.

Fig. 5. Dilatometric and temperature curves for the NiAl specimen

sintered at 10�C/min under vacuum.


also detected. The 10�C/min was the highest heatingrate adopted for ``C'' composition and it was su�cientto induce a satisfactory sample densi®cation, whereas aresidual expansion was noticed at 7.5�C/min. The lowerreactivity of the ``C'' powder mixture, owing to its lower

aluminium content, leads to a signi®cant concentrationof nickel (34 wt% nearly) in the bulk specimen. Ni3Al(39 wt%), NiAl (23 wt%) and Mo (4 wt%) were theother phases detected in the di�raction pattern (Table3).

4. Discussion

The present investigation is part of an alloy develop-ment programme, aiming at the attainment of NiAl-base alloys through a PM route. The idea is to improvefracture toughness of these materials by a secondaryductile phase dispersion. For this reason two- and three-phase compositions, NiAl, Ni3Al and Mo, have beeninvestigated, following an approach which has alreadyproved of some interest not only in toughening NiAl[17±18] and other intermetallic systems [19], but alsocomposites featuring a brittle matrix [20].Reaction-sintering was chosen to prepare the bulk

specimens in view of the interesting possibilities thismethod a�ords for the preparation of refractory andbrittle intermetallic materials. The main limits of thistechnique are probably related to the di�culty of con-trolling the phase formation reactions and the sinteringprocesses to get high quality, pore free and homo-geneous materials.

Fig. 7. Optical micrographs of the NiAl specimen sintered under

vacuum at 4�C/min.

Table 2

Sintering parameters and resulting phases for the samples having NiAl

compositiona

Heating

rate

(�C/min)

Compaction

pressure

(MPa)

Exothermic

peak

(�C)

Phases (wt%)

NiAl Ni3Al Ni2Al3

2 300 NO 100 0 0

3 300 473 100 0 0

4 300 477 100 0 0

10 300 484 100 0 0

2 400 632 89 4 7

a Vacuum was the sintering atmosphere in all cases.

Table 3

Summary of the results for ternary alloysa

Sample

composition

Heating

rate

(�C/min)

Exothermic

peak

(�C)

Phases (wt%)

Ni NiAl Ni3Al Mo AlMo3

A 2 635 1 89 1 9 0

A 2.5 618 0 90 0 10 0

B 7.5 620 6 75 11 8 0

B 10 575 1 84 4 10 1

C 10 580 33 24 39 4 0

a Processing conditions: atmosphere: vacuum; compaction pressure:

300 MPa.

Fig. 8. XRD pattern of the NiAl sample heated at 2�C/min (pre-

sintering compaction pressure 400 MPa). The experimental (dots) and

simulated (line) patterns are displayed.

Fig. 6. XRD pattern of the NiAl sample heated at 10�C/min (pre-

sintering compaction pressure 300 MPa). The experimental (dots) and

simulated (line) patterns are displayed.


In the present study, experimental observations onbinary Ni±Al samples were mainly meant to provideindications on the kinetics of the reactions occurringduring the dilatometric runs and on the in¯uence thetreatment parameters may have on the microstructureof the ®nal products. On this basis, more favourableprocessing parameters for the ternary alloys could beselected.According to well established combustion synthesis

theories [21], as powder compacts are heated up, solidstate reactions start to occur at the interfaces of powdergrains to form intermetallic phases. These exothermicreactions, if su�ciently powerful, will induce the for-mation of a liquid phase in the intergranular regions.This liquid phase will spread through porosity andspeed up all di�usive processes needed both to form thetarget intermetallics and to promote powder densi®ca-tion. In the Ni3Al binary composition an exothermicpeak (onset at 494�C, Fig. 1) was recorded in the tem-perature curves of the samples for which a heating rateof at least 10�C/min was used. A sharp expansion in the

dilatometric curve was observed at the same temperature,which was not fully recovered when cooling the sampledown to room temperature. The exothermic peak in thetemperature curve can be associated with the formationreaction of Ni3Al. The heat output induces the forma-tion of a liquid phase and a corresponding volumeexpansion. The liquid, if present to a su�cient extent,further enhances the formation reactions and the densi-®cation of the materials. Too low heating rates allowthe solid state reactions to proceed to such an extentthat the amount of liquid phase, which will form in theend, may not be su�cient to achieve a good densi®ca-tion and/or phase formation. This situation could beavoided when high rates, e.g. 20�C/min or more, wereadopted for the Ni3Al composition (Fig. 2(a) and (b)).It is worth mentioning other two processing parameterswhich were considered with reference to their e�ects onsintering process of the Ni3Al powders: sintering atmo-sphere and compaction pressure. A hydrogen ¯ux wasused, an alternative to vacuum, with the idea of redu-cing the oxide layers present on the grains of powders,which might prevent both formation and sinteringreactions. From the dilatometric curves and di�ractionanalyses (not included herein) it turned out that suchatmosphere played not a positive role. Even thoughunder hydrogen a comparable densi®cation as that forthe vacuum sintered materials was observed, far less wasthe content of Ni3Al found in the products, featuringinstead a signi®cant concentration of unreacted nickel(Table 1).Two main reasons can be put forward to justify this

