sintered/solutionized at 1300oC for 100 h and quenched. y-Ni3Mo was ... The mechanical alloying process, cold compaction and sintering were used to prepare.
Materials Science and Technology (MS&T) 2014 October 12-16, 2014, Pittsburgh, Pennsylvania, USA Copyright © 2014 MS&T14®
BULK PROCESSING OF AND REVERSE PERITECTOID PHASE TRANSFORMATION IN Ni3Mo ALLOY I. K h a lfa lla h , A . A n in g M aterials Science and E ngineering Departm ent, V irg in ia P olytechnic, Institute and State U n ive rsity, 213 H olden H a ll, B lacksburg, V A , 24061, U S A K eyw ords: Phase transform ation, N i3M o alloy, Reverse peritectoid A b s tra c t The m icrostructural evolutio n and m icrohardness o f the product o f reverse peritectoid reaction in N i 3M o a llo y was studied. H igh-energy b a ll m illin g was used to produce a p a rtia lly alloyed elemental n icke l and m olybdenum o f N i3M o com position, cold-com pacted and then sintered/solutionized at 1300oC fo r 100 h and quenched. y-N i3M o was form ed iso the rm ally at 600oC. Reverse peritectoid transform ation was perform ed between 910oC and 1050oC. Structural and m orphological characterization o f the m ille d powder, sintered and heat-treated samples were perform ed using X R D , optical and scanning m icroscopies. Hardness values increased q u ic k ly in itia lly to as h igh as tw ice that o f the tra nsform ing N i3M o in te rm e ta llic at 910oC, b ut then decreased gradually w ith both transform ation tim es and temperatures. In tr o d u c tio n M icro stru ctu ra l design in alloys u sua lly in vo lves co oling the a llo y fro m a single phase region to a m u ltip le phase region, a typ ica l exam ple being the heat treatm ent o f steel. It is not com m on to heat up a single a llo y to design m icrostructure in a two-phase region at a higher temperature outside pre cipita tio n in v o lv in g a supersaturated solution or transform ation o f other unstable phase structures e xem plified by spherodite or tempered martensite. This w o rk reports on m icrostructural e vo lu tio n d uring reverse perictectoid transform ation in the N i- M o a llo y system and th e ir corresponding mechanical properties. Here, the reaction is given by y -N i3M o — a -N i + S -N iM o. The mechanical a llo yin g process, cold com paction and sintering were used to prepare b u lk samples o f the in term e ta llic, y -N i3M o , and this has already been reported [1 ]. In that w o rk supersaturated N i-2 5 a t% M o a llo y transform ed to y -N i3M o b e lo w the peritectoid temperature. X R D pro files showed that the solid solution a -N i sequentially transform ed to N i2M o , P -N i4M o and then y -N i3M o upon prolonged aging. W h ile there have been a num ber o f studies on the structural developm ent o f y -N i3M o [1 -4 ] and P -N i4M o [5-7] fro m a -N i solid solution and SN iM o [8 -1 0 ], the developm ent o f a -N i + S -N iM o structure is not w e ll know n. This study, thereby, reports the study o f the m icrostructural evolutio n and the related properties resulting fro m the transform ation o f y -N i3M o to a -N i + S -N iM o. E x p e rim e n ta l P ro ced u re s Elem ental blend o f N i-2 5 at% M o was m echanically alloyed, cold compacted, and sintered. The p u rity o f nicke l pow der was 99 w t.% and o f m olybdenum , 99.9 w t.% , both purchased fro m A lfa Aesar. The powders were reduced under flo w in g hydrogen at 500°C fo r 1 hour. The blended m ixtures were m echanically alloyed b y m illin g in a tungsten carbide m edia in a SPEX 8000M
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M ix e r /M ill® fo r 10 h, pressed at 550 M P a in to coins o f 19 m m in diam eter and 4 m m th ic k using a steel u niaxia l m old. S intering/hom ogenizing o f the cold compacted samples was perform ed under hydrogen in a tube furnace at 1300°C fo r 25 h. The cold com paction and the sintering routines were repeated fo r total o f 100 h sintering fo llo w e d b y quenching, in order to im prove density. O ne-m m th ic k sample slices were heat-treated in a salt bath at 600°C fo r 100 h, in order to fo rm the e q u ilib riu m y -N i3M o phase. The reverse peritectoid transform ations were perform ed w ith the y -N i3M o samples in salt baths between 910°C and 1050°C fo r up to 10 h. Figure 1 shows sample preparation and heat-treatm ent steps. Heat-treated samples were characterized using X -ra y d iffra ctio n , optical and scanning electron m icroscopies (S E M ). V icke rs m ic ro hardness values were obtained fo r all samples.
Ni
25 Atomic Perecnt of Mo
50
Figure 1 Part o f N i- M o b in ary e q u ilib riu m phase diagram w ith the heat-treatm ent steps. R esults a nd discussion Figure 2 shows the X R D patterns o f blended N i- M o pow der (see Figure 2a), p a rtia lly m echanically alloyed N i- M o pow der (Figure 2b), a com plete solid solution (F igure 2c) after successive com paction and then sintering routines at 1300°C fo r total o f 100 h and transform ed
o
y -N i3M o sample at 600 C fo r 100 h (Figure 2d). The (111) peak o f the fcc N i shifts to the lo w e r angles as the solid solution form s by m echanical a llo yin g as w e ll as sintering (Figures 2b and 2c). This effect is due to the increase o f the lattice parameter o f N i caused b y the dissolution o f larger size m olybdenum atom in a -N i phase. Figures 3 (a-b) shows the X R D p ro files o f heattreated samples at 910°C fo r 5 m in up to 10 h. The transform ation fro m y -N i3M o to a -N i is observed; the small am ount o f S -N iM o present is not picked up in the X R D . The sh iftin g o f the a -N i peaks to higher angles is evidence that the M o concentration in a -N i is decreasing, and hence fo rm a tio n o f the S -N iM o phase. X R D fo r reverse peritectoid transform ations at 970°C and 1050°C are shown in Figures 4 and 5, respectively. The transform ation sequencing is sim ila r to the one at 910°C except that this tim e the S -N iM o peaks are observed. S E M m icrographs o f the sintered and representative heat-treated samples are shown in Figures 6 to 8. Figure 6a shows the m icrostructure o f the cold-com pacted and sintered sample at 1300°C fo r 100 h and fo llo w e d by
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quenching, i.e., supersaturated solid solution. I t is a little porous, has equiaxed grain structure and includes substantial annealing tw ins. The m icrostructure o f the e q u ilib riu m y -N i 3M o , after heat-treatm ent at 600°C fo r 100 h, is shown in Figure 6b. The developm ent o f y -N i3M o form ed fro m solid solution a -N i has been reported on pre viou sly [1 ]. H eat-treatm ent at 910°C on the w h o le reveals fe w y -N i3M o regions, and they tend to be found at grain boundaries, as seen in Figures 7a and 7b. H ow ever, in a ve ry fe w regions a p e a rlite -like structure (see Figure 6 c) is found. This n on -u n ifo rm m icrostructure is probably due to non-hom ogeneity in the M A and sintered M 3M 0 alloy.
