dissertationHydrogen sintering and phase transformation (HSPT) is a novel low-cost process for powder metallurgy (PM) titanium alloys. The microstructure obtained from the HPST process has very fine Œ± grains, as well as other common features of a PM product, such a

Cao, Fei

English

dissertationHydrogen sintering and phase transformation (HSPT) is a novel low-cost process for  powder  metallurgy  (PM)  titanium  alloys.  The  microstructure  obtained  from  the  HPST  process has very fine α grains, as well as other common features of a PM product, such as  residual pores and large α grains. This research is aimed at evaluating the microstructure  and mechanical behavior of Ti-6Al-4V alloy made from TiH2 powders with different size  distributions  and  a  variety  of  processing  parameters.  Pneumatic  isostatic  forging  (PIF)  was  carried  out  under  β-tarsus  to  eliminate  residual  porosity  without  coarsening  the  grains. Tensile, fatigue, and crack growth properties of HSPT processed alloys, as well as  wrought Ti-6Al-4V alloys, were studied in this research to investigate the microstructure- property  relationships,  with  an  emphasis  on  the  effect  of  microstructural  inhomogeneity  on fatigue behavior.  Tensile  properties of  the HSPT Ti-6Al-4V  alloys  were  found  equivalent  to  or  exceeded  those  of  mill-annealed  alloy.  The  low  cycle  fatigue  (LCF)  life  was  also  found  equivalent to that of wrought Ti-6Al-4V after PIF. However, the high cycle fatigue (HCF)  performance  was  still  inferior  to  that  of  wrought  material  due  to  the  large  α  phase  grain/grain boundary α phase plate, which was found to initiate cracks from inside of the  specimen in HCF range. It is also shown that with the optimization of the distribution of  powder sizes and the processing parameters, the properties of HSPT processed Ti-6Al-4V  alloys  are  much  superior  to  those  of  Ti-6Al-4V  alloys  made  by  other  conventional      iv    blended  elemental  (BE)  methods,  and  equivalent  to  best  properties  found  in  alloys  produced  by  pre-alloyed  method  in  hot  iso-static  pressed  (HIPed)  and  heat  treated  condition.  The S-N behaviors of different alloys were studied. Crack initiation types, sizes, and  locations were found to alter the fatigue failure mechanism and the shape of S-N curves.  The  quantitative  correlations  between  crack  initiation  size,  location,  and  the  S-N  curve  shape were also revealed in this study. Fatigue crack growth tests were carried out using  miniature  single  edge-notched  tension  (SENT)  specimens  to  evaluate  the  crack  growth  behavior. Crack growth stages were classified based on the characteristics of fractographs  of  both  fatigue  failed  samples  and  crack  growth  samples.  Microstructurally  small  crack  growth rate was calculated from rough area size and fatigue life consumed in this region.  The  predicted  S-N  curves  at  HCF  region  based  on  crack  growth  rate  at  different  stages  for Ti-6Al-4V alloys with different crack initiator sizes match very well with the current  HCF data

The University of Utah: J. Willard Marriott Digital Library

Doctor of Philosophy

https://collections.lib.utah.edu/dl_files/ed/56/ed566828c8b30ec34ac204dd3e136a228d0d2ee4.pdf

Doctor of Philosophy

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