sort of behaviour and it cannot be ruled out both ofthem will tend to reduce the overall reaction e�ciency.Hydrogen ¯ux would act as an e�ective reactionenthalpy remover. In this way too a little fraction of thetotal heat developed by the solid state formation reac-tions is available to trigger the liquid phase formation.The high concentration of apparently unreacted nickelin the bulk specimens can be also due to nickel hydride(e.g. NiH2) formation reactions, occurring at high tem-peratures in competition with those for the intermetallicphase. Hydride(s), not stable at room temperature, wasnot found in the sintering products. Therefore, for allother binary and ternary materials vacuum sinteringtreatments were adopted. As to the compaction pres-sure, the experiments carried out with the Ni3Al andNiAl binary compositions provided two opposite indi-cations, in that the yield of the reactions for the forma-tion of the above intermetallic phases increased anddecreased respectively as the compaction pressure wasincreased (Tables 1 and 2). A reasonable explanation isa general enhancement of the kinetics of the solid statereactions brought about by a higher compaction pres-sure, through the increase in the density of the ``green''compacts. For the Ni3Al composition the solid-statereactions will then occur to the right extent to get the

Fig. 9. Dilatometric and temperature curves for the `À'' composition

specimen sintered under vacuum at (a) 2�C/min and (b) 2.5�C/min.


formation of a liquid phase. In the NiAl system thesolid-state processes turn out to be more favouritereaction routes when a high density of the ``green'',possibly, in conjunction with higher concentrations ofdeformation defects, is present. In view of these resultsthe lower pre-sintering compaction pressure, i.e. 300MPa, was actually adopted for all ternary materials.Another useful indication, coming from the sintering

of the binary NiAl alloys, then extended to ternarycompositions, regards an upper limit which is to beconsidered for the heating rate. It has already beenpointed out that there is a threshold value for heatingrate necessary to get a su�cient amount of the liquid phase.On the other hand, when exceedingly high rates wereused, resulting in the formation of too large an amount ofthe liquid phase, samples completely lost their shape inthe course of the dilatometry runs. Another noticeableaspect is the signi®cantly higher reaction temperaturesfor the ternary specimens as compared to the binaryones. This e�ect, which will be investigated further, ismost likely determined by the presence of molybdenumin the starting powder mixtures. This element increasesthe eutectics of the system and reduces the di�usioncoe�cients both in the liquid and solid phases.

5. Conclusions

Reaction sintering of Ni±Mo±Al alloys was investi-gated, with particular reference to the e�ects of proces-sing parameters, like compaction pressure of powdermixtures, sintering atmosphere and heating rate.Binary Ni3Al and NiAl compositions were considered

®rst and provided useful data for the processing ofternary alloys. The main indications for an e�cient sin-tering reaction to occur may be summarised as follows:

(i) vacuum is to be preferred as sintering atmo-sphere. The other one considered in this study,i.e. hydrogen, inhibited the intermetallic phaseformation reactions;

(ii) a minimum heating rate has to be used to havethe formation of a su�cient amount of liquidphase and to make liquid phase sintering com-petitive to solid state reactions. Nonetheless,these are important to initiate the whole process;

(iii) heating rate cannot be arbitrarily increased, asan exceedingly large amount of liquid phaseresults in the loss of sample geometry duringsintering;

(iv) the higher the compaction pressure, the higheris the rate of all solid-state reactions. This is apositive factor for Ni3Al materials, whereas itturns out to be a problem for the NiAl com-position. In this case, in fact, solid state reac-tions become the most favoured ones andresult in the formation of other intermetallicphases than the expected one.

For ternary composition alloys, the best conditionsfor intermetallic formation and densi®cation were easilymet and the only signi®cant di�erence with respect tothe binary alloys is in the higher reaction temperatures.

Acknowledgements

The authors are grateful to Dr. N. Orsi for his ener-getic contribution to the research. Financial support hasbeen obtained from the Italian Ministero della RicercaScienti®ca e Tecnologica in the framework of a NationalResearch Project entitled ``Leghe e composti intermetallici.StabilitaÁ termodinamica, proprietaÁ ®siche e reattivitaÁ ''.

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Reaction-sintering of intermetallic alloys of the Ni–Al–Mo system