Precipitation Hardening in the 90W-5Ni-5Fe Heavy Alloy
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PRECIPITATION HARDENING IN THE 90W-5Ni-5Fe HEAVY ALLOY
J.B. POSTHILL AND D.V. EDMONDS Department of Metallurgy & Science of Materials, University of Oxford, Parks Road, Oxford OXI 3PH
ABSTRACT Precipitation hardening in the W-phase of the 90W-5Ni-5Fe alloy has been identified and monitored by transmission electron microscopy and mechanical tests. Deformation of the alloy prior to aging is observed to accelerate the precipitation reaction. Possible precipitate origin and geometry are discussed in terms of the TEM observations.
INTRODUCTION Tungsten heavy alloys are a group of materials based on tungsten (80-97wt%) and produced by liquid-phase sintering in the presence of nickel and other element(s) (e.g. Cu, Fe, Cr). The resulting microstructure basically consists of spheroidal tungsten-rich bcc grains (20-30lm diameter) embedded in a fcc matrix (Fig.1). This structure leads to an interesting and technologically important combination of properties, viz.(I) 3 2 high density (16-18.5g/cm ), (2) high tensile strength (800-1200N/mm ), and (3) good tensile elongation (10-30%). When extra strength is required, the alloy can be cold worked to achieve strain hardening. Preliminary studies of the effect of cold work also identified a hardening response in the W-phase following aging in the temperature range 600-750eC. A description of this age hardening, in both cold worked and undeformed 90W-5Ni-5Fe alloys, is the subject of the present paper.
Fig.l. Optical micrograph of the two-phase 90W-5Ni-5Fe heavy alloy.
Mat.
Res. Soc. Symp.
Proc. Vol.
21 (1984) QElsevier Science Publishing Co..
Inc.
812
RESULTS Mechanical Testing Figure 2 shows the microhardness of the W-phase as a function of aging temperature for material reduced 10.1% by cold rolling (annealing time = I The material hour) and undeformed material (annealing time = 100 hours). with prior deformation, while exhibiting higher overall hardness, also shows 600'C for I hour) than a much more well defined W-phase hardness peak (at the undeformed material.
10.1% rolled (t=1 hr)
540 52o
Prestrained (t=100 hrs)
980 960
500 E 940 E Z0)920
E 480 - 0460
Prestrained
c)440
+
>L 420 400 X
380 as rec. 400 as rolled
X X X Undeformed
900 X
(t=100 hrs)
(t=100 hrs) 500
600 700 T(oC)
Undeformed
X
880 800
Fig.2. Microhardness of W-phase vs. aging temperature for 10.1% rolled and undeformed heavy alloy,
900
as rec.
500
600 0 T( C)
700
800
Fig.3. Ultimate tensile strength vs. aging temperature for undeformed heavy alloy. Also shown are - 8% prestrained and aged specimens.
Tensile strength results obtained for the undeformed material clearly show that a strengthening response is present (Fig.3). Shown also are UTS measurements for specimens that were first prestrained in tension to - 8% 0 plastic strain, unloaded, annealed at 600 C for times of I hour and 100 With prestraining and aging at 600'C for I hours and subsequently tested. hour the UTS is as high as the peak strength obtained upon aging undeformed material for 100 hours. The UTS after prestra
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