T. Waitz et al. Acta Materialia 52(2004)137-14 01 MT1 101)MT2 nm. Fig 9. Ni-50.3at %Ti. Nanocrystalline structure; martensite HRTEM image showing(00 1) compound twins having a minimum width of 4 lattice planes(0.9 nm)only (near A). The(001)MT1.2 twin boundary plane and the(10 1).] planes of the twin related martensite variants MTl and MT2 are indicated by a dashed and a full line, respectively(BD=[O1OMT12) gradually decreases until only isolated nanocrystals are 340C nanocrystallization occurs since the retained left embedded heterogeneously in an amorphous matrix. crystallites can act as heterogeneous nucleation sites Finally, caused by the plastic deformation of the leading to a high nucleation rate although the rate of amorphous matrix the retained nanocrystals dissolve in growth is low since Ta/ Tm=0.39 is small(Im melting he amorphous phase until at S=7.3 only a few very temperature). In agreement with the present conclusion small nanocrystals survive. Since most of them have the it was proposed that the devitrification process should B2 structure it is concluded that the austenite is more proceed with the largest nucleation rate and slowest stable against amorphization than the b19 martensite. growth rate to obtain a nanocrystalline structure; during This is in agreement with previous results [13]. In the low temperature annealing retained crystallites embed present case it is concluded that the nanocrystalline ded in the amorphous phase can act as nuclei since they debris contains austenite that was retained during the do not dissolve and can exceed the critical size for het thermally induced B2 to B19 transformation [14] prior erogeneous nucleation [17] to HPT. Still their might be another explanation: the It is concluded that the nucleation rate depends on observed B2 nanocrystals could be formed by a stress the HPt strain and decreases with increasing S(as can induced reverse transformation B19 to B2 during HPt be seen by an increase of Tx and Tp: cf. Fig 3). This is That might be caused by a local increase of temperature explained as follows: in the alloy deformed up to S=7.3 exceeding the austenite start temperature(As a 110C in the nucleation rate is lower since a only a small volume the present case). Similar results of a deformation in- fraction <1% of tiny crystallites <15 nm is retained(cf duced reverse transformation were reported by [15] Fig. 2). Contrary to this, in the case of s=6.7 nucle ation is triggered by numerous retained crystallites with 4.2. Nanocrystallization a diameter up to about 30 nm having a high nucleation potency(cf. Fig. I; a retained crystalline volume fraction The results of Figs. 3 and 4 show that during an- Ver of about 18% is deduced using Ver =1-(4H6.7/ ealing in HPT induced amorphous Ni-50. 3at %T1 △H7:3) loys heterogeneous polymorphous crystallization of the In the amorphous matrix the growth of the nuclei B2 phase occurs about 150C lower as compared to thin ceases when the advancing interfaces of neighboring amorphous NiTi films processed by sputtering or melt crystallites impinge on each other leading to grain spinning(Tr510C[16). It is concluded that the low boundaries. As the density of the nuclei increases their thermal stability of the amorphous phase formed by mean separation and therefore the final grain size will HPT is caused by the retained nanocrystalline debris decrease. Since the density of the nuclei is lower in the already at an annealing temperature as low as te, case of S=7.3 annealing at Ta=340C is leading to a triggering crystallization(see Figs. I and 2). Therefor larger grain size as compared to the alloy deformed upgradually decreases until only isolated nanocrystals are left embedded heterogeneously in an amorphous matrix. Finally, caused by the plastic deformation of the amorphous matrix the retained nanocrystals dissolve in the amorphous phase until at S ¼ 7:3 only a few very small nanocrystals survive. Since most of them have the B2 structure it is concluded that the austenite is more stable against amorphization than the B190 martensite. This is in agreement with previous results [13]. In the present case it is concluded that the nanocrystalline debris contains austenite that was retained during the thermally induced B2 to B190 transformation [14] prior to HPT. Still their might be another explanation: the observed B2 nanocrystals could be formed by a stress induced reverse transformation B190 to B2 during HPT. That might be caused by a local increase of temperature exceeding the austenite start temperature (As  110 C in the present case). Similar results of a deformation in￾duced reverse transformation were reported by [15]. 4.2. Nanocrystallization The results of Figs. 3 and 4 show that during an￾nealing in HPT induced amorphous Ni–50.3at.%Ti al￾loys heterogeneous polymorphous crystallization of the B2 phase occurs about 150 C lower as compared to thin amorphous NiTi films processed by sputtering or melt spinning (Tx510 C [16]). It is concluded that the low thermal stability of the amorphous phase formed by HPT is caused by the retained nanocrystalline debris triggering crystallization (see Figs. 1 and 2). Therefore, already at an annealing temperature as low as Ta ¼ 340 C nanocrystallization occurs since the retained crystallites can act as heterogeneous nucleation sites leading to a high nucleation rate although the rate of growth is low since Ta=Tm ¼ 0:39 is small (Tm melting temperature). In agreement with the present conclusion it was proposed that the devitrification process should proceed with the largest nucleation rate and slowest growth rate to obtain a nanocrystalline structure; during low temperature annealing retained crystallites embed￾ded in the amorphous phase can act as nuclei since they do not dissolve and can exceed the critical size for het￾erogeneous nucleation [17]. It is concluded that the nucleation rate depends on the HPT strain and decreases with increasing S (as can be seen by an increase of Tx and Tp; cf. Fig. 3). This is explained as follows: in the alloy deformed up to S ¼ 7:3 the nucleation rate is lower since a only a small volume fraction <1% of tiny crystallites <15 nm is retained (cf. Fig. 2). Contrary to this, in the case of S ¼ 6:7 nucle￾ation is triggered by numerous retained crystallites with a diameter up to about 30 nm having a high nucleation potency (cf. Fig. 1; a retained crystalline volume fraction Vcr of about 18% is deduced using Vcr ¼ 1
ðDH6:7= DH7:3Þ. In the amorphous matrix the growth of the nuclei ceases when the advancing interfaces of neighboring crystallites impinge on each other leading to grain boundaries. As the density of the nuclei increases their mean separation and therefore the final grain size will decrease. Since the density of the nuclei is lower in the case of S ¼ 7:3 annealing at Ta ¼ 340 C is leading to a larger grain size as compared to the alloy deformed up Fig. 9. Ni–50.3at.%Ti. Nanocrystalline structure; martensite. HRTEM image showing (0 0 1) compound twins having a minimum width of 4 lattice planes (0.9 nm) only (near A). The ð001ÞMT1;2 twin boundary plane and the ð1 0
1ÞMT1;2 planes of the twin related martensite variants MT1 and MT2 are indicated by a dashed and a full line, respectively (BD ½010MT1;2). 144 T. Waitz et al. / Acta Materialia 52 (2004) 137–147