PECULIARITIES OF CHANGE IN STRUCTURE AND PROPERTIES OF SOLID-PHASE COMPOUND OF OT4-1ALLOY OBTAINED BY COLD FORCED FIT AND FURTHER THERMAL TREATMENT

Regularities of the effect of maximum value of stress-strain state,  obtained under conditions of cold forced fit of blanks of the shaft-hole  system during formation of solid-phase diffusion bond (SDB) and subsequent  modes  of  temperature  action  in  autonomous  vacuum,  are  investigated on evolution of structural changes and properties of contact  area  (CA)  of  solid  joints  of  OT4-1  alloy.  It  is  shown  that  under  cold  plastic  deformation  of  OT4-1  alloy,  formation  of  SDB  in  microstructure of CA leads to generation of deformation relief (traces of sliding  along  the  grain  boundaries),  decrease  in  contact  surfaces,  and  to  volume interaction, both in the plane of contact (curvature of grains) and  in  volume  of  contact  zone  (outbreaks  of  dislocations).  The  main  parameters  (specific  parameter  of  structure  organization,  grains  density,  average density of grain boundaries, development of grain boundaries)  exceed  those  of  the  initial  state  of  base  metal  in  10,  4,  1.8,  1.5 times  respectively. Temperature influences under conditions of autonomous  vacuum in the interval of phase transformations α → β lead to staging  of structural changes, both in the main metal and in SDB contact area.  At the initial moment, globular component appears in microstructure,  which  again  goes  back  to  acicular  structure  of  the  initial  state  (with  some  increase  in  microhardness)  with  increase  in  holding  time,  and  also  with  increase  in  temperature.  For  the  first  time,  phenomenon  of  appearance of the globular structure formation stage during heating of  plastically  deformed  metal  is  established  not  only  under  temperature  and  time  conditions  of  phase  transformation,  but  also  under  elevated temperatures; and the higher heating temperature is, the shorter is lifetime  of  the  stage.  Moreover  with  less  degree  of  plastic  deformation,  stage of structure globularization is observed at temperatures close to  T_pt  and  shorter  exposures.  For  base  metal  (degree  of  deformation  is  insignificant),  globular  structure  disappears  almost  completely  after  heating for 10 min at 950 °C. For stress-strain state of cold-deformed  SDB, globular structure disappears when heated: for 1 hour at 950 °C;  for  40  min  at  975 °C;  for  20  min  at  1000 °С. At  these  temperatures,  process of discontinuities “healing” is almost completed, i.e. bond line  disappears,  and  solid  metal  is  formed  along  the  microstructure  of  the  CA,  not  differing  from  the  basic  metal  with  insignificant  increase  in  microstructure. Quantitative assessment of structural changes in basic  parameters  of  interface  of  structural  state  makes  it  possible  to  reveal  mechanism,  kinetics  and  structural  dependence  on  degree  of  plastic  deformation  and  heat  treatment  regimes,  that  ensure  discontinuities  “healing”,  disappearance  of  interfaces  and  provision  of  SDB  properties no worse than those of basic metal.

structure, solid-phase connection, forced fit, heat treatment, titanium alloy, structural state interface

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